MR Montage Resources Corporation

ABOUT THIS PROXY STATEMENT/PROSPECTUS

This document, which forms part of a registration statement on Form S-4 filed with the U.S. Securities and Exchange Commission (the “SEC”) by Southwestern, constitutes a prospectus of Southwestern under Section 5 of the Securities Act of 1933 (as amended, the “Securities Act”) with respect to the shares of Southwestern Common Stock to be issued to Montage stockholders pursuant to the Merger Agreement. This document also constitutes a proxy statement of Montage under Section 14(a) of the Securities Exchange Act of 1934 (as amended, the “Exchange Act”). It also constitutes a notice of meeting with respect to the Montage Special Meeting.

You should rely only on the information contained in or incorporated by reference into this proxy statement/prospectus. Southwestern and Montage have not authorized anyone to provide you with information that is different from that contained in or incorporated by reference into this proxy statement/prospectus. This proxy statement/prospectus is dated October 6, 2020. The information contained in this proxy statement/prospectus is accurate only as of that date or, in the case of information in a document incorporated by reference, as of the date of such document, unless the information specifically indicates that another date applies. Neither the mailing of this proxy statement/prospectus to Montage stockholders nor the issuance by Southwestern of shares of Southwestern Common Stock pursuant to the Merger Agreement will create any implication to the contrary.

This proxy statement/prospectus does not constitute an offer to sell, or a solicitation of an offer to buy, any securities, or the solicitation of a proxy, in any jurisdiction to or from any person to whom it is unlawful to make any such offer or solicitation in such jurisdiction. Southwestern has supplied all information contained in this proxy statement/prospectus relating to Southwestern, and Montage has supplied all such information relating to Montage. Southwestern and Montage have both contributed to the information related to the merger contained in this proxy statement/prospectus.

All references in this proxy statement/prospectus to “Southwestern” refer to Southwestern Energy Company, a Delaware corporation. All references in this proxy statement/prospectus to “Montage” refer to Montage Resources Corporation, a Delaware corporation. All references in this proxy statement/prospectus to “Southwestern Common Stock” refer to the common stock of Southwestern, par value $0.01 per share, and all references in this proxy statement/prospectus to “Montage Common Stock” refer to the common stock of Montage, par value $0.01 per share. All references in this proxy statement/prospectus to “Merger Agreement” refer to the Agreement and Plan of Merger, dated as of August 12, 2020, by and between Southwestern and Montage, as it may be amended, a copy of which is attached as Annex A to this proxy statement/prospectus and incorporated by reference herein. All references in this proxy statement/prospectus to the “Exchange Ratio” refer to the ratio of 1.8656 shares of Southwestern Common Stock per outstanding share of Montage Common Stock that will be issued to Montage stockholders in connection with the merger.

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ADDITIONAL INFORMATION

This proxy statement/prospectus incorporates by reference important business and financial information about Southwestern and Montage from other documents that are not included in or delivered with this proxy statement/prospectus. For a listing of the documents incorporated by reference into this proxy statement/prospectus, see “Where You Can Find More Information” beginning on page 185.

You can obtain any of the documents incorporated by reference into this proxy statement/prospectus free of charge by requesting them by telephone at (832) 796-4068 or in writing at the following address: Investor Relations, 10000 Energy Drive, Spring, Texas 77389.

To receive timely delivery of the documents in advance of the Montage Special Meeting, you should make your request no later than November 4, 2020, which is five business days before the Montage Special Meeting.

You may also obtain any of the documents incorporated by reference into this proxy statement/prospectus without charge through the SEC’s website at www.sec.gov. In addition, you may obtain copies of documents filed by Southwestern with the SEC by accessing Southwestern’s website at https://ir.swn.com/corporateprofile/sec-filings. You may also obtain copies of documents filed by Montage with the SEC by accessing Montage’s website at https://ir.montageresources.com/financial-information/sec-filings.

We are not incorporating the contents of the websites of the SEC, Southwestern, Montage or any other entity into this proxy statement/prospectus. We are providing the information about how you can obtain certain documents that are incorporated by reference into this proxy statement/prospectus at these websites only for your convenience.

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Table of Contents

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QUESTIONS AND ANSWERS

The following questions and answers briefly address some commonly asked questions about the Montage Special Meeting and the merger. They may not include all the information that is important to Montage stockholders. Montage stockholders should carefully read this entire proxy statement/prospectus, including the annexes and the other documents referred to herein.

You are receiving this proxy statement/prospectus because Southwestern and Montage have agreed to combine in an all-stock merger transaction. The combined company will retain the Southwestern Energy Company corporate name and the company’s listing will remain on the NYSE under the ticker “SWN.” As referred to in this proxy statement/prospectus, the “Effective Time” means the date and time when the certificate of merger has been duly filed with the Secretary of State of the State of Delaware, or such later date and time as may be agreed by Southwestern and Montage and specified in the certificate of merger. The Merger Agreement governs the terms of the merger of Southwestern and Montage and is attached to this proxy statement/prospectus as Annex A.

In order to complete the merger, among other things, Montage stockholders must adopt the Merger Agreement in accordance with the General Corporation Law of the State of Delaware (the “DGCL”).

This proxy statement/prospectus serves as both the proxy statement through which Montage will solicit proxies to obtain the necessary stockholder approvals for the merger and the prospectus by which Southwestern will issue shares of Southwestern Common Stock as consideration in the merger.

This proxy statement/prospectus, which you should carefully read in its entirety, contains important information about the Montage Special Meeting, the merger and other matters.

The Merger Agreement is attached to this proxy statement/prospectus as Annex A. For a more complete discussion of the proposed merger, its effects and the other transactions contemplated by the Merger Agreement, please see “The Merger” elsewhere in this proxy statement/prospectus.

Montage stockholders will also be asked to approve, on an advisory (non-binding) basis, the payments that will or may be paid to Montage’s named executive officers in connection with the merger.

Montage stockholders will also be asked to approve the proposal to adjourn the Montage Special Meeting to solicit additional proxies if there are not sufficient votes at the time of the Montage Special Meeting to approve the Merger Proposal or to ensure that any supplement or amendment to this proxy statement/prospectus is timely provided to Montage stockholders.

Your vote is very important, regardless of the number of shares that you own. The approval of the Merger Proposal is a condition to the obligations of Southwestern and Montage to complete the merger.

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Even if you plan to attend the Montage Special Meeting, Montage recommends that you vote your shares in advance as described below so that your vote will be counted if you later decide not to or become unable to attend the Montage Special Meeting. Shares held in “street name” may be voted by you at the Montage Special Meeting only if you obtain a signed legal proxy from your bank, broker or other nominee giving you the right to directly vote the shares.

Approval of the Merger Proposal requires the affirmative vote of holders of a majority of the outstanding shares of Montage Common Stock entitled to vote thereon. Accordingly, a Montage stockholder’s abstention from voting or the failure of a Montage stockholder to vote (including the failure of a Montage stockholder who holds shares in “street name” through a bank, broker or other nominee to give voting instructions to that bank, broker or other nominee) will have the same effect as a vote “against” the Merger Proposal.

Approval of the Adjournment Proposal requires the affirmative vote of holders of a majority of the shares of Montage Common Stock present in person or represented by proxy at the Montage Special Meeting and entitled to vote thereat. Assuming a quorum is present, approval of the Advisory Compensation Proposal requires the affirmative vote of holders of a majority of the shares of Montage Common Stock present in person or represented by proxy at the Montage Special Meeting and entitled to vote thereon. Accordingly, with respect to a Montage stockholder who is present in person or represented by proxy at the Montage Special Meeting, a Montage stockholder’s abstention from voting or the failure of a Montage stockholder to vote will have the same effect as a vote “ against” the Adjournment Proposal and the Advisory Compensation Proposal. The failure of a Montage stockholder who holds shares in “street name” through a bank, broker or other nominee to give voting instructions to the bank, broker or other nominee will have no effect on the outcome of the Advisory Compensation Proposal or the Adjournment Proposal. However, if a Montage stockholder instructs its bank, broker or other nominee regarding some but not all of the proposals, such broker non-vote will have the same effect as voting “against” the Adjournment Proposal but will have no effect on the outcome of the Advisory Compensation Proposal. Regardless of whether there is a quorum, the chairman of the Montage Special Meeting may also adjourn the Montage Special Meeting.

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Because Southwestern will issue a fixed number of shares of Southwestern Common Stock in exchange for each share of Montage Common Stock, the value of the merger consideration that Montage stockholders will receive in the merger will depend on the market price of shares of Southwestern Common Stock at the Effective Time. The market price of shares of Southwestern Common Stock that Montage stockholders receive at the Effective Time could be greater than, less than or the same as the market price of shares of Southwestern Common Stock on the date of this proxy statement/prospectus or at the time of the Montage Special Meeting. Accordingly, you should obtain current market quotations for Southwestern Common Stock and Montage Common Stock before deciding how to vote with respect to the Merger Proposal. Southwestern Common Stock is traded on the NYSE under the symbol “SWN.” Montage Common Stock is traded on the NYSE under the symbol “MR.” The common stock of the combined company will continue to trade on the NYSE under the symbol “SWN.”

For more information regarding the merger consideration to be received by Montage stockholders if the merger is completed, please see “The Merger Agreement—Merger Consideration.”

At the Effective Time, each Montage RSU Award (as defined in “The Merger Agreement—Treatment of Montage Equity Awards”) that is outstanding immediately prior to the Effective Time will be converted into an Assumed RSU Award (as defined in “The Merger—Interests of Montage’s Directors and Executive Officers in the Merger”), with respect to a number of shares of Southwestern Common Stock equal to the product obtained by multiplying (i) the applicable number of shares of Montage Common Stock subject to such Montage RSU Award by (ii) the Exchange Ratio. For each holder of a Montage RSU Award, any fractional shares resulting from the conversion of his or her Montage RSU Awards will be rounded to the nearest whole share. Except as otherwise provided in the Merger Agreement, each Assumed RSU Award will continue to have, and will be subject to, the same terms and conditions (including time vesting conditions and, if applicable, any accelerated vesting in connection with a termination of service) that applied to the underlying Montage RSU Award immediately prior to the Effective Time, except that Southwestern (x) may modify terms rendered inoperative by reason of the transactions contemplated by the Merger Agreement or for such other immaterial administrative or ministerial changes as in the reasonable and good faith determination of Southwestern are appropriate to effectuate the administration of the Assumed RSU Award, and (y) may settle the Assumed RSU Award upon vesting in shares of Southwestern Common Stock or cash.

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The Merger Agreement provides that Montage shall take all necessary and appropriate actions so that, prior to the Effective Time, each then outstanding Montage PSU Award (as defined in “The Merger Agreement—Treatment of Montage Equity Awards”) will be terminated and vested in accordance with its terms. Southwestern will settle such Montage PSU Awards in Southwestern Common Stock (determined in the same manner as described above for converting Montage RSU Awards into Assumed RSU Awards) or cash.

The Merger Agreement further provides that Montage shall take all necessary and appropriate actions so that prior to the Effective Time each Montage Restricted Stock Award (as defined in “The Merger Agreement—Treatment of Montage Equity Awards”) granted to a Montage non-employee director shall vest. Shares of Montage Common Stock attributable to such Montage Restricted Stock Awards will be treated in the manner as applies to shares of Montage Common Stock, as set forth in the Merger Agreement, at the Effective Time.

The directors of Southwestern immediately prior to the Effective Time shall, from and after the Effective Time, be the directors of the surviving corporation until their successors shall have been duly elected or appointed and qualified or until their earlier death, resignation or removal in accordance with Southwestern’s certificate of incorporation and bylaws.

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Shares in “street name” - Shares held in “street name” may be voted by you at the Montage Special Meeting only if you obtain a signed legal proxy from your bank, broker or other nominee giving you the right to vote the shares. If you choose to vote your shares in street name in person at the Montage Special Meeting, please bring that signed legal proxy along with proof of identification.

Even if you plan to attend the Montage Special Meeting, Montage recommends that you vote your shares in advance as described below so that your vote will be counted even if you later decide not to or become unable to attend the Montage Special Meeting.

Additional information on attending the Montage Special Meeting can be found under “The Montage Special Meeting.”

Additional information on voting procedures can be found under “The Montage Special Meeting.”

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The effect of not instructing your bank, broker or other nominee how you wish to vote your shares will be the same as a vote “against” the Merger Proposal and will not have any effect on the outcome of the Adjournment Proposal or the Advisory Compensation Proposal. However, if you instruct your bank, broker or other nominee on how you wish to vote your shares on some but not all proposals, such broker non-vote will have the same effect as voting “against” the Merger Proposal and the Adjournment Proposal but will not have any effect on the outcome of the Advisory Compensation Proposal.

Record Holders. For shares held directly, please complete, sign, date and return each proxy card, or you may cast your vote by telephone or Internet as provided on each proxy card, or otherwise follow the voting instructions provided in this proxy statement/prospectus in order to ensure that all of your shares of Montage Common Stock are voted.

“Street name” Holders. For shares held in “street name” through a bank, broker or other nominee, you should follow the procedures provided by your bank, broker or other nominee to vote your shares.

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Your attendance at the Montage Special Meeting will not revoke your proxy unless you give written notice of revocation to Montage’s Corporate Secretary before your proxy is exercised or unless you vote your shares in person at the Montage Special Meeting.

Execution or revocation of a proxy will not in any way affect your right to attend the Montage Special Meeting and vote. Written notices of revocation and other communications with respect to the revocation of proxies should be addressed to:

Montage Resources Corporation

Attn: Corporate Secretary

122 W. John Carpenter Freeway

Suite 300

Irving, Texas 75039

For more information, please see “The Montage Special Meeting—Revocation of Proxies.”

A: If your shares are held in the name of a bank, broker or other nominee and you previously provided voting instructions to your bank, broker or other nominee, you should follow the instructions provided by your bank, broker or other nominee to revoke or change your voting instructions.

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Southwestern and Montage also may be required to reimburse banks, brokers and other custodians, nominees and fiduciaries or their respective agents for their expenses in forwarding proxy materials to beneficial owners of Montage Common Stock. Southwestern’s directors, officers and employees and Montage’s directors, officers and employees also may solicit proxies by telephone, by electronic means or in person. They will not be paid any additional amounts for soliciting proxies.

For a more complete discussion of the U.S. federal income tax consequences of the merger, see the section entitled “Material U.S. Federal Income Tax Consequences of the Merger.”

