The FTC filed a complaint alleging that Scott Sheffield attempted to collude with representatives of OPEC to reduce oil and gas output to increase prices at the pump and inflate Pioneer's profits. "The FTC has a responsibility to refer potentially criminal behavior and takes that obligation very seriously," spokesman Doug Farrar told CNBC. In response, Exxon agreed to keep Sheffield off its board, the oil major said in a statement Thursday. The FTC alleged that Sheffield repeatedly held private conversations with high-ranking OPEC representatives to assure them that Pioneer and its competitors in the Permian Basin were working to keep oil output artificially low. "Notwithstanding, Pioneer and Mr. Sheffield are not taking any steps to prevent the merger from closing," the company said in the statement.

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Persons: Scott Sheffield, Doug Farrar, Exxon, Sheffield, Sheffield's, Lina Khan, — CNBC's Pippa Stevens, Mary Catherine Wellons, Lina Khan's Organizations: Federal Trade Commission, Natural Resources, OPEC, Exxon Mobil, Pioneer, FTC, Justice Department, Wall Street, CNBC, Exxon, Sheffield