Exxon Mobil is expected to say on Wednesday it will buy U.S. rival Pioneer Natural Resources for about $60 billion, a deal that puts it atop the largest U.S. oilfield and secures a decade of low-cost production, according to people familiar with the matter.

Exxon, which was valued at $442 billion on Tuesday, is expected to make a pure stock offer valued at more than $250 a share for Pioneer, the people said on condition of anonymity because the details were not public.

Pioneer shares, which closed at $237.41 on Tuesday, were up 2.7% at $244 in premarket trading. Exxon shares were trading marginally lower.

It would be the largest acquisition by any company this year and Exxon’s biggest since its $81 billion purchase of Mobil Oil in 1998.

Exxon declined to comment on “market speculation,” while Pioneer did not immediately respond to a request for comment.

The deal will leave four of the largest U.S. oil companies in control of much of the Permian Basin shale field and its extensive oilfield infrastructure.