Depositional Systems and Oil and Gas Plays in the Cretaceous Olmos Formation, South Texas

The Upper Cretaceous Olmos Formation in South Texas continues to be an active exploration target 60 years after oil was first discovered in this clastic assemblage. The shallow, oil-bearing formation was deposited on a broad, wave-influenced shelf. Sand accumulated in two depocenters. Initial deposition took place in a western depocenter during an early phase of wave-dominated deltaic sedimentation. Deposition of the strike-elongate delta preceded two cycles of high-constructive deltaic deposition, during which time sediments prograded seaward over the Lower Cretaceous shelf edge. These three deltaic complexes together compose the Catarina delta system. The focus of sedimentation then shifted eastward to the Big Foot delta system, where again wave-dominated deltaic sedimentation was followed by two episodes of high-constructive but wave-modified delta formation. Sands not retained in the Big Foot delta system migrated alongshore to the west, where they formed a thick retrogradational coastal/interdeltaic complex, named the Rocky Creek barrier/strandplain system. Regional uplift with concomitant erosion removed much of the updip facies tracts of the Olmos. This truncated section was then unconformably covered by Escondido shelf mudstones, thereby creating conditions favorable for stratigraphic entrapment of hydrocarbons migrating updip from the deeper basin through permeable deltaic sandstones.The resulting stratigraphic trap play is the most prolific of the seven oil and gas plays in the Olmos Formation. Six other plays produce oil and gas from a variety of structural traps, but most of the remaining oil production is from the Charlotte Fault Zone. Youthful shelf-edge gas, condensate, and oil plays are highly productive, and together with wildcat prospects further basinward offer the best potential for continued high-level production from this mature province.