Oil giant Exxon Mobil announced this morning it was moving deeper into the natural-gas business by acquiring XTO Energy Inc. in an all-stock deal worth about $31 billion. The transaction, one of the largest of the year, was struck amid Senate talks about imposing a federal tax on carbon emissions. Natural gas is considered among the cleanest-burning fossil fuels, which means that it wouldn't be taxed as heavily as oil or coal.

It’s Exxon’s biggest deal since its merger with Mobil 10 years ago, and it amounts to a huge bet on natural gas as a growing energy source for the future.

"They're going to in effect invest $41 billion in good old North American natural gas," Bruce Bullock, director of the Maguire Energy Institute at Southern Methodist University, told the Dallas Business Journal.

Exxon Mobil will issue 0.7098 common shares for each common share of XTO. Exxon will also take on $10 billion in XTO debt. The price is a 25 percent premium for XTO shareholders.

“XTO is a leading U.S. unconventional natural-gas producer, with an outstanding resource base, strong technical expertise, and highly skilled employees,” said Exxon chairman and CEO Rex Tillerson in a release. “XTO’s strengths, together with Exxon Mobil’s advanced R&D and operational capabilities, global scale, and financial capacity, should enable development of additional supplies of unconventional oil and gas resources, benefiting consumers both here in the United States and around the world.”

XTO is one of the leading companies at extracting natural gas by cracking shale deposits, a relatively new method of getting at natural gas and one that has vastly increased the estimates of how much gas there is in the United States—with new deposits identified in Texas, Appalachia, and Louisiana. The deal also comes at a time when a price on carbon emissions is becoming more likely in the United States. With natural gas being the cleanest burning of the fossil fuels, its attractiveness for use in electricity generation and even transportation is likely to increase.

The U.S. EPA recently identified carbon dioxide and other greenhouse-gas emissions as a threat to public safety, the first step toward regulating those emissions. At the same time, the U.S. House has passed legislation placing a price on carbon emissions, and similar legislation is pending in the Senate.

"Clearly natural gas is an attractive fuel as a price for carbon might be put in place," said Tillerson during a morning press conference. But he also pointed out that, though Exxon Mobil is getting further into the business of natural-gas extraction, the acquisition of XTO doesn't mean a shift away from oil for the nation's largest oil company.

"We don't have any particular strategy to change the balance of oil versus natural gas," Tillerson said.