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Structural Wedge Model And The Antelope Uplift, West Side Of The San Joaquin Basin, California, And Hydrocarbon Potential


The west side of the San Joaquin basin, California, is a geologically complex area whose potential for new plays and discoveries is considered high due to rich source rocks, and structural and stratigraphic complexities. One such structural play is based on a crustal-scale, structural-wedge being driven eastward into the undeformed San Joaquin basin with shallow-level backthrusts that dip basinward. The wedge model was first proposed by Namson and Davis in 1988 to explain the characteristics of the 1983 Coalinga earthquake (Mw=6.5) and the development of the Coalinga anticline (Namson, J.S. and Davis, T.L., Seismically active fold and thrust belt in the San Joaquin Valley, California, Geological Society of America Bulletin, v. 100, p. 257-273). In the interpretation presented here, the wedge model is applied to the Antelope uplift, an extensive area of older rocks at shallow depths, located between the northern Temblor Range and North Belridge oil field. There the wedge creates a triangle-zone geometry with a subthrust anticline(s) involving Tertiary-age strata with hydrocarbon potential, as opposed to the wedge at Coalinga that involves mostly Cretaceous- and Jurassic-age rocks with much less potential. The subthrust anticline(s) is bounded on the west by the westward-dipping Temblor Range thrust and on the east by the eastward-dipping Shale Hills thrust, and the Antelope uplift is a broad hanging-wall anticline to the Shale Hills thrust (roof thrust). The opposing thrust sheets conceal the subthrust anticline(s) that could have up to 10,000 acres of closure. This interpretation, while not unique, and untested by drilling, is supported by reprocessed 2D seismic lines tied to well data, Bouguer gravity, and ties to the surface geology. The stratigraphy of the subthrust anticline(s) can be postulated by two methods: First, geologic units within the subthrust block have approximately the same structural relief as the undeformed units to the east (Buttonwillow depocenter); and second, the stratigraphy of the northern Cymric oil field can be mapped northward, using deep well data and strike seismic lines, beneath the lateral ramp forming the southern edge of the Antelope uplift, and into the subthrust block. These methods suggest that subthrust anticline(s) should involve known reservoirs and source rock of late Cretaceous through Miocene age, and the prospectivity is further enhanced by a local petroleum system indicated by several nearby oil fields and numerous oil and gas shows in wells. Hydrocarbon generation and migration are most likely from the Buttonwillow depocenter (lateral migration) and deep subthrust areas (vertical migration). The structure of the entire Temblor Range suggests the wedge model could be used in additional locations to explore for large subthrust structures.