By Sabrina Valle and David French

After brief informal conversations earlier this year, Exxon approached Pioneer about a deal last month, the sources said. It was from a position of strength. Exxon's shares were hovering near record highs, and it had a cash pile of about $30 billion.

Sheffield said he was approached by Woods two weeks ago with an offer that was beneficial both to shareholders and Pioneer's employees. The Permian operations will be led by a mix of staff from both companies.

"Darren has been very fair in the negotiations," Sheffield told a small group of reporters.

Woods had already capitalized on elevated energy prices to streamline Exxon's operations and focus on highly profitable oil and gas production.

This allowed Exxon to enter the deal negotiations with its stock coveted by investors as a valuable currency. The stakes were high. The acquisition would turn Exxon into the largest shale producer in the Permian basin, itself the biggest and most lucrative U.S. oilfield.

Pioneer agreed to an all-stock deal, even though Exxon could afford to pay all-cash through a combination of its cash on hand and borrowing, the sources said. The deal structure allowed the two companies to reconcile their price disagreements, according to the sources.

"We basically closed this deal fairly quickly," Woods told reporters on a conference call on Wednesday.

Exxon could boast it clinched the agreement by paying a relatively small 16% premium to Pioneer's unaffected share price, while Pioneer could allow its shareholders to benefit from the deal's cost and revenue synergies by giving them more than 10% of the combined company, the sources said.

From Pioneer's side, the view was that shareholders would be willing to accept a skinny takeover premium in return for a stake in the industry's behemoth, whose shares are perceived as the gold standard by many traditional oilmen, including Sheffield.

The combined company will also have more resources to develop international projects, such as its Guyana operations, the sources added.

For his part, Sheffield was ready to sell Pioneer, the sources said. The 70-year-old wildcatter has led the company since its founding in 1997, with a brief hiatus between 2016 and 2019. He had said he planned to retire in January.

He now stands to receive a $29 million payout and join Exxon's board as a director.

"When Scott and I sat down and started talking about the complementary nature of both of our businesses, it became very obvious very early on in those discussions that there's a big opportunity here," Woods said.