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Montage Resources Corporation

122 W. John Carpenter Freeway

Suite 300

Irving, Texas 75039

Attention: Investor Relations

(469) 444-1736

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SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA OF SOUTHWESTERN

The following table presents selected historical consolidated financial data for the periods indicated below. Southwestern derived the selected historical statements of earnings data for the years ended December 31, 2019, 2018 and 2017 and the balance sheet data as of December 31, 2019 and 2018, from its audited consolidated financial statements included in its Annual Report on Form 10-K for the fiscal year ended December 31, 2019, which is incorporated by reference into this document. The selected historical consolidated financial data for the six months ended June 30, 2020 and June 30, 2019, and as of June 30, 2020, are derived from Southwestern’s unaudited interim consolidated financial statements and related notes thereto contained in Southwestern’s Quarterly Report on Form 10-Q for the quarter ended June 30, 2020, which is incorporated by reference into this proxy statement/prospectus.

The selected historical consolidated financial data for the years ended December 31, 2016 and 2015 and as of December 31, 2017, 2016 and 2015, have been derived from Southwestern’s audited consolidated financial statements as of and for such years, which have not been included or incorporated by reference into this proxy statement/prospectus. The selected historical consolidated financial data as of June 30, 2019, have been derived from Southwestern’s unaudited interim consolidated financial statements and related notes thereto contained in Southwestern’s Quarterly Report on Form 10-Q for the quarter ended June 30, 2019, which has not been incorporated by reference into this proxy statement/prospectus.

In presenting the selected historical consolidated financial data in conformity with GAAP, Southwestern is required to make estimates and assumptions that affect the amounts reported. See “Management’s Discussion and Analysis of Financial Condition and Results of Operations—Critical Accounting Policies and Estimates,” included in its Annual Report on Form 10-K for the fiscal year ended December 31, 2019, and its Quarterly Report on Form 10-Q for the quarter ended June 30, 2020, which are incorporated by reference into this document, for a detailed discussion of the accounting policies that Southwestern believes require subjective and complex judgments that could potentially affect reported results. The unaudited financial statements as of and for the periods described above have been prepared on the same basis as the audited consolidated financial statements incorporated by reference in this proxy statement/prospectus and include all adjustments, consisting only of normal recurring adjustments, necessary for a fair statement of the information for the periods presented.

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The selected historical consolidated financial data set forth below are only a summary and are not necessarily indicative of future results of Southwestern and should be read together with the other information contained in “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and the consolidated financial statements and related notes of Southwestern, included in its Annual Report on Form 10-K for the fiscal year ended December 31, 2019 and its Quarterly Report on Form 10-Q for the quarter ended June 30, 2020, which are incorporated by reference into this proxy statement/prospectus.

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SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA OF MONTAGE

The following table presents selected historical consolidated financial data for the periods indicated below. The selected historical consolidated financial data for the years ended December 31, 2019, 2018 and 2017 and as of December 31, 2019 and 2018, have been derived from Montage’s audited consolidated financial statements included in its Annual Report on Form 10-K for the fiscal year ended December 31, 2019, which is incorporated by reference into this proxy statement/prospectus. The selected historical consolidated financial data for the six months ended June 30, 2020 and June 30, 2019, and as of June 30, 2020, are derived from Montage’s unaudited interim consolidated financial statements and related notes thereto contained in Montage’s Quarterly Report on Form 10-Q for the quarter ended June 30, 2020, which is incorporated by reference into this proxy statement/prospectus.

The selected historical consolidated financial data for the years ended December 31, 2016 and 2015 and as of December 31, 2017, 2016 and 2015, have been derived from Montage’s audited consolidated financial statements as of and for such years, which have not been included or incorporated by reference into this proxy statement/prospectus. The selected historical consolidated financial data as of June 30, 2019, have been derived from Montage’s unaudited interim consolidated financial statements and related notes thereto contained in Montage’s Quarterly Report on Form 10-Q for the six months ended June 30, 2019, which has not been included or incorporated by reference into this proxy statement/prospectus.

In presenting the selected historical consolidated financial data in conformity with GAAP, Montage is required to make estimates and assumptions that affect the amounts reported. See “Management’s Discussion and Analysis of Financial Condition and Results of Operations—Critical Accounting Estimates,” included in its Annual Report on Form 10-K for the fiscal year ended December 31, 2019 and its Quarterly Report on Form 10-Q for the quarter ended June 30, 2020, which are incorporated by reference into this proxy statement/prospectus, for a detailed discussion of the accounting policies that Montage believes require subjective and complex judgments that could potentially affect reported results. The unaudited financial statements as of and for the periods described above have been prepared on the same basis as the audited consolidated financial statements incorporated by reference in this proxy statement/prospectus and include all adjustments, consisting only of normal recurring adjustments, necessary for a fair statement of the information for the periods presented.

The selected historical consolidated financial data set forth below are only a summary and are not necessarily indicative of future results of Montage and should be read together with the other information contained in “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and the consolidated financial statements and related notes of Montage, included in its Annual Report on Form 10-K for the fiscal year ended December 31, 2019 and its Quarterly Report on Form 10-Q for the quarter ended June 30, 2020, which are incorporated by reference into this proxy statement/prospectus. Montage completed its business combination with Blue Ridge Mountain Resources, Inc. (“BRMR”) on February 28, 2019. The results and other financial information of BRMR are not included in the table below for the periods or dates prior to February 28, 2019. For additional information, see the section entitled “Where You Can Find More Information” beginning on page 185.

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SELECTED UNAUDITED PRO FORMA CONDENSED COMBINED FINANCIAL INFORMATION

The following selected unaudited pro forma condensed combined statements of operations data for the six months ended June 30, 2020 and year ended December 31, 2019 have been derived from the “Unaudited Pro Forma Condensed Combined Financial Information” and related notes included in this proxy statement/prospectus and have been prepared to give effect to the merger and related equity and debt issuances along with the use of proceeds therefrom, as if they had been completed on January 1, 2019. The unaudited pro forma condensed combined consolidated balance sheet data at June 30, 2020 has been prepared to give effect to the merger and related equity and debt issuances, along with the use of proceeds therefrom, as if they were completed on June 30, 2020.

The following selected unaudited pro forma condensed combined consolidated financial information is not necessarily indicative of the results that might have occurred had the merger taken place on January 1, 2019 for statements of operations purposes, and on June 30, 2020 for balance sheet purposes, and is not intended to be a projection of future results. Future results may vary significantly from the results reflected because of various factors, including those discussed in the section entitled “Risk Factors.” The following selected unaudited pro forma condensed combined consolidated financial information should be read in conjunction with the section entitled “Unaudited Pro Forma Condensed Combined Financial Information” and related notes included in this proxy statement/prospectus.

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SUMMARY UNAUDITED PRO FORMA COMBINED OIL, NATURAL GAS AND NGL RESERVE INFORMATION AND PRODUCTION DATA

The following tables present a summary of the estimated pro forma combined net proved developed and undeveloped oil, natural gas and NGL reserves as of December 31, 2019, as well as pro forma production data, which have been derived from “Note 4. Supplemental Pro Forma Oil and Natural Gas Reserve Information” included in the section entitled “Unaudited Pro Forma Condensed Combined Financial Statements” within this proxy statement/prospectus and give effect to the merger as if the transaction had occurred on January 1, 2019.

The following estimated pro forma combined net proved developed and undeveloped natural gas, oil and NGL reserve information and production data is not necessarily indicative of the results that might have occurred had the merger taken place on January 1, 2019, and is not intended to be a projection of future results. Future results may vary significantly from the results reflected because of various factors, including those discussed in the section entitled “Risk Factors.” The following summary estimated unaudited pro forma combined net proved developed and undeveloped natural gas, oil and NGL reserve information and production data should be read in conjunction with the section entitled “Unaudited Pro Forma Condensed Combined Financial Information” and related notes included in this proxy statement/prospectus.

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COMPARATIVE HISTORICAL AND UNAUDITED PRO FORMA PER SHARE INFORMATION

The following table presents Southwestern’s and Montage’s historical and pro forma per share data as of and for the year ended December 31, 2019 and as of and for the six months ended June 30, 2020. The pro forma per share data as of and for the year ended December 31, 2019 and as of and for the six months ended June 30, 2020 is presented as if the merger and related common stock and debt issuances, along with the use of the proceeds therefrom, had been completed on January 1, 2019. The information provided in the table below is unaudited. This information should be read together with the historical consolidated financial statements and related notes of Southwestern and Montage, filed by each with the SEC, and incorporated by reference into this proxy statement/prospectus, and with the unaudited pro forma condensed combined financial statements included in the section entitled “Unaudited Pro Forma Condensed Combined Financial Statements” beginning on page 132. The pro forma information is not necessarily indicative of the operating results or financial position that would have occurred if the merger had been completed as of the beginning of the period.

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COMPARATIVE PER SHARE MARKET PRICE AND DIVIDEND INFORMATION

Market Price

The Southwestern Common Stock is traded on the NYSE under the symbol “SWN” and the Montage Common Stock is traded on the NYSE under the symbol “MR.”

The high and low trading prices for the Southwestern Common Stock on August 11, 2020, the last trading day immediately before the public announcement of the merger, were $3.25 and $3.01, respectively. The high and low trading prices for the Montage Common Stock on August 11, 2020, the last trading day immediately before the public announcement of the merger, were $6.48 and $5.93, respectively.

As of September 30, 2020, the last date before the date of this proxy statement/prospectus for which it was practicable to obtain this information, there were 605,546,429 shares of Southwestern Common Stock outstanding and approximately 2,287 holders of record of Southwestern Common Stock, and 36,022,393 shares of Montage Common Stock outstanding and approximately 18 holders of record of Montage Common Stock.

The following table sets forth the closing sale price per share of Southwestern Common Stock and Montage Common Stock as reported on the NYSE as of August 11, 2020, the last trading day prior to the public announcement of the merger, and on September 30, 2020, the last practicable trading day before the date of this proxy statement/prospectus. The table also shows the estimated implied value of the merger consideration proposed for each share of Montage Common Stock as of the same two dates. The implied value was calculated by multiplying the closing price of a share of Southwestern Common Stock on the relevant date by the Exchange Ratio of 1.8656 shares of Southwestern Common Stock for each share of Montage Common Stock.

The market prices of Southwestern Common Stock and Montage Common Stock have fluctuated since the date of the announcement of the Merger Agreement and will continue to fluctuate prior to the completion of the merger. No assurance can be given concerning the market prices of Southwestern Common Stock or Montage Common Stock before completion of the merger or of Southwestern Common Stock after completion of the merger. Because the Exchange Ratio, which determines the merger consideration, is fixed and will not be adjusted for changes in the market prices of either Southwestern Common Stock or Montage Common Stock, the market price of Southwestern Common Stock (and, therefore, the value of the merger consideration) when received by Montage stockholders after the merger is completed could be greater than, less than or the same as shown in the table above. Accordingly, these comparisons may not provide meaningful information to stockholders in determining how to vote with respect to the proposals described in this proxy statement/prospectus. We urge you to obtain current market quotations for Southwestern Common Stock and Montage Common Stock and to review carefully the other information contained in this proxy statement/prospectus. Please see “Risk Factors—Risks Relating to the Merger—Because the Exchange Ratio is fixed and will not be adjusted in the event of any change in either Southwestern’s or Montage’s stock price, the value of the merger consideration is uncertain.”

For more information on the market for Southwestern’s or Montage’s common equity, related stockholder matters and issuer purchases of equity securities, see Part II, Item 5 “Market for Registrant’s Common Equity, Related Stockholder Matters and Issuer Purchases of Equity Securities” of Southwestern’s Annual Report on Form 10-K for the fiscal year ended December 31, 2019, or Montage’s Annual Report on Form 10-K for the fiscal year ended December 31, 2019, which are each incorporated by reference into this proxy statement/prospectus.

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Dividend Information

Southwestern currently does not pay dividends on Southwestern Common Stock, and does not anticipate paying any cash dividends in the foreseeable future. All decisions regarding the declaration and payment of dividends and stock repurchases are at the discretion of the Southwestern Board and will be evaluated regularly in light of Southwestern’s financial condition, earnings, growth prospects, funding requirements, applicable law and any other factors that the Southwestern Board deems relevant. The terms of the combined company’s debt agreements will contain restrictions on its ability to pay dividends. The terms of agreements governing debt that the combined company may incur in the future may also limit or prohibit dividend payments. Accordingly, Southwestern cannot assure you that it will either pay dividends in the future or continue to pay any dividend that it may commence in the future.

Montage has not paid any cash dividends since its inception, and it does not anticipate declaring or paying any cash dividends to holders of Montage Common Stock. The terms of the Merger Agreement limit the ability of Montage to declare or pay dividends prior to the completion of the merger. In addition, certain of Montage’s debt instruments place restrictions on its ability to pay cash dividends.

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RISK FACTORS

Risks Relating to the Merger

Because the Exchange Ratio is fixed and will not be adjusted in the event of any change in either Southwestern’s or Montage’s stock price, the value of the merger consideration is uncertain.

Upon completion of the merger, each share of Montage Common Stock outstanding immediately prior to the merger will be converted into and become exchangeable for 1.8656 shares of Southwestern Common Stock. The Exchange Ratio is fixed in the Merger Agreement and will not be adjusted for changes in the market price of either Southwestern Common Stock or Montage Common Stock. The market prices of Southwestern Common Stock and Montage Common Stock have fluctuated prior to and after the date of the announcement of the Merger Agreement and will continue to fluctuate from the date of this proxy statement/prospectus to the date of the Montage Special Meeting and the date the merger is consummated, and the market price of Southwestern Common Stock will continue to fluctuate thereafter.

Because the value of the merger consideration will depend on the market price of Southwestern Common Stock at the time the merger is completed, Montage stockholders will not know, or be able to determine, at the time of the Montage Special Meeting the market value of the merger consideration they would receive upon completion of the merger.

Stock price changes may result from a variety of factors, including, among others, general market and economic conditions, changes in Southwestern’s and Montage’s respective businesses, operations and prospects, reductions or changes in U.S. government spending or budgetary policies, market assessments of the likelihood that the merger will be completed, interest rates, general market, industry and economic conditions, such as oil prices and demand for natural gas, oil and NGLs, federal, state and local legislation, governmental regulation and legal developments in the industry segments in which Southwestern or Montage operate, the effects of the coronavirus pandemic (COVID-19) and governmental and business responses to the pandemic, the timing of the merger and other factors generally affecting the respective prices of Southwestern Common Stock or Montage Common Stock.

