NEW YORK (Bloomberg) — Exxon Mobil Corp., the biggest US oil company, agreed to buy XTO Energy Inc. for $31 billion in a bet that US emissions restrictions will spur increased demand for natural gas. Owners of Fort Worth, Texas-based XTO will get 0.7098 share of Exxon for each of their shares, the companies said yesterday in a statement. The transaction, the largest energy acquisition since 2006 and Irving, Texas-based Exxon's biggest takeover since the purchase of Mobil Corp. in 1999, values XTO at $51.69 a share, 25 percent higher than its last closing price.

"This says that corporate M&A is alive and well in the exploration and production sector," said Curtis Trimble, an analyst at Natixis Bleichroeder Inc. in Houston. "It also says that Exxon isn't shy about stepping up their exposure to the natural-gas market. Almost certainly, we will see some more follow-the-leader type transactions."

Exxon, which also will assume $10 billion in debt, will get the largest producer of US natural gas. Demand for the fuel will grow as US carbon legislation prompts power producers to switch from coal, Kenneth Cohen, Exxon's vice president for government affairs, said in a December 7 interview.