ExxonMobil has agreed to acquire rival oil exploration firm XTO Energy, in an all-stock transaction valued at 41 billion dollars, the companies said Monday.

The biggest US oil group said the deal would "enhance ExxonMobil's position in the development of unconventional natural gas and oil resources."

The transaction includes the assumption of 10 billion dollars in XTO debt and offers shareholders a 25 percent premium to the most recent stock price.

"XTO is a leading US unconventional natural gas producer, with an outstanding resource base, strong technical expertise and highly skilled employees," said Rex Tillerson, ExxonMobil's chairman and chief executive.

"XTO's strengths, together with ExxonMobil's advanced R&D (research and development) 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."

The deal, one of the biggest of 2009, has been approved by the boards of both firms and requires XTO stockholder approval and regulatory clearance.

XTO has rights to some 45 trillion cubic feet of gas including shale gas, tight gas, coal bed methane and shale oil.

These will complement ExxonMobil's holdings in the United States, Canada, Germany, Poland, Hungary and Argentina.

Company officials hope to close the deal in the second quarter of 2010 and then establish a new upstream organization to manage production of unconventional resources and use of new oil exploration technology.

Bob Simpson, chairman and founder of XTO, said the firm has establish a resource base of energy across the United States over the company's 23 years.

"XTO has a proven ability to profitably and consistently grow production and reserves in unconventional resources," said Simpson.

"As the world's leading energy company, ExxonMobil will build on our success and open new opportunities for the development of natural gas and oil resources on a global basis."