Many of these factors are beyond Southwestern’s and Montage’s control, and neither Southwestern nor Montage is permitted to terminate the Merger Agreement solely due to a decline in the market price of the common stock of the other party. You are urged to obtain current market quotations for Southwestern Common Stock and Montage Common Stock in determining whether to vote for the Merger Proposal. In addition, see “Comparative Historical and Unaudited Per Share Information” and “Comparative Per Share Market Price and Dividend Information.”

The merger may not be completed and the Merger Agreement may be terminated in accordance with its terms.

The merger is subject to a number of conditions that must be satisfied or waived (to the extent permissible), including the approval of the Merger Proposal prior to the completion of the merger. These conditions are described in “The Merger Agreement—Conditions to the Completion of the Merger.” These conditions to the completion of the merger, some of which are beyond the control of Southwestern and Montage, may not be satisfied or waived in a timely manner or at all, and, accordingly, the merger may be delayed or not completed. Additionally, either Southwestern or Montage may terminate the Merger Agreement under certain circumstances, including, among other reasons, if the merger is not completed by February 12, 2021 (subject to certain exceptions). Montage will be required to pay to Southwestern a termination fee of $9.7 million if the Merger Agreement is terminated: (i) by Southwestern if Montage has entered into any agreement with respect to an Acquisition Proposal (other than the merger or an Acceptable Confidentiality Agreement as permitted by the Merger Agreement), the Montage Board’s approval or recommendation of (or, in the case of a committee, a proposal to the Montage Board to approve or recommend) any Acquisition Proposal other than the merger, or the Montage Board’s resolution to do any of the foregoing; (ii) by Southwestern with respect to an Adverse

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Recommendation Change; (iii) by Montage in order to enter into a definitive agreement with respect to a Superior Proposal or (iv) (A) an Acquisition Proposal has been publicly proposed by any person (other than Southwestern or any of its affiliates) or an Acquisition Proposal has otherwise become known to Montage’s directors or officers, or its stockholders generally, (B) thereafter the Merger Agreement is terminated by either Southwestern or Montage due to the merger having not been consummated by 11:59 p.m., Central Time, on February 12, 2021 or Montage stockholders failing to approve the Merger Proposal at the Montage Special Meeting (or a postponement or adjournment of that meeting) or by Southwestern for a breach of or any inaccuracy in any of the representations or warranties or a material breach of any of the covenants or agreements in the Merger Agreement by Montage, and (C) within 12 months after the termination of the Merger Agreement, Montage or any of its subsidiaries enters into any definitive agreement providing for an Acquisition Proposal, or an Acquisition Proposal is consummated; provided that, for purposes of this paragraph only, any reference in the definition of “Acquisition Proposal” to “20% or more” will be deemed to be references to “50% or more.” For more information, see “The Merger Agreement—Termination of the Merger Agreement” and “The Merger Agreement—Termination of the Merger Agreement—Termination Fee.”

The termination of the Merger Agreement could negatively impact Southwestern or Montage.

If the merger is not completed for any reason, including as a result of a failure to obtain approval of the Merger Proposal, the ongoing businesses of Southwestern and Montage may be adversely affected and, without realizing any of the benefits of having completed the merger, Southwestern and Montage may experience certain negative effects, including:

If the merger is not consummated, neither Southwestern nor Montage can assure you that the risks described above will not negatively impact its business, financial results and ability to pay dividends, if any, to its respective stockholders.

The market price for shares of Southwestern Common Stock following the completion of the merger may be affected by factors different from, or in addition to, those that historically have affected or currently affect the market prices of shares of Southwestern Common Stock and Montage Common Stock.

Upon consummation of the merger, Southwestern stockholders and Montage stockholders will both hold shares of Southwestern Common Stock. Southwestern’s businesses differ in some regards from those of Montage, and Montage’s businesses differ in some regards from those of Southwestern, and, accordingly, the results of operations of Southwestern following completion of the merger will be affected by some factors that are different from those currently or historically affecting the results of operations of Southwestern and those currently or historically affecting the results of operations of Montage. The results of operations of Southwestern following completion of the merger may also be affected by factors different from those that currently affect or have historically affected either Southwestern or Montage. In addition, following completion of the merger, Southwestern may seek to raise additional equity financing through one or more underwritten offerings, private placements and/or rights offerings, or issue stock in connection with acquisitions, which may result in downward pressure on the share price of Southwestern Common Stock. For a discussion of the businesses of each of

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Southwestern and Montage and some important factors to consider in connection with those businesses, please see “The Parties to the Merger” and the documents and information included elsewhere in incorporated by reference into this proxy statement/prospectus.

The shares of Southwestern Common Stock to be received by Montage stockholders as a result of the merger will have rights different from the shares of Montage Common Stock.

Upon consummation of the merger, the rights of Montage stockholders, who will become stockholders of Southwestern, will be governed by the Southwestern certificate of incorporation and the Southwestern bylaws. The rights associated with Montage Common Stock are different from the rights associated with Southwestern Common Stock. See “Comparison of Stockholders’ Rights” for a discussion of these rights.

Southwestern stockholders and Montage stockholders will each have reduced ownership and voting interest in and will exercise less influence over management of Southwestern following completion of the merger.

Southwestern stockholders currently have the right to vote in the election of the Southwestern Board and on other matters affecting Southwestern, and Montage stockholders currently have the right to vote in the election of the Montage Board and on other matters affecting Montage. Upon consummation of the merger, each Southwestern stockholder and each former Montage stockholder will be a stockholder of Southwestern with a percentage ownership of Southwestern that is smaller than such stockholder’s percentage ownership of Southwestern or Montage, as applicable, immediately prior to the merger. As of the date of this proxy statement/prospectus, based on the Exchange Ratio, the number of outstanding shares of Montage Common Stock (plus the number of shares underlying outstanding Montage restricted stock units and performance units) and the number of outstanding shares of Southwestern Common Stock (on a fully diluted basis), it is estimated that Southwestern stockholders will own approximately 90% and Montage stockholders will own approximately 10% of the issued and outstanding shares of Southwestern Common Stock on a fully diluted basis immediately following the effective time of the merger. Because of this, each share of Southwestern Common Stock and each share of Montage Common Stock will represent a smaller percentage ownership of Southwestern than it represented in Southwestern and Montage, respectively.

Until the completion of the merger or the termination of the Merger Agreement in accordance with its terms, Southwestern and Montage are each prohibited from entering into certain transactions and taking certain actions that might otherwise be beneficial to Southwestern or Montage and their respective stockholders.

From and after the date of the Merger Agreement and prior to completion of the merger, the Merger Agreement restricts Southwestern and Montage from taking specified actions without the consent of the other party and generally requires that the business of each company and its respective subsidiaries be conducted in all material respects in the ordinary course of business consistent with past practice. These restrictions may prevent Southwestern or Montage from making appropriate changes to their respective businesses or organizational structures or from pursuing attractive business opportunities that may arise prior to the completion of the merger, and could have the effect of delaying or preventing other strategic transactions. Adverse effects arising from the pendency of the merger could be exacerbated by any delays in consummation of the merger or termination of the Merger Agreement. Please see “The Merger Agreement—Covenants—Conduct of Business Prior to the Effective Time.”

Obtaining required approvals and satisfying closing conditions may prevent or delay completion of the merger.

The merger is subject to a number of conditions to closing as specified in the Merger Agreement. These closing conditions include, among others, obtaining stockholder approval of the Merger Proposal, approval for listing on the NYSE of the shares of Southwestern Common Stock issuable in accordance with the Merger

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Agreement, the absence of governmental restraints or prohibitions preventing the consummation of the merger, the effectiveness of Southwestern’s registration statement on Form S-4 registering the Southwestern Common Stock issuable pursuant to the Merger Agreement and the absence of any stop order or proceedings by the SEC with respect thereto. The obligation of each of Southwestern and Montage to consummate the merger is also conditioned on, among other things, (i) the accuracy of the representations and warranties as set forth by the other party in the Merger Agreement, (ii) the performance by the other party, in all material respects, of its obligations under the Merger Agreement required to be performed at or prior to the Effective Time and (iii) the delivery by the other party of a certificate of an authorized officer certifying that the required conditions have been satisfied. The required stockholder consents and approvals may not be obtained and the required conditions to closing may not be satisfied, and, if all required consents and approvals are obtained and the conditions are satisfied, no assurance can be given as to the terms, conditions and timing of such consents and approvals. Any delay in completing the merger could cause Southwestern and Montage not to realize, or to be delayed in realizing, some or all of the benefits that Southwestern and Montage expect to achieve if the merger is successfully completed within its expected time frame. For a more complete summary of the conditions that must be satisfied or waived prior to completion of the merger, please see “The Merger Agreement—Conditions to the Completion of the Merger.”

Under the terms of the Credit Agreement Amendment, failure to redeem at least $450 million of the Montage Notes may lead to an accelerated maturity of the Southwestern Credit Agreement.

Southwestern entered into an amendment to its credit agreement to, among other things, permit the merger and the assumption of the Montage Notes (as defined in “The Merger—Treatment of Indebtedness”). Unless, on or prior to the date that is 91 days before the scheduled maturity date of the Montage Notes, at least $450 million of the Montage Notes have been either redeemed or refinanced (such refinance subject to certain criteria) or amended such that their maturity date, or the maturity date of such refinancing indebtedness, as applicable, is extended to a date at least 91 days after the Southwestern Credit Agreement maturity date of April 26, 2024, the springing maturity included in the Credit Agreement Amendment will be triggered 91 days prior to the maturity date of the Montage Notes. For a more complete summary of the terms of the Credit Agreement Amendment, please see “The Merger—Treatment of Indebtedness—Credit Agreement Amendment.”

Completion of the merger will trigger change in control or other provisions in certain agreements to which Montage is a party, which may have an adverse impact on Southwestern’s business and results of operations following completion of the merger.

The completion of the merger will trigger change in control and other provisions in certain agreements to which Montage is a party. If Southwestern or Montage is unable to negotiate waivers of those provisions, counterparties may exercise their rights and remedies under the agreements, potentially terminating the agreements or seeking monetary damages or equitable remedies. Even if Southwestern and Montage are able to negotiate consents or waivers, the counterparties may require a fee for such waivers or seek to renegotiate the agreements on terms less favorable to Montage or the combined company. Any of the foregoing or similar developments may have an adverse impact on Southwestern’s business and results of operations following completion of the merger.

Failure to attract, motivate and retain executives and other key employees could diminish the anticipated benefits of the merger.

The success of the merger will depend in part on the retention of personnel critical to the business and operations of the combined company due to, for example, their technical skills or management expertise. Competition for qualified personnel can be intense.

Current and prospective employees of Southwestern and Montage may experience uncertainty about their future role with Southwestern, Montage or the combined company until strategies with regard to these employees are announced or executed, which may impair Southwestern’s and Montage’s ability to attract, retain and motivate key management, sales, marketing, technical and field personnel, prior to and following the merger.

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Employee retention may be particularly challenging during the pendency of the merger, as employees of Southwestern and Montage may experience uncertainty about their future roles with Southwestern. If Southwestern and Montage are unable to retain personnel, including Southwestern’s and Montage’s key management, who are critical to the successful integration and future operations of the companies, Southwestern and Montage could face disruptions in their operations, loss of existing customers or loss of sales to existing customers, loss of key information, expertise or know-how and unanticipated additional recruitment and training costs.

In addition, the loss of key personnel could diminish the anticipated benefits of the merger. If key employees of Southwestern or Montage depart, the integration of the companies may be more difficult and Southwestern’s business following the merger may be harmed. Furthermore, Southwestern may have to incur significant costs in identifying, hiring and retaining replacements for departing employees and may lose significant expertise and talent relating to the business of each of Southwestern and Montage, and the combined company’s ability to realize the anticipated benefits of the merger may be adversely affected. In addition, there could be disruptions to or distractions for the workforce and management associated with integrating employees into Southwestern. Following completion of the merger, Southwestern may not be able to attract or retain key employees of Southwestern and Montage to the same extent that those companies have been able to attract or retain their own employees in the past.

The merger, and uncertainty regarding the merger, may cause customers or suppliers to delay or defer decisions concerning Southwestern and Montage and adversely affect each company’s ability to effectively manage its respective business.

The merger will be consummated only if the stated conditions are satisfied or waived, including approval of the Merger Proposal, among other conditions. Many of the conditions are outside the control of Southwestern and Montage, and both parties also have certain rights to terminate the Merger Agreement. Accordingly, there may be uncertainty regarding the completion of the merger. This uncertainty may cause customers, suppliers, vendors or others that deal with Southwestern or Montage to delay or defer entering into contracts with Southwestern or Montage or making other decisions concerning Southwestern or Montage or seek to change or cancel existing business relationships with Southwestern or Montage, which could negatively affect their respective businesses. Any delay or deferral of those decisions or changes in existing agreements could have an adverse impact on the respective businesses of Southwestern and Montage, regardless of whether the merger is ultimately completed.

The opinion rendered to the Montage Board from its financial advisor will not reflect changes in circumstances between the date of such opinion and the completion of the merger.

Montage has received an opinion from its financial advisor, Barclays, in connection with the signing of the Merger Agreement, but Montage has not requested an updated opinion as of the date of this proxy statement/prospectus from Barclays, and the Montage Board does not expect to request an updated opinion prior to the completion of the merger. Barclays does not assume any obligation to update, revise or reaffirm its opinion. Barclays’ opinion was necessarily based on economic, monetary, market and other conditions existing on, and the information made available to Barclays, only as of, the date of the opinion of Barclays. Its opinion does not address the fairness of the Exchange Ratio, from a financial point of view, at the time the merger is completed. Changes in the operations and prospects of Southwestern or Montage, general economic, monetary, market and other conditions and other factors that may be beyond the control of Southwestern or Montage, and on which the opinion of Barclays was based, may alter the value of Southwestern or Montage or the prices of shares of Southwestern Common Stock or Montage Common Stock by the time the merger is completed. The opinion of Barclays does not speak as of any date other than the date of such opinion. For a description of the opinion that Montage received from its financial advisor, please see “The Merger—Opinion of Montage’s Financial Advisor.” The full text of Barclays’ written opinion, dated as of August 12, 2020, is attached as Annex B to this proxy statement/prospectus.

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Southwestern and Montage may be adversely affected by negative publicity related to the proposed merger and in connection with other matters.

