Marathon Oil Corp. will lay off a chunk of its workforce after closing on its merger with ConocoPhillips by year-end, according to a filing with the Texas Workforce Commission.

The Houston-based E&P is planning for “a mass layoff at the Houston Facility wherein at least 500 employees will experience an employment loss” during the 12 months following closing, said Jill Ramshaw, senior vice president for human resources, in the Oct. 28 Worker Adjustment and Retraining Notification (WARN) letter.

Marathon and ConocoPhillips announced the $22.5 billion merger in May. The Houston address listed in the WARN notice is that of Marathon’s corporate headquarters.

Marathon reported a worldwide workforce of 1,681 with 74% based in the U.S. at the end of 2023 in its annual filing with the U.S. Securities and Exchange Commission.

Terminated employees will be notified of specific end dates within a month of closing, and “many will be retained for transition roles,” according to the WARN notice.

Those transition positions are still being finalized, but Marathon said more than 50% of them will last more than six months. The Houston office will not close entirely, the filing said.

Hart Energy’s call to Marathon for additional information wasn’t immediately returned.