A private-equity firm is urging oil producer Resolute Energy Corp. to merge with a rival, the latest salvo in a growing campaign by some investors to force shale drillers to consolidate.

Kimmeridge Energy Management Co. told Resolute’s board of directors in a letter Friday that it was stepping up call for changes at the company, saying Resolute had failed to follow through on a strategic review to explore a merger or potential asset sale announced in May after investor pressure.

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