From time to time, political and public sentiment in connection with the proposed merger and in connection with other matters could result in a significant amount of adverse press coverage and other adverse public statements affecting Southwestern and Montage. Adverse press coverage and other adverse statements, whether or not driven by political or public sentiment, may also result in investigations by regulators, legislators and law enforcement officials or in legal claims. Responding to these investigations and lawsuits, regardless of the ultimate outcome of the proceeding, can divert the time and effort of senior management from the management of Southwestern’s and Montage’s respective businesses. Addressing any adverse publicity, governmental scrutiny or enforcement or other legal proceedings is time consuming and expensive and, regardless of the factual basis for the assertions being made, can have a negative impact on the reputation of Southwestern and Montage, on the morale and performance of their employees and on their relationships with their respective regulators. It may also have a negative impact on their ability to take timely advantage of various business and market opportunities. The direct and indirect effects of negative publicity, and the demands of responding to and addressing it, may have a material adverse effect on Southwestern’s and Montage’s respective businesses, financial condition, results of operations and cash flows.

The directors and executive officers of Montage have interests and arrangements that may be different from, or in addition to, those of Montage stockholders generally.

When considering the recommendation the Montage Board with respect to the proposals described in this proxy statement/prospectus, stockholders should be aware that the directors and executive officers of Montage have interests in the merger that are different from, or in addition to, those of Montage stockholders generally. These interests include the treatment in the merger of outstanding equity, equity-based and incentive awards, severance arrangements, other compensation and benefit arrangements, and the right to continued indemnification of former Montage directors and officers by Southwestern.

Montage stockholders should be aware of these interests when they consider the recommendation of the Montage Board and when they vote on the Merger Proposal. The Montage Board was aware of these interests when it adopted, approved and declared advisable the Merger Agreement, the merger and the transactions contemplated thereby on the terms and subject to the conditions set forth in the Merger Agreement, determined that the Merger Agreement, the merger and the transactions contemplated by the Merger Agreement were advisable and fair to, and in the best interests of, Montage and Montage stockholders and recommended that Montage stockholders approve the Merger Proposal. The interests of Montage directors and executive officers are described in more detail in “The Merger—Interests of Montage’s Directors and Executive Officers in the Merger.”

Southwestern or Montage may waive one or more of the closing conditions without re-soliciting stockholder approval.

Southwestern or Montage may determine to waive, in whole or part, one or more of the conditions to closing the merger prior to Southwestern or Montage, as the case may be, being obligated to consummate the merger. Montage currently expects to evaluate the materiality of any waiver and its effect on Montage stockholders in light of the facts and circumstances at the time, to determine whether any amendment of this proxy statement/prospectus or any re-solicitation of proxies is required in light of such waiver. Any determination whether to waive any condition to the merger or to re-solicit stockholder approval or amending or supplementing this proxy statement/prospectus as a result of a waiver will be made by Montage at the time of such waiver based on the facts and circumstances as they exist at that time.

Montage stockholders will not be entitled to appraisal rights in the merger.

Under Delaware law, holders of Montage Common Stock do not have appraisal rights in connection with the merger, as more fully described in “The Merger—No Appraisal Rights.”

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There are various provisions of the Merger Agreement and related documents that restrict the ability of Montage to seek alternative transactions or to terminate the merger.

The Merger Agreement contains “no shop” provisions that restrict Montage’s ability to, among other things, solicit or pursue alternative Acquisition Proposals, as described in “The Merger Agreement—Covenants—No Solicitation of Acquisition Proposals.” There are only limited circumstances under which the Merger Agreement would permit the Montage Board to withhold, withdraw, qualify or modify the Montage recommendation (as defined in “The Merger Agreement—Representations and Warranties”). The Merger Agreement also provides that in certain circumstances, Montage may be required to pay Southwestern a termination fee of $9.7 million if the Merger Agreement is terminated, as more fully described in “The Merger Agreement—Termination of the Merger Agreement.”

The EnCap Support Agreement includes covenants that, with limited exceptions, require EnCap (which collectively owns approximately 39.0% of the outstanding shares of Montage Common Stock as of September 30, 2020) to vote all of the shares of Montage Common Stock held by EnCap on such date in favor of the Merger Proposal and against actions that may impair or impede the transactions contemplated by the Merger Agreement. For specific details, please see “The Merger Agreement—EnCap Support Agreement.”

These provisions could discourage a potential competing acquirer from considering or proposing an acquisition or merger, even if it were prepared to pay consideration with a higher value than that implied by the Exchange Ratio, or might result in a potential competing acquirer proposing to pay a lower per share price than it might otherwise have proposed to pay because of the added expense of the termination fee provisions of the Merger Agreement.

Each of Southwestern and Montage will incur transaction and merger-related expenses in connection with the merger.

Southwestern and Montage have incurred and expect to incur a number of non-recurring costs associated with consummating the merger and combining the operations of the two companies. These costs and expenses include fees paid to financial, legal and accounting advisors, potential employment-related costs, filing fees, printing expenses and other related charges. Some of these costs are payable by Southwestern or Montage regardless of whether the merger is completed.

Following completion of the merger, Southwestern will also incur integration costs related to the merger. There are a large number of processes, policies, procedures, operations, technologies and systems that must be integrated in connection with the merger and the integration of the two companies’ businesses. Although Southwestern and Montage expect that the elimination of duplicative costs and the realization of other efficiencies related to the integration of the businesses may offset incremental transaction and merger-related costs over time, any net benefit may not be achieved in the near term or at all. Many of these costs will be borne by Southwestern or Montage even if the merger is not completed. While both Southwestern and Montage have assumed that certain expenses would be incurred in connection with the merger and the other transactions contemplated by the Merger Agreement, there are many factors beyond their control that could affect the total amount or the timing of the integration and implementation expenses.

Southwestern and Montage may be targets of securities class action and derivative lawsuits which could result in substantial costs and may delay or prevent the merger from being completed.

Securities class action lawsuits and derivative lawsuits are often brought against companies that have entered into merger agreements. Defending against these claims can result in substantial costs and divert management time and resources, even if the lawsuits are without merit. An adverse judgment could result in monetary damages, which could have a negative impact on Southwestern’s and Montage’s respective businesses, results of operations and financial condition. Additionally, if a plaintiff is successful in obtaining an injunction

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prohibiting completion of the merger, the injunction may delay or prevent the merger from being completed, which may adversely affect Southwestern’s and Montage’s respective businesses, results of operations and financial condition.

Risks Relating to the Combined Company

Recent declines in crude oil prices to record low levels as a result of the outbreak of the novel strain of coronavirus (“COVID-19”) and a significantly oversupplied crude oil market have negatively impacted demand for the products of Southwestern and Montage, and are expected to continue to negatively impact demand for the products of Southwestern, Montage and of the combined company, which may result in a material negative impact on the combined company’s results of operations, financial position and liquidity.

The COVID-19 outbreak in the United States and globally, together with the recent significant decline in commodity prices due, in significant part, to the recent actions of OPEC and other oil producing nations (“OPEC+”), have adversely affected, and are expected to continue to adversely affect, both the price of and demand for crude oil and the continuity of the combined company’s business operations. Oil demand significantly deteriorated as a result of the COVID-19 pandemic and corresponding preventative measures taken around the world to mitigate its spread, including “shelter-in-place” orders, quarantines, executive orders and similar government orders and restrictions for their residents to control the spread of COVID-19.

In the midst of the ongoing COVID-19 pandemic, OPEC+ were unable to reach an agreement on production levels for crude oil, at which point Saudi Arabia and Russia initiated efforts to aggressively increase production. The convergence of the COVID-19 pandemic and the crude oil production increases caused the significant dual impact of global oil demand decline and the risk of a substantial increase in supply. While OPEC+ agreed in April 2020 to cut production, downward pressure on commodity prices has remained and could continue for the foreseeable future.

While the U.S. Department of Homeland Security and various local orders have identified the energy industry as critical to the U.S. infrastructure, generally allowing certain of the operations of Southwestern, Montage and their customers to continue, the operations of Southwestern and Montage, and those of their respective customers, have been and will likely continue to be disrupted in various ways. The recent decline in commodity prices has and could continue to adversely affect the demand and pricing for the production of Southwestern, Montage, and of the combined company. Additionally, demand for the combined company’s production will likely be significantly affected in the event of a global recession due to the reduction in oil prices and the ongoing effects of COVID-19, with the possibility of numerous bankruptcies of exploration and production (“E&P”) companies during 2020.

Additionally, in an effort to minimize the spread of illness, Southwestern and Montage have implemented various worksite restrictions in order to minimize contact among personnel. Certain travel restrictions and flight cancellations have also slowed personnel travel and equipment delivery to certain locations.

The COVID-19 pandemic, coupled with the global crude oil supply and demand imbalance and resulting decline in crude oil prices, has significantly impacted the value of Southwestern Common Stock and of Montage Common Stock, which may reduce the ability of Southwestern, Montage or the combined company to access capital in the bank and capital markets, which could in the future negatively affect the combined company’s liquidity. In addition, a recession or long-term market correction resulting from the COVID-19 pandemic could in the future further materially affect the value of Southwestern Common Stock, affect the combined company’s access to capital and affect the combined company’s business in the near and long-term. The borrowing base of the combined company’s credit facilities will be dependent upon its receivables, which may be significantly lower in the future due to reduced activity levels or decreases in pricing for its production and services. In addition, if the combined company’s customers experience financial distress due to the current market conditions, they could default on their payments owed to the combined company and create a credit risk on collecting receivables.

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The COVID-19 pandemic continues to rapidly evolve. The extent to which COVID-19 and depressed crude oil prices will impact the combined company’s results, financial position and liquidity will depend on future developments, which are highly uncertain and cannot be predicted.

The combined company may not be able to retain customers or suppliers, and customers or suppliers may seek to modify contractual obligations with the combined company, either of which could have an adverse effect on Southwestern’s business and operations. Third parties may terminate or alter existing contracts or relationships with Southwestern or Montage.

As a result of the merger, the combined company may experience impacts on relationships with customers and suppliers that may harm the combined company’s business and results of operations. Certain customers or suppliers may seek to terminate or modify contractual obligations following the merger whether or not contractual rights are triggered as a result of the merger. There can be no guarantee that customers and suppliers will remain with or continue to have a relationship with the combined company or do so on the same or similar contractual terms following the merger. If any customers or suppliers seek to terminate or modify contractual obligations or discontinue their relationships with the combined company, then the combined company’s business and results of operations may be harmed. Furthermore, the combined company will not have long-term arrangements with many of its significant suppliers. If the combined company’s suppliers were to seek to terminate or modify an arrangement with the combined company, then the combined company may be unable to procure necessary supplies from other suppliers in a timely and efficient manner and on acceptable terms, or at all.

Southwestern and Montage also have contracts with vendors, landlords, licensors and other business partners which may require Southwestern or Montage, as applicable, to obtain consent from these other parties in connection with the merger. If these consents cannot be obtained, the combined company may suffer a loss of potential future revenue, incur costs, and lose rights that may be material to the business of the combined company. In addition, third parties with whom Southwestern or Montage currently have relationships may terminate or otherwise reduce the scope of their relationship with either party in anticipation of the merger. Any such disruptions could limit the combined company’s ability to achieve the anticipated benefits of the merger. The adverse effect of any such disruptions could also be exacerbated by a delay in the completion of the merger or by a termination of the Merger Agreement.

The combined company may fail to realize the anticipated benefits of the merger.

The success of the merger will depend on, among other things, the combined company’s ability to combine the Southwestern and Montage businesses in a manner that realizes anticipated synergies and meets or exceeds the forecasted stand-alone cost savings anticipated by each company. On a combined basis, the combined company expects to benefit from significant synergies, based on, among other things, increased scale. If the combined company is not able to successfully achieve these objectives, or the cost to achieve these synergies is greater than expected, then the anticipated benefits of the merger may not be realized fully or at all or may take longer to realize than expected.

The combined company must achieve the anticipated savings and synergies in a timely manner and without adversely affecting current revenues and investments in future growth. In addition, the combined company must successfully combine the businesses of Southwestern and Montage in a manner that permits the anticipated savings and synergies to be realized. If the combined company is not able to successfully achieve these objectives, the anticipated benefits of the merger may not be realized fully or at all or may take longer to realize than expected. A variety of factors may adversely affect the combined company’s ability to realize the currently expected operating synergies, savings and other benefits of the merger.

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The failure to successfully integrate the businesses and operations of Southwestern and Montage in the expected time frame may adversely affect the combined company’s future results.

Southwestern and Montage have operated and, until the completion of the merger, will continue to operate independently, however, their businesses may not be integrated successfully. It is possible that the integration process could result in the loss of key Southwestern employees or key Montage employees, the loss of customers, the disruption of either company’s or both companies’ ongoing businesses, inconsistencies in standards, controls, procedures and policies, unexpected integration issues, higher than expected integration costs and an overall post-completion integration process that takes longer than originally anticipated. Specifically, the following issues, among others, must be addressed in integrating the operations of Southwestern and Montage in order to realize the anticipated benefits of the merger so the combined company performs as expected:

In addition, at times the attention of certain members of either company’s or both companies’ management and resources may be focused on completion of the merger and the integration of the businesses of the two companies and diverted from day-to-day business operations or other opportunities that may have been beneficial to such company, which may disrupt each company’s ongoing business and the business of the combined company.

The unaudited pro forma condensed combined financial information contained in this proxy statement/prospectus may not be an indication of the combined company’s results of operations or financial condition following the closing of the merger.

This proxy statement/prospectus includes unaudited pro forma condensed combined financial information for the combined company, which give effect to the merger and should be read in conjunction with the financial statements and accompanying notes of Southwestern and Montage, which are incorporated by reference into this proxy statement/prospectus. The unaudited pro forma condensed combined financial information contained in this proxy statement/prospectus should not be considered to be an indication of the combined company’s results of operations or financial condition following the closing of the merger. The unaudited pro forma condensed combined financial information has been derived from the historical financial statements of Southwestern and

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Montage and adjustments, assumptions and preliminary estimates have been made in connection with the preparation of this information. The information upon which these adjustments and assumptions have been made is preliminary, and these kinds of adjustments, assumptions and estimates are difficult to make with accuracy.

Moreover, the unaudited pro forma condensed combined financial information does not reflect all costs that are expected to be incurred by the combined company in connection with the merger. For example, the impact of any incremental costs incurred in coordinating the operations of Southwestern and Montage are not reflected in the unaudited pro forma condensed combined financial information. In addition, the unaudited pro forma condensed combined financial information does not include, among other things, estimated cost synergies, adjustments related to restructuring or integration activities, future acquisitions or disposals not yet known or probable, or impacts of merger-related change in control provisions that are currently not factually supportable or probable of occurring.

