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EP Energy and XCL jointly accounted for about a third of the 92,000 barrels per day Uinta oil production in October, according to data provider Enverus.

The regulator is concerned that EnCap - which owns XCL Resources, another Utah oil producer - would become too dominant a player in Utah's Uinta Basin, the sources added.

EP Energy and XCL did not respond to requests for comment.

XCL and all five refiners are senior members, with EP Energy also in the group, according to UPA's website.

Paramount moved into the Nevada gold district in June with the acquisition of Canada's X-Cal Resources XCL.TO in an all-stock deal worth C$32 million.