XTO made several acquisitions last year totaling $4.2 billion including Hunt Petroleum Corp. last year and subsequently saw an output jump of 23 percent to the equivalent of 2.95 billion cubic feet of gas a day. XTO raised its forecast for full-year production growth to 23 percent from an August estimate of 20 percent. Third quarter revenue rose 8 percent to $2.29 billion.

XTO stockholder approval and regulatory clearance, XTO owners will get 0.7098 share of  Exxon for each of their shares, the companies said in a statement. The transaction, the largest energy acquisition since 2006 and Irving-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.

The transaction value includes $10 billion of existing XTO debt and is based on the closing share prices of Exxon Mobil and XTO on Dec. 11.

“We are pleased that Exxon Mobil and XTO have reached this agreement,” said Rex W. Tillerson, chairman and CEO of Irving-based Exxon Mobil Corp.

The purchase is scheduled to close in the second quarter, the companies said.

“XTO is a leading U.S. unconventional natural gas producer, with an outstanding resource base, strong technical expertise and highly skilled employees,” Tillerson said. “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.”

Tillerson said the agreement is good news for the U.S. economy and energy security, as it will enhance opportunities for job creation and investment in the production of America’s own clean-burning natural gas resources.

In a release, XTO chairman and founder Bob R. Simpson said that during the company’s 23-year history, XTO has developed technical expertise and has assembled a substantial, high-quality and diverse resource base in producing basins across the United States, beginning with the Barnett Shale in Fort Worth.

“XTO has a proven ability to profitably and consistently grow production and reserves in unconventional resources,” Simpson said in the release. “As the world’s leading energy company, Exxon Mobil will build on our success and open new opportunities for the development of natural gas and oil resources on a global basis.”