As a result, the actual results of operations and financial condition of the combined company following the closing of the merger may not be consistent with, or evident from, the unaudited pro forma condensed combined financial information. The assumptions used in preparing the unaudited pro forma condensed combined financial information may not prove to be accurate, and other factors may affect the combined company’s results of operations or financial condition following the closing of the merger. Any potential decline in the combined company’s financial condition or results of operations may cause significant variations in the price of the Southwestern Common Stock following the closing of the merger.

The unaudited pro forma condensed combined financial information in this proxy statement/prospectus is based on the best information available, which in part includes a number of estimates and assumptions. These estimates and assumptions may prove not to be accurate, and accordingly, the unaudited pro forma condensed combined financial information should not be assumed to be indicative of what the combined company’s financial condition, results of operations or cash flows actually would have been as a stand-alone company or to be a reliable indicator of what the combined company’s financial condition or results of operations may actually be in the future.

The financial analyses and forecasts considered by Montage and its financial advisor may not be realized, which may adversely affect the market price of the Southwestern Common Stock following the closing of the merger.

In performing its financial analysis and rendering its opinion regarding the fairness, from a financial point of view, of the merger consideration, Montage’s financial advisor relied on, among other things, internal stand-alone financial analyses and forecasts provided by Montage and Southwestern. None of these analyses or forecasts were prepared with a view towards public disclosure or compliance with the published guidelines of the SEC, U.S. GAAP or the guidelines established by the American Institute of Certified Public Accountants for preparation and presentation of financial forecasts. These forecasts are inherently based on various estimates and assumptions that are subject to the judgment of those preparing them. These forecasts are also subject to significant economic, competitive, industry and other uncertainties and contingencies, all of which are difficult or impossible to predict and many of which are beyond the control of Southwestern and Montage. Important factors that may affect the actual results of Southwestern and Montage and cause the internal financial forecasts to not be achieved include risks and uncertainties relating to Southwestern’s and Montage’s businesses, industry performance, the regulatory environment, general business and economic conditions and other factors described under the section entitled “Cautionary Statement Regarding Forward-Looking Statements” in this proxy statement/prospectus.

In addition, the financial forecasts also reflect assumptions that are subject to change and do not reflect revised prospects for Southwestern’s and Montage’s businesses, changes in general business or economic conditions or any other transaction or event that has occurred or that may occur and that was not anticipated at the time the financial forecasts were prepared. There can be no assurance that Southwestern’s, Montage’s or the

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combined company’s financial condition or results of operations will be consistent with those set forth in such analyses and forecasts. For more information please see “The Merger—Certain Unaudited Forecasted Financial Information.”

Any acquisitions, partnerships or joint ventures that the combined company enters into could disrupt its operations and have a material adverse effect on its business, financial condition and results of operations.

Southwestern’s strategy is to evaluate potential strategic acquisitions of businesses, including partnerships or joint ventures with third parties. Southwestern may not be successful in identifying acquisition, partnership and joint venture candidates. In addition, Southwestern may not be able to continue the operational success of such businesses or successfully finance or integrate any businesses that it acquires or with which it forms a partnership or joint venture. The process of integrating an acquired business may involve unforeseen costs and delays or other operational, technical and financial difficulties and may require a disproportionate amount of management attention and financial and other resources. Any acquisition, partnership or joint venture may not be successful, may reduce Southwestern’s cash reserves, may negatively affect its earnings and financial performance and, to the extent financed with the proceeds of debt, may increase its indebtedness. Southwestern cannot ensure that any acquisition, partnership or joint venture it makes will not have a material adverse effect on its business, financial condition and results of operations.

If the combined company fails to continue to meet all applicable listing requirements, the Southwestern Common Stock may be delisted from the New York Stock Exchange, which could have an adverse impact on the liquidity and trading price of the Southwestern Common Stock.

The Southwestern Common Stock is currently listed on the NYSE, which has qualitative and quantitative listing criteria. If the combined company is unable to meet any of the NYSE listing requirements in the future, including, for example, if the closing bid price for the Southwestern Common Stock falls below $1.00 per share for 30 consecutive trading days, the NYSE could determine to delist the Southwestern Common Stock. A delisting of the Southwestern Common Stock could negatively impact the combined company by, among other things, reducing the liquidity and market price of Southwestern Common Stock, reducing the number of investors willing to hold or acquire Southwestern Common Stock, which could negatively impact the combined company’s ability to raise equity financing or to use Southwestern Common Stock as consideration for acquisitions, decreasing the amount of news and analyst coverage of the combined company, and limiting the combined company’s ability to issue additional securities or obtain additional financing in the future.

The trading price and volume of the Southwestern Common Stock may be volatile following the merger.

The trading price and volume of the Southwestern Common Stock may be volatile following completion of the merger. The stock markets in general have experienced extreme volatility that has often been unrelated to the operating performance of particular companies. These broad market fluctuations may adversely affect the trading price of the Southwestern Common Stock. As a result, you may suffer a loss on your investment.

The market for Southwestern Common Stock will depend on a number of conditions, most of which the combined company cannot control, including:

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These and other factors may impair the market for the Southwestern Common Stock and the ability of investors to sell shares at an attractive price. These factors also could cause the market price and demand for the Southwestern Common Stock to fluctuate substantially, which may negatively affect the price and liquidity of the Southwestern Common Stock. Many of these factors and conditions are beyond the control of the combined company or the combined company stockholders.

Securities class action litigation has often been instituted against companies following periods of volatility in the overall market and in the market price of a company’s securities. Such litigation, if instituted against the combined company, could result in very substantial costs, divert management’s attention and resources and harm the combined company’s business, operating results and financial condition.

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Future sales or issuances of Southwestern Common Stock could depress the market price for shares of Southwestern Common Stock.

The Southwestern Common Stock that Southwestern will issue to Montage stockholders if the merger is consummated generally may be sold immediately in the public market. It is possible that some Montage stockholders will decide to sell some or all of the shares of Southwestern Common Stock that they receive in the merger. Any disposition by a significant stockholder of Southwestern Common Stock, or the perception in the market that such dispositions could occur, may cause the price of Southwestern Common Stock to fall. Any such decline could impair the combined company’s ability to raise capital through future sales of Southwestern Common Stock. Further, Southwestern Common Stock may not qualify for investment indices and any such failure may discourage new investors from investing in Southwestern Common Stock.

Combined company stockholders may experience dilution in the future.

The percentage ownership of combined company stockholders may be diluted in the future because of equity issuances for acquisitions, capital market transactions or otherwise, including, without limitation, equity awards that the combined company may grant to its directors, officers and employees. Such issuances may have a dilutive effect on the combined company’s earnings per share, which could adversely affect the market price of the Southwestern Common Stock.

Certain employees of Montage will have rights to purchase or receive shares of Southwestern Common Stock after the merger as a result of the conversion of their Montage equity awards into Southwestern equity awards. The conversion of these Montage equity awards into Southwestern equity awards is described in further detail in the section entitled “The Merger Agreement—Treatment of Montage Equity Awards.” The issuance of shares of Southwestern Common Stock pursuant to these awards will dilute the percentage ownership of combined company stockholders. It is also expected that, from time to time after the closing of the merger, the Southwestern Compensation Committee will grant additional equity awards to employees and directors of the combined company under the combined company’s compensation and employee benefit plans. These additional equity awards will have a dilutive effect on the combined company’s earnings per share, which could adversely affect the market price of the Southwestern Common Stock.

In addition, the combined company’s certificate of incorporation will authorize the combined company to issue, without the approval of stockholders, one or more classes or series of preferred stock having such designations, powers, preferences and relative, participating, optional and other special rights, including preferences over Southwestern Common Stock with respect to dividends and distributions, as the Southwestern Board generally may determine. The terms of one or more classes or series of preferred stock could dilute the voting power or reduce the value of the Southwestern Common Stock. For example, the repurchase or redemption rights or liquidation preferences that could be assigned to holders of preferred stock could affect the residual value of the Southwestern Common Stock. For more information, see “Description of Southwestern Capital Stock.”

Certain provisions contained in Southwestern’s amended and restated certificate of incorporation and amended and restated bylaws, and certain provisions of Delaware law may prevent or delay an acquisition of the combined company or other strategic transactions, which could decrease the trading price of the Southwestern Common Stock.

The Southwestern certificate of incorporation and the Southwestern bylaws contain, and Delaware law contains, provisions that are intended to deter coercive takeover practices and inadequate takeover bids and to encourage prospective acquirers to negotiate with the Southwestern Board rather than to attempt a hostile takeover.

In addition, because Southwestern has not chosen to be exempt from Section 203 of the DGCL, this provision could also delay or effectively prevent a change of control that some stockholders may favor. In

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general, Section 203 provides that persons that acquire ownership of 15% or more of the outstanding voting stock of a Delaware corporation, or an affiliate or associate of the corporation that within the prior three years did own 15% or more of the corporation’s outstanding voting stock, and the affiliates and associates of such persons (each an “interested stockholder”), shall not engage in any “business combination” with that corporation or its subsidiaries, including any merger, sales and leases of assets, issuances of securities or other similar transactions, for a three-year period following the date on which that person became the owner of 15% or more of such corporation’s outstanding voting stock unless one of the following exceptions applies: (i) the Southwestern Board approved the business combination or the transaction that resulted in the person becoming an interested stockholder prior to the time that the person became an interested stockholder, (ii) upon consummation of the transaction that resulted in the person becoming an interested stockholder such person owned at least 85% of the outstanding voting stock of the corporation, excluding, for purposes of determining the voting stock outstanding, voting stock owned by directors who are also officers and certain employee stock plans, or (iii) the transaction is approved by the Southwestern Board and by the affirmative vote of two-thirds of the outstanding voting stock which is not owned by the interested stockholder.

Southwestern believes these provisions could help to protect its stockholders from coercive or otherwise unfair takeover tactics by requiring potential acquirers to negotiate with the Southwestern Board and by providing the Southwestern Board with more time to assess any acquisition proposal. These provisions are not intended to make the combined company immune from takeovers. However, these provisions will apply even if the offer may be considered beneficial by some stockholders and could delay or effectively prevent an acquisition that the Southwestern Board determines is not in the best interests of the combined company and its stockholders. These provisions may also prevent or discourage attempts to remove and replace incumbent directors.

Southwestern’s current and future levels of indebtedness may adversely affect the combined company’s results and limit its growth.

As of June 30, 2020, Southwestern had long-term indebtedness of $2,440 million and approximately $209 million of letters of credit under its credit agreement. On August 18, 2020, Southwestern commenced an offering for $350 million aggregate principal amount of its 8.375% Senior Notes due 2028. Further, in connection with the merger, Southwestern will assume the amounts of outstanding Montage senior notes ($510.5 million as of June 30, 2020) and expects to borrow funds under the Southwestern credit agreement to repay the amounts outstanding under Montage’s revolving credit agreement ($160.0 million as of June 30, 2020). The terms of the indentures relating to Southwestern’s outstanding senior notes, its credit agreement, and the master lease agreements relating to its drilling rigs and other equipment (collectively, the “financing agreements”) impose restrictions on Southwestern’s ability and, in some cases, the ability of its subsidiaries to take a number of actions that they may otherwise desire to take, which may include, without limitation, one or more of the following:

Southwestern’s credit agreement contains customary representations, warranties and covenants including, among others, the following covenants:

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Although Southwestern does not anticipate any violations of its financial covenants, Southwestern’s ability to comply with such financial covenants depends in part on the success of its development program and upon factors beyond its control, such as the market prices for natural gas, oil and NGLs.

Although the indentures governing Southwestern’s outstanding notes contain covenants limiting liens and sale and leaseback transactions, those covenants contain exceptions that would allow Southwestern to create, grant or incur certain liens or security interests. Moreover, the indentures do not contain any limitations on the ability of Southwestern or its subsidiaries to incur debt, pay dividends or make investments, or limit the ability of its subsidiaries to make distributions to Southwestern. Such activities may, however, be limited by Southwestern’s other financing agreements in certain circumstances.

Southwestern’s level of indebtedness and off-balance sheet obligations, and the covenants contained in its financing agreements, could have important consequences for its operations, including:

If securities or industry analysts do not publish research or reports about the combined company’s business, if they adversely change their recommendations regarding the combined company’s common stock or if the combined company’s operating results do not meet their expectations, the Southwestern Common Stock price and trading volume could decline.

The trading market for Southwestern Common Stock following the consummation of the merger will depend in part on the research and reports that securities or industry analysts publish about the combined company or its businesses. While securities and industry analysts currently cover Southwestern and Montage,

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securities and industry analysts may not publish research on the combined company. If no securities or industry analysts provide coverage of the combined company, the trading price for the Southwestern Common Stock would likely be negatively impacted. In the event securities or industry analysts initiate coverage, if one or more of the analysts who cover the combined company downgrade its securities or publish inaccurate or unfavorable research about its businesses, or if the combined company’s operating results do not meet analyst expectations, the trading price of Southwestern Common Stock would likely decline. If one or more of these analysts cease coverage of the combined company or fail to publish reports on the combined company regularly, demand for Southwestern Common Stock could decrease, which might cause the Southwestern Common Stock price and trading volume to decline.

Declaration, payment and amounts of dividends, if any, distributed to stockholders of Southwestern will be uncertain.

Southwestern does not currently intend to pay dividends. Whether any dividends are declared or paid to stockholders of Southwestern following the merger, and the amounts of any such dividends that are declared or paid, are uncertain and depend on a number of factors. If dividends are paid to stockholders of Southwestern, they may not be of the same amount as any dividends paid by Southwestern to its stockholders prior to the Effective Time. The Southwestern Board will have the discretion to determine the dividend policy of Southwestern, which may be impacted by any of the following factors:

Stockholders should be aware that they have no contractual or other legal right to dividends that have not been declared.

The combined company may record goodwill and other intangible assets that could become impaired and result in material non-cash charges to the results of operations of the combined company in the future.

The combined company will account for the merger as an acquisition of a business in accordance with GAAP. Under the acquisition method of accounting, the assets and liabilities of Montage and its subsidiaries will be recorded, as of completion, at their respective fair values and added to Southwestern’s. The combined company’s reported financial condition and results of operations for periods after completion of the merger will reflect Montage’s balances and results after completion of the merger but will not be restated retroactively to reflect the historical financial position or results of operations of Montage and its subsidiaries for periods prior to the merger.

Under the acquisition method of accounting, the total purchase price is allocated to Montage’s identifiable tangible and intangible assets acquired and liabilities assumed based on their respective fair market values as of the date of completion of the merger, with any excess purchase price allocated to goodwill. To the extent the value of goodwill or intangibles, if any, becomes impaired in the future, the combined company may be required to incur material non-cash charges relating to such impairment. The combined company’s operating results may be significantly impacted from both the impairment and the underlying trends in the business that triggered the impairment.

