HOUSTON, Reuters Exxon Mobil agreed to buy U.S. rival Pioneer Natural Resources in an allstock deal valued at 59.5 billion that would make it the biggest producer in the largest U.S. oilfield and secure a decade of lowcost production.

The deal, valued at 253 a share, combines the largest U.S. oil company with one of the most successful names to emerge from the shale revolution that turned the U.S. into the world39;s largest oil producer in little more than a decade.

Exxon Chief Executive Darren Woods said in a media briefing the combination provides a big opportunity for synergies between the companies.

We basically closed this deal fairly quickly, Woods said after approaching Pioneer CEO Scott Sheffield two weeks ago. It became very obvious very early on in those discussions that there39;s a big opportunity here.

The merged company could add 700,000 barrels per day of new oil and gas boepd within four years of the deal closing, raising output to 2 million boepd. It also aims to cut greenhouse gas emissions and increase oil output per well by combining Exxon technology with Pioneer39;s lower cost of operations, Exxon said.

The offer represents a 9 premium to Pioneer39;s average price for the 30 days prior to Oct. 5, when reports of deal talks surfaced. Pioneer shares closed up 1.4 at 240.82 trading. Exxon shares fell 3.6.

The closing price implies investors see a 72 probability the deal will be completed, based on Pioneer39;s closing share price on Oct. 5.

The…