(Bloomberg) -- Exxon Mobil Corp. plans to lay off 397 employees from Pioneer Natural Resources Co. after purchasing the Permian Basin operator for $63 billion earlier this year.

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The cuts representing almost 20% of Pioneer’s pre-merger staff will come in seven stages between now and May 2026, Exxon said in a notice to the Texas Workforce Commission released on Wednesday. They will mostly affect employees in Pioneer’s offices in the Dallas suburb of Irving and the West Texas town of Midland, according to the notice.

The Pioneer acquisition increased Exxon’s oil and natural gas output to the highest in more than a decade and made it the top producer in America’s biggest shale basin. Cost savings will be “considerably higher” than first anticipated, Exxon Chief Executive Officer Darren Woods said during a call with analysts Nov. 1.

Exxon plans to release its long-term plan for the Permian region as well as the integration of Pioneer in a Dec. 11 presentation.

“The success of this merger depends heavily on the retention of Pioneer’s talented workforce,” Exxon wrote in the notice. “More than 1,900 Pioneer employees were offered jobs as part of the merger, with well over a majority accepting their offer of employment.”

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