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Other Risk Factors

Southwestern’s and Montage’s businesses are and will be subject to the risks described above. In addition, Southwestern and Montage are, and will continue to be, subject to the risks described in, as applicable, the Southwestern annual report on Form 10-K for the fiscal year ended December 31, 2019, and the Montage annual report on Form 10-K for the fiscal year ended December 31, 2019, as updated by subsequent Quarterly Reports on Form 10-Q and Current Reports on Form 8-K, all of which are filed with the SEC and incorporated by reference into this proxy statement/prospectus. See “Where You Can Find More Information” for the location of information incorporated by reference into this proxy statement/prospectus.

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CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

Certain statements and information in this proxy statement/prospectus may constitute “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995. The words “believe,” “expect,” “anticipate,” “plan,” “intend,” “foresee,” “predict,” “budget,” “should,” “would,” “could,” “attempt,” “appears,” “forecast,” “outlook,” “estimate,” “continue,” “project,” “projection,” “goal,” “model,” “target,” “potential,” “may,” “will,” “objective,” “guidance,” “outlook,” “effort,” “are likely” and other similar expressions are intended to identify forward-looking statements, which are generally not historical in nature. These forward-looking statements are based on Southwestern’s and Montage’s current expectations and beliefs concerning future developments and their potential effect on their respective businesses.

The forward-looking statements contained in this document are largely based on Southwestern’s and Montage’s expectations for the future, which reflect certain estimates and assumptions made by their respective managements. These estimates and assumptions reflect Southwestern’s and Montage’s best judgment based on currently known market conditions, operating trends, and other factors. Although Southwestern and Montage believe such estimates and assumptions to be reasonable, they are inherently uncertain and involve a number of risks and uncertainties that are beyond Southwestern’s and Montage’s control. As such, managements’ assumptions about future events may prove to be inaccurate. For a more detailed description of the risks and uncertainties involved, see “Risk Factors” in Southwestern’s and Montage’s most recently filed Annual Reports on Form 10-K, subsequent Quarterly Reports on Form 10-Q, Current Reports on Form 8-K, and other SEC filings. Southwestern and Montage do not intend to publicly update or revise any forward-looking statements as a result of new information, future events, changes in circumstances, or otherwise.

These cautionary statements qualify all forward-looking statements attributable to Southwestern or Montage, or persons acting on either’s behalf. Southwestern management and Montage management caution you that the forward-looking statements contained in this proxy statement/prospectus are not guarantees of future performance, and neither Southwestern nor Montage can assure you that such statements will be realized or that the events and circumstances they describe will occur. Factors that could cause actual results to differ materially from those anticipated or implied in the forward-looking statements herein include, but are not limited to:

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All written and oral forward-looking statements attributable to Southwestern or Montage are expressly qualified in their entirety by this cautionary statement. For further discussion of these and other risks, contingencies and uncertainties applicable to Southwestern and Montage, please see “Risk Factors.”

All subsequent written or oral forward-looking statements attributable to Southwestern or Montage or any person acting on its or their behalf are expressly qualified in their entirety by the cautionary statements contained or referred to in this section. Neither Southwestern nor Montage is under any obligation, and each expressly disclaims any obligation, to update, alter, or otherwise revise any forward-looking statements, whether written or oral, that may be made from time to time, whether as a result of new information, future events or otherwise, except as may be required by law.

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THE PARTIES TO THE MERGER

Southwestern Energy Company

10000 Energy Drive

Spring, Texas 77389-4954

(832) 796-4068

Southwestern is an independent energy company engaged in exploration, development and production activities, including the related marketing of natural gas, oil and NGLs produced in its operations. Southwestern is a holding company whose assets consist of direct and indirect ownership interests in, and whose business is conducted substantially through, its subsidiaries. Currently, it operates exclusively in the United States. Shares of Southwestern Common Stock are listed and traded on the NYSE under the ticker symbol “SWN.” Additional information about Southwestern and its subsidiaries, including, but not limited to, information regarding its business, properties, legal proceedings, financial statements, financial condition and results of operations, market risk, executive compensation and related party transactions is set forth in Southwestern’s Annual Report on Form 10-K for the fiscal year ended December 31, 2019, and Southwestern’s Quarterly Reports on Form 10-Q for the quarters ended March 31, 2020 and June 30, 2020, which are each incorporated by reference into this proxy statement/prospectus.

For more information about Southwestern, please visit Southwestern’s website at www.swn.com. The information contained on Southwestern’s website or accessible through it does not constitute a part of this proxy statement/prospectus.

Montage Resources Corporation

122 W. John Carpenter Freeway

Suite 300

Irving, Texas 75039

(469) 444-1647

Montage is an independent exploration and production company engaged in the acquisition and development of oil and natural gas properties in the Appalachian Basin. As of December 31, 2019, Montage had assembled an acreage position approximating 233,800 net surface acres in Eastern Ohio, 38,700 net surface acres in Pennsylvania, and 49,700 net surface acres in West Virginia, which excludes any acreage pending title. Shares of Montage Common Stock are listed and traded on the NYSE under the ticker symbol “MR.” Additional information about Montage and its subsidiaries, including, but not limited to, information regarding its business, properties, legal proceedings, financial statements, financial condition and results of operations, executive compensation and related party transactions is set forth in Montage’s Annual Report on Form 10-K for the fiscal year ended December 31, 2019 and Montage’s Quarterly Reports on Form 10-Q for the quarters ended March 31, 2020 and June 30, 2020, which are each incorporated by reference into this proxy statement/prospectus.

For more information about Montage, please visit Montage’s website at www.montageresources.com. The information contained on Montage’s website or accessible through it does not constitute a part of this proxy statement/prospectus.

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THE MERGER

The following discussion contains certain information about the proposed merger. This discussion is subject, and qualified in its entirety by reference, to the Merger Agreement attached as Annex A to this proxy statement/prospectus. You are urged to carefully read this entire proxy statement/prospectus, including the Merger Agreement, before making any investment or voting decision.

Transaction Structure

Upon satisfaction or waiver of the conditions to closing in the Merger Agreement, at the Effective Time, Montage will merge with and into Southwestern, with Southwestern continuing as the surviving corporation. For further discussion regarding the circumstances in which Southwestern may change the method and structure of effecting the merger, please see “The Merger Agreement—Structure of the Merger.”

Consideration to Montage Stockholders

At the Effective Time, each share of Montage Common Stock issued and outstanding immediately prior to the Effective Time (excluding any Excluded Shares) shall be converted into 1.8656 shares of Southwestern Common Stock. All Montage Common Stock (if any) held in treasury or owned directly or indirectly by Montage or any of its wholly owned subsidiaries or by Southwestern or any of its wholly owned subsidiaries as of immediately prior to the Effective Time (other than any shares held in a fiduciary capacity), will automatically be cancelled and no consideration will be paid or delivered in exchange therefor.

In addition, each outstanding Montage equity award in respect of Montage Common Stock will be treated as described in “The Merger Agreement—Treatment of Montage Equity Awards.”

Background of the Merger

The terms of the merger are the result of arm’s-length negotiations between Montage and Southwestern. The following is a summary of the events leading up to the agreement to merge and the key meetings, negotiations, discussions and actions by and between Montage and Southwestern and their respective advisors that preceded the public announcement of the merger.

As part of Montage’s ongoing strategic planning process, the Montage Board, together with Montage’s executive management team, regularly reviews and assesses Montage’s long-term strategic plans and goals, opportunities, overall industry trends, the competitive environment in which Montage operates and Montage’s short-term and long-term performance. As part of these reviews, the Montage Board, with the assistance of Montage’s advisors, considered whether various strategic actions, including potential business combination transactions and acquisition opportunities, would be in the best interests of Montage and would enhance value for Montage’s stockholders.

The Montage Board and Montage management have long recognized the benefits provided to large operators in the upstream industry and the associated competitive advantages those benefits provide, as well as the challenges presented by operating as a small-scale exploration and production company in a continued low commodity price environment. In early 2018, Montage began a process to identify strategic alternatives, which led to the completion of Montage’s business combination with BRMR on February 28, 2019 (the “BRMR Merger”). The BRMR Merger significantly increased the scale of Montage’s operations through the consolidation of Montage’s and BRMR’s contiguous and complementary acreage positions to create one of the largest Utica-focused public operators. Following the completion of the BRMR Merger and during the spring of 2019, Montage management and the Montage Board had ongoing general discussions regarding opportunities to continue enhancing value for Montage’s stockholders, with a particular focus on ways to further increase scale and address the risks associated with restricted access to capital and Montage’s limited trading liquidity.

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On May 24, 2019, John K. Reinhart, Montage’s President and Chief Executive Officer, had discussions with Michael L. Hodges, Montage’s Executive Vice President and Chief Financial Officer, regarding an internal strategic assessment of a potential combination of Montage and Company A, a publicly traded independent exploration and production company, which Montage management understood to be considering various strategic options.

On June 12, 2019, a financial consultant of Montage, hired to compile publicly available data in a format for preliminary assessment, provided Mr. Reinhart a preliminary background analysis of Company A.

On July 1, 2019, Mr. Reinhart contacted Company A’s Chief Executive Officer regarding an introductory meeting.

On August 8, 2019, Mr. Reinhart and Company A’s Chief Executive Officer met and discussed Company A’s strategy moving forward, potential synergies that could be realized in a combination of Montage and Company A, the importance of scale in the industry, and potential consolidation opportunities.

On August 13, 2019, Mr. Reinhart had a telephone call with D. Martin Phillips and Mark E. Burroughs, Jr., directors of Montage and principals of EnCap, Montage’s largest beneficial stockholder, regarding the stated intent of Company A’s Chief Executive Officer to reach out to Messrs. Phillips and Burroughs for discussions. Following this call, on August 14, 2019, Company A’s Chief Executive Officer had a discussion with Messrs. Phillips and Burroughs regarding potential acquisitions of Montage and other portfolio companies of EnCap. This discussion resulted in minimal continued dialogue until late January 2020, as discussed below.

On December 6, 2019, representatives of Company B, a publicly traded independent exploration and production company, and its financial advisor met in person with representatives of EnCap, which meeting was arranged by Company B’s financial advisor, to discuss general matters and a potential acquisition of, or merger with, Montage. At the meeting, representatives of EnCap deferred conversation regarding any potential strategic transaction involving Montage to Mr. Reinhart.

On December 10, 2019, the Chief Executive Officer of Company B invited Mr. Reinhart to meet in person, and on December 12, 2019, Mr. Reinhart accepted the invitation.

On January 9, 2020, Mr. Reinhart, Mr. Hodges, and Company B’s Chief Executive Officer met in person. At this meeting, Company B’s Chief Executive Officer presented the concept of a potential merger with, or acquisition of, Montage using Company B’s stock as consideration. Discussions at the meeting centered around Company B’s strategy and the information required to assess potential synergies that could be realized in the transaction and associated business combination metrics.

Following a request for a meeting from J. David Cecil, Southwestern’s then-Executive Vice President, Corporate Development, Mr. Burroughs and other representatives of EnCap met with Mr. Cecil on January 15, 2020 and discussed a potential combination between Southwestern and Montage and/or other portfolio companies of EnCap.

Following the meeting between Mr. Burroughs and Mr. Cecil, Mr. Cecil contacted Mr. Reinhart by email on January 24, 2020 and requested an introductory telephone call. The introductory call between Mr. Reinhart and Mr. Cecil was held on January 27, 2020, during which the parties discussed industry consolidation and scale benefits, as well as Southwestern’s desire to meet with Montage executives and learn more about Montage.

On January 27, 2020, Messrs. Reinhart and Hodges also had a telephone call with representatives of Company B’s financial advisor to discuss the potential merger or acquisition opportunity presented during the January 9^th meeting between Mr. Reinhart and Company B’s Chief Executive Officer. Following this call, the parties exchanged emails regarding data requests in connection with assessment of the potential merger or acquisition opportunity and a draft confidentiality agreement from Company B.

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On January 29, 2020, Mr. Reinhart had a telephone call with Company B’s Chief Executive Officer to further discuss the data requests in connection with an assessment of a merger or acquisition opportunity between Montage and Company B, as well as the form of confidentiality agreement to be used.

On January 30, 2020, in order to permit Montage and Company B to begin possible due diligence investigations, Mr. Reinhart sent a draft mutual confidentiality agreement to Company B’s Chief Executive Officer. Over the next several days, the parties negotiated the terms of the mutual confidentiality agreement, with the participation of representatives from Norton Rose Fulbright US LLP (“Norton Rose Fulbright”), Montage’s outside counsel.

Additionally, on January 30, 2020, Mr. Phillips and another representative of EnCap met again with the Chief Executive Officer of Company A to discuss a combination between Company A and Montage and/or other portfolio companies of EnCap.

Also on January 30, 2020, Mr. Cecil contacted Mr. Reinhart to initiate the process for entering into a mutual confidentiality agreement, as well as begin coordination of an introductory meeting between Southwestern and Montage executives.

The next day, Mr. Cecil sent a draft mutual confidentiality agreement to Mr. Reinhart to facilitate further discussions about a possible transaction. Over the course of the next several days, the parties negotiated the terms of the confidentiality agreement between Southwestern and Montage, including the terms of a customary standstill provision binding on both parties, with participation by representatives of Norton Rose Fulbright and Skadden, Arps, Slate, Meagher & Flom LLP (“Skadden”), Southwestern’s outside counsel. The confidentiality agreement contained a provision prohibiting either party from publicly requesting or proposing any waiver, amendment or termination of the standstill provision. The confidentiality agreement also contained a “fall away” provision rendering the standstill provision inoperative and of no force or effect with respect to a party if (i) any other person or “group” (as defined in the Exchange Act) acquired or publicly proposed to acquire more than 50% of the outstanding voting securities of the other party or (ii) such other party or any of its affiliates entered into an agreement with a person other than a party to the confidentiality agreement or any of its affiliates, providing for a merger, share exchange, asset or equity sale or other business combination transaction pursuant to which the voting securities of such other party outstanding immediately prior to such transaction would constitute less than 50% of the outstanding voting securities of such other party immediately following such transaction (or if such other party would not be the survivor or successor owner of its material assets after the merger, share exchange, asset or equity sale or other business combination, the voting securities of such other party outstanding immediately prior to such transaction would be converted into, or exchanged for, in such transaction, less than 50% of the outstanding voting securities of the survivor or successor or its publicly traded direct or indirect parent company).

