Structural Wedge Model and the Antelope Uplift, West-Side of the San Joaquin Basin, California: The Possibility of Additional Large Hydrocarbon Traps

Abstract

The west-side of the San Joaquin basin, California, is a geologically complex area whose potential for additional discoveries is considered high due to rich source rock units and structural and stratigraphic complexity. Recent exploration interest has focused on “unconventional” shale plays, but conventional plays and prospects, those involving migrated hydrocarbons, continue to play an important role. New conventional plays require some unconventional thinking too, as the basin has been intensely explored for over 130 years. One such 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 (1988) and Namson, et al. (1990) to explain the characteristics of the 1983 Coalinga earthquake (M_w=6.5), local structural geometry and kinematics, and the development of the Coalinga anticline. A similar wedge model was subsequently employed by Guzofski, et al. (2007) to explain the earthquake and anticline. The wedge model is applied, further south, to the Antelope uplift, an extensive area of older rocks at shallow depths, located between the northern Temblor Range and the North Belridge oil field. There the wedge creates a triangle-zone geometry with subthrust anticlines involving Tertiary-age strata with hydrocarbon potential, as opposed to the wedge at Coalinga that involves mostly Cretaceous and Jurassic age rocks with lesser hydrocarbon potential. The subthrust anticlines are bounded on the west by the westward-dipping Temblor Range thrust and on the east by the eastward-dipping Shale Hills thrust. The Antelope uplift is a broad hanging-wall anticlinorium above the Shale Hills thrust (roof thrust). The opposing thrust sheets conceal anticlinal traps 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 surface geology. Structural relief and strike seismic lines suggest that the subthrust anticlines should involve known reservoirs and source rocks of late Cretaceous through Miocene age, and the prospectivity is further enhanced by a prolific petroleum system proven by numerous moderate to large oil fields. Further south, the structure of the central and southern Temblor Range suggests the wedge model could assist exploration for additional large subthrust structures.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018