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Exxon Mobil Corp. plans to lay off 397 Pioneer Natural Resources Co. employees after buy the Permian Basin operator for $63 billion earlier this year.

The reductions representing nearly 20% of Pioneer’s pre-merger workforce will occur in seven stages between now and May 2026, Exxon said in a notice to the Texas Workforce Commission released Nov. 13. They will mainly affect employees at Pioneer’s offices in the Dallas suburbs. of Irving and the West Texas city of Midland, according to the notice.

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The acquisition of Pioneer brought Exxon’s oil and natural gas production to its highest level in more than a decade and made it the top producer in America’s largest shale basin. The cost savings will be “significantly higher” than initially expected, Exxon CEO Darren Woods said on a Nov. 1 call with analysts.

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

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“The success of this merger is highly dependent 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, and a large majority of them accepted their job offers.