On February 3, 2020, Mr. Reinhart had a telephone call with Mr. Cecil to schedule an introductory meeting and further discuss Southwestern’s interest in a potential transaction with Montage.

On February 4, 2020, Montage and Company B executed a mutual confidentiality agreement, and Montage provided Company B with access to an electronic data-room. The confidentiality agreement contained a customary reciprocal standstill provision and a provision prohibiting either party from requesting or proposing to waive, terminate or amend the standstill provision.

On February 6, 2020, Mr. Reinhart provided a January 2020 update to the Montage Board which included a strategic action update outlining Montage’s discussions with Company B and the entry into the mutual confidentiality agreement, as well as the informal introductory communication with Southwestern and the forthcoming confidentiality agreement process. Mr. Reinhart’s communication to the Montage Board also included an update regarding potential business development opportunities and other strategic acquisition opportunities, including an entity or asset-level acquisition of an exploration and production company operating

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in the Appalachian Basin, the upcoming divestiture of an Appalachian Basin-based exploration and production company by its private equity owners, and a potential divestiture by a global energy company of its Appalachian production and assets. The Montage Board was supportive of Montage management continuing dialogue to assess ongoing opportunities.

On February 10, 2020, Mr. Reinhart, Mr. Hodges, Oleg E. Tolmachev, Montage’s then Executive Vice President and Chief Operating Officer, and Matthew H. Rucker, Montage’s then-Executive Vice President, Resource Planning and Development, met with William J. Way, Southwestern’s President and Chief Executive Officer, Julian M. Bott, Southwestern’s Executive Vice President and Chief Financial Officer, Clayton A. Carrell, Southwestern’s Executive Vice President and Chief Operating Officer, and Mr. Cecil for a general introductory meeting.

On February 11, 2020, Mr. Reinhart contacted Randall M. Albert, Chairman of the Montage Board, and Mr. Burroughs to discuss the meeting with representatives of Southwestern the previous night and provide an update on the status of discussions with Company A and Company B.

On February 12, 2020, Mr. Reinhart contacted Mr. Cecil regarding a potential joint management presentation providing an overview of Montage and Southwestern, including a discussion of the expected date and content of the presentation, and provided Mr. Cecil with an outline of potential materials recommended to be provided by each side for corporate review. Mr. Reinhart subsequently contacted Mr. Burroughs to provide a summary of the topics expected to be reviewed in the joint management presentation with Southwestern and a potential management presentation with Company A.

On February 13, 2020 and February 14, 2020, Mr. Reinhart and Company B’s Chief Executive Officer discussed the status of the assessment of the merger or acquisition opportunity. Company B’s Chief Executive Officer relayed that Company B’s assessment of Montage and the potential merger or acquisition opportunity was ongoing, and the parties had a further general discussion surrounding Montage’s fit with Company B’s strategic corporate objectives.

Also on February 14, 2020, Montage and Southwestern executed the mutual confidentiality agreement.

On February 17, 2020, Company B’s Chief Executive Officer contacted Mr. Reinhart and expressed that Company B’s assessment of Montage was ongoing but a transaction with Montage was likely not a priority for Company B’s ongoing business strategy.

Also on February 17, 2020, Mr. Reinhart and Company A’s Chief Executive Officer met in person. Discussions during this meeting centered around the potential benefits of a merger between Montage and Company A.

Messrs. Reinhart and Hodges also contacted the Montage Board on February 17, 2020 to outline materials necessary for the Montage Board to facilitate the hiring of a strategic advisor.

Also on February 17, 2020, Mr. Reinhart contacted Mr. Cecil to confirm the date and location of the joint management presentation for March 11, 2020 at Southwestern’s principal executive offices in Spring, Texas. As part of that correspondence, Mr. Reinhart also confirmed certain matters related to an electronic data-room for due diligence information sharing.

On February 18, 2020, Mr. Reinhart sent the Montage Board a summary of plans to assess the engagement of a financial advisor, subject to approval of the Montage Board, and again provided the Montage Board the January 2020 update. The Montage Board provided feedback regarding a potential strategic financial advisor to consider.

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Also on February 18, 2020, Mr. Reinhart sent a draft mutual confidentiality agreement to Company A’s Chief Executive Officer. Over the next several days, the parties negotiated the terms of the mutual confidentiality agreement, with the participation of representatives from Norton Rose Fulbright and Vinson & Elkins L.L.P. (“Vinson & Elkins”), outside counsel to EnCap.

On February 20, 2020, Mr. Reinhart contacted Andy Huggins, Vice President, Commercial Development of Southwestern, to advise him that, with the exception of the management presentation, the preparation for which was still in progress, the Montage data requested by Southwestern had been posted to the Montage electronic data-room for Southwestern’s review.

On February 27, 2020, Montage and Company A executed the mutual confidentiality agreement, and the parties began scheduling initial management presentations for both companies. The confidentiality agreement contained a customary reciprocal standstill provision, a provision prohibiting either party from publicly requesting or proposing any waiver, amendment or termination of the standstill provision, and a customary “fall away” provision rendering the standstill provision inoperative and of no force or effect under certain circumstances. Over the next few days, Mr. Reinhart sent representatives of Company A certain requested data regarding Montage, as well as a copy of the Montage management presentation outline for Company A’s review.

On March 2, 2020, the Montage Board held a regularly scheduled, in-person meeting, which was attended by members of Montage management. Following the meeting, the Montage Board met in executive session to discuss the status of potential strategic transactions. Discussions during the executive session generally centered on industry consolidation and scale, potential counterparties for strategic transactions in Montage’s applicable region, and status updates on Company A, Company B and Southwestern. In addition to a consolidation scenario, the Montage Board analyzed numerous considerations and risks related to a stand-alone scenario during the discussions, including discussions regarding the upcoming maturity of Montage’s outstanding unsecured senior notes due 2023, Montage’s limited access to the debt and equity capital markets as a small-cap exploration and production company going forward, rising leverage and Montage’s muted near-term organic growth prospects considering the aforementioned items. The Montage Board also discussed the ongoing process for selecting a strategic financial advisor. Following this session, the Montage Board was supportive of Montage management continuing the strategic discussions with Company A, Company B and Southwestern and asked management to consider those options while also considering alternative stand-alone options in light of evolving commodity prices, capital markets, investor sentiment and other factors.

On March 3, 2020, Mr. Hodges engaged in discussions with representatives of Barclays Capital Inc. (“Barclays”) and two other potential financial advisors (collectively with Barclays, the “Sell-Side Financial Advisor Candidate Firms”) regarding financial advisory services and the associated confidentiality agreement process and sent draft confidentiality agreements to each of them. Over the next few days, the parties negotiated the terms of the confidentiality agreements.

On March 4, 2020, a representative of Company A contacted Mr. Reinhart to provide a revised due diligence request list and advise Mr. Reinhart that a Company A electronic data-room had been opened.

On March 5, 2020, Montage entered into confidentiality agreements with Barclays and each of the two other Sell-Side Financial Advisor Candidate Firms so that Montage could hold more in-depth discussions with such advisors for the purposes of evaluating their potential role as a financial advisor to Montage.

On March 9, 2020, Mr. Hodges had a telephone call with Company B’s financial advisor to discuss the status of the assessment of the merger or acquisition opportunity. During this call, Company B’s financial advisor indicated that Company B’s analysis of Montage was ongoing, but a transaction with Montage was one of many opportunities that Company B was assessing, and it was likely not a strategic fit for Company B.

Between March 10, 2020 and March 12, 2020, Barclays and the two other Sell-Side Financial Advisor Candidate Firms each gave a presentation to members of Montage management.

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On March 11, 2020, Messrs. Reinhart, Hodges, Rucker, and Tolmachev and other members of Montage management met with Messrs. Way, Bott and Carrell and other members of Southwestern management at Southwestern’s principal executive offices in Spring, Texas for initial joint management presentations. At this meeting, management of each company gave corporate overviews of their company which covered multiple functions, including assets, strategy, operations, financial positions, marketing and midstream, business development, and commercial agreements.

On March 12, 2020, Mr. Reinhart and Mr. Hodges met in person with Company A’s Chief Executive Officer and Chief Financial Officer. During this meeting, Company A’s Chief Executive Officer relayed that Company A was working with financial advisors regarding its strategic and financial alternatives, including the current discussions with Montage. The parties reviewed certain initial and high-level due diligence materials from Company A in context of analyzing the potential benefits of a merger of Company A and Montage.

On March 14, 2020, Montage provided Company A with access to an electronic data-room and notified Company A that materials were being uploaded in accordance with the previously provided request list.

On March 16, 2020, Messrs. Reinhart and Hodges received an unsolicited communication from a financial advisor providing information and assessments on potential mergers and acquisitions synergies related to Company A.

Also on March 16, 2020, Company A’s Chief Executive Officer contacted Mr. Reinhart and Mr. Hodges to schedule initial management presentations by Montage and Company A for March 19, 2020, noting that the data for the Company A management presentation was in the process of being finalized.

That same day, Mr. Reinhart and Mr. Cecil discussed the evaluation process and next steps following the previous Montage and Southwestern joint management presentations that had occurred on March 11, 2020.

Following completion by Montage management of its due diligence and interview process with Barclays and the two other Sell-Side Financial Advisor Candidate Firms, Montage management sent the Montage Board a presentation in support of its recommendation to engage Barclays. After review of this presentation, the Montage Board, acting by unanimous written consent effective as of March 17, 2020, authorized and approved the engagement of Barclays as Montage’s sole financial advisor to assist Montage in the evaluation of potential business combination transactions or other strategic alternatives, based on Barclays’ qualifications, expertise and reputation, and authorized Mr. Reinhart and Mr. Hodges to negotiate, execute and deliver an engagement letter with Barclays. Montage management worked with Barclays to finalize the engagement letter over the next several days with the participation of representatives from Norton Rose Fulbright.

On March 18, 2020, Mr. Hodges contacted Mr. Cecil regarding initial questions related to Southwestern data that was shared with or presented to Montage by Southwestern and its representatives.

On March 19, 2020, Montage and Company A each gave management presentations and conducted initial discussions regarding a potential transaction between the companies via videoconference. The attendees at this meeting included senior leadership of each company, but did not include Mr. Reinhart or the Chief Executive Officer of Company A. During these presentations, management of each company gave corporate overviews of their company which covered multiple functions, including assets, strategy, operations, financial positions, marketing and midstream, business development, and commercial agreements.

On March 21, 2020, Messrs. Reinhart and Hodges received unsolicited materials from a financial advisor regarding an update of the opportunity with Company A.

On March 23, 2020, representatives of Montage held a due diligence session with representatives of Southwestern by telephone. Over the next several months, Montage and Southwestern and their respective advisors continued to conduct due diligence investigations of each other and their respective operations and

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assets. As part of this process, each party continued to upload documents and other materials to their respective electronic data-rooms. Both parties submitted multiple follow-up due diligence requests to each other during the due diligence process, which continued throughout the period leading up to the signing of the Merger Agreement.

On March 24, 2020, Messrs. Reinhart and Hodges scheduled a telephone call with Company A’s Chief Executive Officer and Chief Financial Officer to discuss next steps in assessing a potential business combination between Montage and Company A.

On March 25, 2020, Montage and Barclays entered into an engagement letter. That same day, representatives of Barclays and Montage management held a kick-off meeting via teleconference to provide a general status update on discussions with Company A and Southwestern.

Also on March 25, 2020, Mr. Reinhart and Mr. Hodges had a telephone call with representatives of Barclays in advance of the telephone call scheduled for March 26, 2020 with Company A’s Chief Executive Officer and Chief Financial Officer.

On March 26, 2020, Messrs. Reinhart and Hodges had a telephone call with Company A’s Chief Executive Officer and Chief Financial Officer. During this call, the parties discussed the potential benefits of a strategic combination between the two companies and the associated financial synergies. Company A’s Chief Executive Officer and Chief Financial Officer provided materials in support of these benefits, which Mr. Hodges subsequently sent to Barclays. During this call, Messrs. Reinhart and Hodges requested an update of the proposed merger structure from Company A’s financial advisor for Montage to review.

Also on March 26, 2020, an informal group of Montage directors consisting of Mr. Burroughs, Mr. Albert, and Richard D. Paterson (collectively, the “Transaction Team”) was formed for the purpose of, with the participation of Mr. Reinhart, facilitating the Montage Board’s involvement in the strategic transaction process and keeping members of the Montage Board updated routinely on developments related to Montage’s potential strategic transactions.

On March 30, 2020, Mr. Reinhart reported to the Transaction Team. As part of this communication, Mr. Reinhart reiterated the intent that the Transaction Team was to be a means of more frequent updates regarding the strategic assessments under review by Montage, as well as to serve as a group that was to be consulted as the strategic reviews reach milestones, decision points or other meaningful assessments. Mr. Reinhart also provided the Transaction Team with an update on the kick-off meeting that Montage management personnel had with Barclays on March 25, 2020, and provided an overview of the status of the ongoing discussions on potential strategic transactions with Southwestern and Company A, including an update on the ongoing due diligence process with each company.

On March 31, 2020, representatives of Montage, Barclays, Company A and Company A’s financial advisors had a telephone call to discuss the merger opportunity between Montage and Company A and initial assessments by Company A’s financial advisors.

On April 1, 2020, representatives of Montage contacted Barclays to provide Montage management’s initial informal estimates of potential synergies in connection with the potential transaction with Company A, based upon internal management assessments regarding how the pro forma company’s assets and operations may be optimized.

During the first half of April 2020, at the direction of Montage management, Barclays held ongoing discussions with Company A management and Company A’s financial advisor to exchange financial information and conduct mutual financial due diligence for a potential transaction between Montage and Company A.

On April 3, 2020, Mr. Hodges had a final telephone call with Company B’s financial advisor, who confirmed that Company B was not interested in pursuing a transaction with Montage.

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On April 6, 2020, Mr. Reinhart had a telephone call with the Transaction Team to provide an update on the status of the financial analyses by Barclays with respect to a strategic transaction with Southwestern and Company A, respectively, and also provided related written materials by email. That same day, at the direction of Montage management, representatives of Barclays had a telephone call with Company A’s financial advisors to discuss potential merger synergies via optimization of the assets and operations of the pro forma company.

On April 13, 2020, Mr. Reinhart reported to the Transaction Team, advising them that Southwestern planned to advise Montage of its intent to continue to assess a potential transaction between Montage and Southwestern over the next few days, as well as providing the Transaction Team with an update on valuation work and merger synergies analysis for the potential Company A transaction.

