BG Group PLC (BG.LN) Tuesday became the latest European energy company to buy into a U.S. shale-rock natural gas field, agreeing to pay $1.06 billion for a stake in EXCO Resources Inc.'s (XCO) acreage.

The proceeds will bolster EXCO's balance sheet and sent shares of natural gas producer soaring. Shares recently traded nearly 18% higher at $13.08 apiece.

The deal underscores how, despite lower oil and gas prices, international companies still view U.S. shale - hydrocarbon-rich rock formations - as economically attractive. It's also an opportunity to test their mettle in a relatively new energy frontier.

This continued interest is a boon to U.S. natural gas producers troubled by the sharp downturn in prices and the tightening of credit markets. The price decline has put pressure on EXCO and others to sell stake in shale fields to shore up their balance sheets and defray drilling costs.

EXCO, like other natural gas producers, "is a company that had dramatic amounts of debt," said Leo Mariani, an analyst with RBC Capital Markets.

The company on Monday agreed to sell some assets in the mid-continent and East Texas to Encore Acquisition Co. (EAC) for $375 million. The BG and the Encore deals will help EXCO cut its debt from $3 billion to $1.6 billion, Mariani said.

BG said that, on top of the $1.06 billion acreage deal, it also had an agreement to negotiate the acquisition of a 50% interest in related EXCO gas-gathering and distribution assets for $249 million.

BG said it would acquire a 50% interest in 120,000 net acres in Texas and Louisiana, adding 2.6 trillion standard cubic feet to its resources.

The properties include a slice of the Haynesville shale project stretching across the two states, which as a whole could hold around 200 trillion cubic feet of natural gas. That's the equivalent of 33 billion barrels of oil, or 18 years' worth of current U.S. oil production.

BG said the acquired properties' current net production of 78 million standard cubic feet a day are expected to increase to around a net 250 million standard cubic feet a day in 2012.

It said it would pay $655 million upfront and the remainder as a cost carry on future costs to develop the Haynesville shale gas.

"BG has a fantastic record of capital investment over the long term and I suspect this will go down as another great deal," said Malcolm Graham-Wood, a director at U.K. broker HansonWesthouse, who rates BG a buy.

BG shares fell 1.8%, or 19 pence, to GBP10.18.

Oil-producing countries have said lower crude prices - still at half their peak of $147 a barrel last year - could make unconventional resources like oil shale uneconomic.

Yet, BG's deal signals that shale basins, or water-tight rock formations, remain financially attractive thanks to new technologies.

The company is joining a stream of international oil companies that have invested in U.S. shale reservoirs amid the sharp downturn in natural gas prices. Natural gas prices have plunged more than 70% since hitting a high last summer above $13 a million British thermal units.

In May, Italy's Eni SpA (E) entered a $280 million alliance with independent producer Quicksilver Resources Inc. (KWK) to develop acreage in the Barnett Shale.

The Barnett Shale gas field in North Texas is credited with a sizable portion of the recent boom in domestic U.S. gas production.

The U.S. Department of Energy said in an April report that by 2011, 50% to 60% of U.S. gas reserves growth will come from unconventional shale reservoirs.

Last year, BP PLC (BP) agreed to buy Chesapeake Energy Corp. (CHK) assets in Oklahoma's Woodford Shale and in Arkansas' Fayetteville Shale for $1.75 billion and $1.9 billion, respectively.

Chesapeake also sold a stake in the Marcellus Shale - a largely undeveloped natural gas field that stretches from West Virginia to New York - to StatoilHydro ASA (STO) for $3.4 billion.

Anglo-Dutch oil major Royal Dutch Shell PLC (RDSB.LN) also continues working on Colorado oil shale, one of its biggest research and development projects to date.

The U.S. shale deal is the latest for BG, which launched a string of large acquisition efforts last year after originally focusing on organic growth.

Though lower than in mid-2008, oil prices have risen from a low of around $32 a barrel in December last year on growing hopes of economic recovery.

So oil companies are trying to secure bargains, from Iraqi Kurdistan to Africa, before asset prices rise again.

Company Web site: http://www.bg-group.com

By Benoit Faucon and Jason Womack, Dow Jones Newswires; +44-20-7842-9266; benoit.faucon@dowjones.com