Alta Mesa Resources Inc. (NASDAQ: AMR) said Dec. 20 that CEO Hal Chappelle, as well as COO Michael E. Ellis, are stepping down from their positions at the pure-play Stack company.

James Hackett, executive chairman of the board, has been named interim CEO of Alta Mesa. Hackett, the well-respected former board chairman and CEO of Anadarko Petroleum Corp. (NYSE: APC), has been with Alta Mesa since the merger of blank-check vehicle Silver Run II with the company and Kingfisher Midstream LLC closed February 2018.

In addition, the leadership team of private equity portfolio advisory firm Meridian Energy LLC has agreed to fill out the remaining C-Suite positions. As a result, Randy L. Limbacher will serve as Alta Mesa’s president, John H. Campbell will serve as the COO of the company’s upstream business and Mark P. Castiglione will serve as the chief of staff to the president on a consulting basis.

Limbacher, Campbell and Castiglione have also worked with Riverstone Holdings LLC since June 2017. Riverstone Holdings, where Hackett serves as a senior adviser, previously backed Silver Run II before its transaction with Alta Mesa and Kingfisher Midstream.

Hackett commented: “We welcome Randy, John and Mark to Alta Mesa Resources, and I look forward to working with them in this transitional period for the company. This senior executive team has deep experience in public energy company leadership and understands the expectations demanded of those companies. All three are trained as petroleum engineers and have significant financial experience.”

Alta Mesa said Michael A. McCabe will remain vice president and CFO as a search continues for his replacement. The new leadership team will share details of the 2019 operating and financial plan early next year.

Chappelle and Ellis also resigned as members of the company’s board of directors, effective immediately.

Chappelle, who has over 30 years of experience in the oil and gas industry, served as Alta Mesa’s CEO since 2004. Ellis founded Alta Mesa in 1987 after beginning his career with Amoco. He built Alta Mesa’s asset base by starting with small earn-in exploitation projects, then progressively grew Alta Mesa with successive acquisitions of fields from major oil companies and consistent success in exploration and development drilling, according to the company website.

Alta Mesa holds about 139,000 net surface acres in the Stack Play, as of Nov. 1. The company’s net production during the third quarter was about 33,400 barrels of oil equivalent per day.