On April 16, 2020, Company A’s financial advisors contacted Mr. Reinhart, Mr. Hodges, and representatives of Barclays and provided a combination overview and non-binding proposal on behalf of Company A based on financial analysis work conducted by the financial advisors. Company A’s proposal entailed a stock for stock merger whereby Montage would merge into Company A, with Company A surviving the merger, along with other financing transactions on which the proposed merger was conditioned. The proposal also included an expected pro forma relative equity ownership and proposed that the combined company’s board of directors would reflect that relative ownership.

On April 17, 2020, the Transaction Team and Mr. Reinhart discussed updates related to the status of the potential Southwestern and Company A transactions, including the status of Company A’s initial proposal and Montage’s preparation of a counter proposal. Mr. Reinhart also relayed that Montage was awaiting feedback from Southwestern.

In response to Company A’s proposal on April 16, 2020, Montage management prepared a pro forma operating plan designed to maximize levered free cash flow, reduce financial leverage and lower operating costs. On April 23, 2020, members of Montage management completed a high-level, non-binding counter proposal regarding a potential transaction between Montage and Company A and sent the proposal to Company A’s financial advisors. Montage’s counter proposal included a merger whereby Company A would merge into Montage, with Montage surviving the merger, along with pro forma optimization suggestions (along with the associated impacts to cash flows), a financing transaction contemplated as part of Montage’s counter proposal and plans for addressing the outstanding debt of both Company A and Montage.

On April 24, 2020, Company A advised Montage that it intended to pursue an alternative strategy and it was not interested in further discussions regarding Montage’s counter proposal at that time.

On April 28, 2020, Mr. Reinhart provided the Transaction Team with an update on the summary and status of the potential Southwestern and Company A transactions and a preview of the materials to be provided to the Montage Board at its meeting on May 1, 2020. Mr. Reinhart relayed that the main variances between the Montage and Company A proposals were the equity ownership split, the surviving entity, pro forma optimization suggestions proposed by Montage management, and other operational matters raised in Montage’s proposal as items for Company A to address. Mr. Reinhart informed the Transaction Team that Company A had informed Montage that it intended to pursue alternative strategies. Following discussions with the Transaction Team, it was determined that a combination with Company A under the terms proposed by Company A was not a strategic fit. Mr. Reinhart also informed the Transaction Team that Southwestern had indicated that their assessment of a potential transaction with Montage was temporarily on-hold due to uncertain market conditions, particularly the recent decline in oil prices and the economic impact of the COVID-19 pandemic.

On May 1, 2020, the Montage Board held a regularly scheduled meeting via teleconference, which teleconference was attended by members of Montage management. During the course of the meeting, the Montage Board discussed, among other things, Montage’s potential strategic transaction opportunities with Southwestern and Company A, including the benefits, considerations and status of each opportunity, and the

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non-binding proposal presented by Company A and Montage’s non-binding counter proposal thereto and reactions to those proposals. Following discussion, the Montage Board was supportive of Montage’s continued pursuit of consolidation opportunities in consideration of Montage’s stand-alone financial position, business development opportunities, and growth opportunities.

On May 6, 2020, at the direction of Montage management, representatives of Barclays had a discussion with representatives of Southwestern regarding the status of the potential transaction between Southwestern and Montage. Following this discussion, representatives of Barclays and Montage discussed the timing and progress of Southwestern’s evaluation and assessments.

On May 7, 2020, Mr. Reinhart had a telephone call with Mr. Cecil to discuss the status of the evaluation of a potential transaction between Southwestern and Montage and next steps to continue the process.

On May 12, 2020, Mr. Hodges had a telephone call with Company A’s financial advisor. During this call, Company A’s financial advisor confirmed that Company A was evaluating other strategic options and did not expect to revisit discussions with Montage in the near term.

On May 20, 2020, Mr. Cecil contacted Mr. Reinhart by email and telephone to discuss an update regarding the discussions of the Southwestern Board and intentions to advance discussions regarding a potential transaction.

On May 22, 2020, Mr. Way sent a letter to Mr. Reinhart confirming Southwestern’s intention of continuing dialogue regarding Southwestern’s potential acquisition of 100% of the issued and outstanding common stock of Montage. In the letter, Mr. Way relayed that Southwestern currently contemplated acquiring Montage in an all-stock transaction based on an “at market” exchange ratio, the Southwestern Board had been informed of Southwestern’s efforts to date, Southwestern had engaged Skadden as its outside counsel in connection with consideration of the potential transaction, and that Southwestern expected to retain outside financial advisors. In addition, Mr. Way’s letter outlined certain additional due diligence items that were important to further Southwestern’s analysis, pro forma business plans and valuation.

On May 26, 2020, Mr. Reinhart provided an update to the Transaction Team, noting that there was no further update regarding any potential transaction with Company A and describing the letter from Mr. Way discussed above. Also that day, Mr. Reinhart contacted Mr. Cecil to inform him that the Montage Board had been updated on the joint discussions between Montage and Southwestern, that the Montage Board was prepared to engage further, and that Montage required delivery of certain high-priority due diligence items in order to continue assessing Southwestern valuation.

On May 27, 2020, Mr. Reinhart had a telephone call with Mr. Cecil to discuss next steps in the evaluation process for a potential “at market” merger transaction.

On May 28, 2020, Mr. Reinhart sent the Transaction Team a copy of the letter from Mr. Way discussed above and informed them that Barclays was undertaking financial analyses regarding a potential merger transaction with Southwestern for review by the Montage Board.

During the month of May 2020, from time to time, Montage management discussed with Barclays the scope of potential partners for a consolidation transaction with Montage. During the same period, as part of its ordinary course of business conversations with various exploration and production companies, Barclays made multiple inquiries about such companies’ interest level in a potential strategic transaction with a small-scale exploration and production company operating in the Appalachian Basin. Only Company C, a publicly traded exploration and production company, indicated some potential interest and Barclays reported the feedback from these conversations to Montage management.

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On June 2, 2020, Mr. Reinhart provided the Transaction Team with an update regarding the ongoing due diligence efforts and scheduled due diligence calls in connection with the potential transaction with Southwestern.

On June 8, 2020, Mr. Way had a telephone call with Mr. Reinhart to provide a general update on discussions that had taken place among the Southwestern Board and advising Mr. Reinhart of certain Southwestern organizational changes.

Also on June 8, 2020, Mr. Reinhart provided the Transaction Team with an update regarding due diligence efforts and ongoing valuation work being performed in connection with the potential Southwestern transaction.

On June 12, 2020, Mr. Reinhart provided the Transaction Team with an update regarding Montage’s ongoing strategic alternatives and assessments, including (i) accretive business development opportunities, (ii) potential acquisition opportunities, including the potential entity or asset-level acquisition of an exploration and production company operating in the Appalachian Basin and the potential divestiture by a global energy company of its Appalachian production and assets, each in lieu of potential business combination transactions, and (iii) options for stand-alone capital market actions.

On June 15, 2020, at the direction of Montage management and pursuant to the general guidance from the Montage Board to explore possible strategic transaction counterparties, Barclays contacted representatives of Company C to discuss whether Company C would be interested in entering into a confidentiality agreement with Montage in order to conduct some initial due diligence on Montage and understand whether a strategic transaction was possible.

On June 16, 2020, Barclays received feedback from a representative of Company C that Company C was interested in starting due diligence with Montage to assess the possibility of a potential transaction, and Barclays provided Mr. Reinhart with contact information for Company C’s Chief Executive Officer in preparation for an introductory call.

On June 17, 2020, Mr. Reinhart provided the Transaction Team with an update. In this update, Mr. Reinhart provided the Transaction Team with a preliminary valuation analysis and related market information regarding a potential transaction with Southwestern. In addition, Mr. Reinhart advised the Transaction Team of Company C’s desire to access an electronic data-room for due diligence purposes, Company C’s request to enter into a mutual confidentiality agreement with Montage, and a telephone call with Company C’s Chief Executive Officer scheduled for later that day.

Later that day, Mr. Reinhart had a telephone call with Company C’s Chief Executive Officer. During this call, the parties discussed the respective businesses of Montage and Company C and potential synergies between the companies, and Company C’s Chief Executive Officer confirmed that Company C would like to assess the potential of a merger with or acquisition of Montage.

Also on June 17, 2020, Mr. Reinhart had a telephone call with Messrs. Way and Bott to discuss the status of ongoing valuation work.

On June 18, 2020, Mr. Reinhart sent a draft mutual confidentiality agreement to a representative of Company C. Over the next several days, the parties negotiated the terms of the mutual confidentiality agreement, with the participation of representatives from Norton Rose Fulbright and Vinson & Elkins.

On June 19, 2020, at the direction of Montage management, representatives of Barclays had an introductory telephone call with representatives of Citigroup Global Markets Inc. (“Citi”) and Goldman Sachs & Co. LLC (“Goldman Sachs”), Southwestern’s financial advisors. Representatives of Barclays provided an update to Messrs. Reinhart and Hodges following this call.

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On June 24, 2020, representatives of Barclays, Citi and Goldman Sachs had an additional, more detailed telephone call during which various financial aspects of the potential transaction, pro forma financial information and due diligence were discussed.

On June 25, 2020, Montage and Company C entered into a mutual confidentiality agreement. The confidentiality agreement contained a customary reciprocal standstill provision, a provision prohibiting either party from publicly requesting or proposing any waiver, amendment or termination of the standstill provision, and a customary “fall away” provision rendering the standstill provision inoperative and of no force or effect under certain circumstances.

On June 26, 2020, Montage provided Company C with access to an electronic data-room and notified Company C of certain key pertinent information that had been uploaded.

Also on June 26, 2020, Mr. Reinhart provided the Transaction Team with an update, relaying that Company C had entered into the mutual confidentiality agreement with Montage and had been provided with access to an electronic data-room. Mr. Reinhart also informed the Transaction Team that Barclays was working with Citi and Goldman Sachs with respect to various financial aspects regarding the potential transaction.

On July 1, 2020, Mr. Reinhart contacted representatives of Company C, inquiring if they had any questions or needed assistance with their ongoing review, and the representatives of Company C responded that they had no requests at that time.

On July 6, 2020, Mr. Reinhart provided the Transaction Team with an update regarding discussions between Barclays and Citi and Goldman Sachs with respect to various financial aspects regarding the potential transaction and due diligence efforts.

On July 8, 2020, at the request of Montage management, representatives of Barclays provided representatives of Montage management with an overview of publicly available information on Company C.

On July 13, 2020, Mr. Reinhart provided the Transaction Team with an update, relaying that feedback from Company C was expected later that week, due diligence efforts with respect to the potential transaction with Southwestern were ongoing, and an update from Southwestern’s financial advisors was expected soon related to a meeting of the Southwestern Board held during the previous week.

On July 14, 2020, Messrs. Reinhart and Hodges contacted Messrs. Way and Bott by email, and followed up with a telephone call, to discuss the pace of valuation work and associated due diligence efforts.

On July 15, 2020, Mr. Reinhart sent the Transaction Team a copy of the overview of publicly available information on Company C previously provided by Barclays, as well as a preliminary relative contribution analysis related to a potential transaction with Company C based on such public information.

On July 16, 2020, Mr. Reinhart contacted Mr. Bott to provide a list of high-priority due diligence items associated with the valuation analysis, pro forma business plan framework and other financial due diligence matters.

On July 20, 2020, Mr. Reinhart provided the Transaction Team with an update to relay feedback, following a meeting of the Southwestern Board, that Southwestern continued to support an “at market” business combination transaction and was also assessing potential associated capital markets transactions to refinance Montage’s unsecured senior notes to be undertaken in connection with the business combination transaction announcement.

Also on July 20, 2020, representatives of Company C requested that Montage provide certain cash flow data of Montage for review by Company C’s management. The next day, representatives of Company C requested

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additional data and financial analyses from Barclays and Montage to compare the cash flow data provided in the Montage data-room. Following a meeting of the Company C board of directors, Montage received feedback that Company C was currently prioritizing other strategic opportunities.

On July 22, 2020, Montage announced preliminary second quarter 2020 production results and its revised expectations for full year 2020 capital expenditures. Additionally, Montage announced that it had entered into a non-binding letter of intent with an international third-party to sell its non-core wellhead gathering infrastructure in the Ohio Utica condensate development area.

On July 24, 2020, Mr. Bott contacted Mr. Reinhart to advise him that an initial pro forma business plan had been delivered to Montage.

On July 27, 2020, Mr. Reinhart had a telephone call with Messrs. Way and Bott to discuss the status of ongoing valuation work associated with the potential transaction.

On July 30, 2020, Messrs. Reinhart and Hodges had a telephone call with Messrs. Way and Bott, during which Messrs. Way and Bott advised Messrs. Reinhart and Hodges of their intention to send a draft of a merger agreement, a capital markets plan to address Montage’s outstanding unsecured notes and an envisioned timeline for the announcement of a potential transaction. Following the call, Mr. Bott provided Messrs. Reinhart and Hodges the preliminary timeline by email, which included a timeline related to potential capital markets transactions to be undertaken shortly after execution of a merger agreement. Also on July 30, 2020, Citi and Goldman Sachs provided updated due diligence items on behalf of Southwestern.

On August 1, 2020, Mr. Bott contacted Mr. Reinhart to advise him of anticipated timing of forthcoming drafts of a merger agreement and an EnCap support agreement.

On August 2, 2020, Mr. Way sent a draft merger agreement and a draft EnCap support agreement to Mr. Reinhart. Among other things, the draft merger agreement included provisions that required Montage to present the Southwestern transaction to Montage’s stockholders for approval even if a third party were to propose an alternative transaction that the Montage Board determined to be a superior proposal (a “Force the Vote Provision”). The Southwestern draft merger agreement also included a termination fee payable by Montage in certain circumstances, including one in which Southwestern elected to terminate the merger agreement after the Montage Board withdrew its recommendation in favor of the Southwestern transaction in response to a superior proposal from a third party, equal to 5.0% of Montage’s total equity value implied by the transaction. In addition, the Southwestern draft merger agreement included a provision requiring Montage to reimburse Southwestern for all of its expenses in the event an alternative acquisition proposal was proposed or announced or otherwise became known to Montage and thereafter the Southwestern merger agreement was terminated under certain circumstances (the “Expense Reimbursement Provision”). Among other things, the draft EnCap support agreement included a provision requiring EnCap to vote its shares of Montage Common Stock against any alternative transaction for a period of 12 months following termination of the merger agreement between Montage and Southwestern in certain circumstances (a “Tail Provision”).