A Thermally Subsiding Basin Model for the Maturation of Hydrocarbons in the Pismo Basin, California

Abstract Previously developed models which describe the formation of a sedimentary basin due to cooling lithospheric material (D. L. Turcotte’s method) and the generation of hydrocarbons as a function of time and temperature (D. W. Waples’ method) have been applied to the Pismo Basin in San Luis Obispo County, California. This basin was studied because of its excellent hydrocarbon potential due to organic rich sediments. The Miocene Monterey Formation contains as much as 5 weight percent organic carbon and is as much as 1500 meters thick in the Pismo Basin. A thermally subsiding basin model was applied to the basin as suggested by the complex tectonic history of the region in order to determine the maturity of the Monterey Formation. Thermal parameters calculated from the subsiding basin model are in agreement with reported values for the area. Calculated parameters include a heat flow value of 105 milliwatts per square meter, a thermal gradient of 50°C per kilometer, and a basin depth of 2.9 kilometers. Applying Waples’ method for the maturation of hydrocarbons to the modeled thermal regime results in the following conclusions:1.In the central portion of the Pismo Basin the Monterey Formation has entered the liquid oil window at 1.6 kilometers below the present ground surface.2.At 2.0 kilometers below the surface the Monterey Formation has entered the zone of intense oil generation.3.At 2.2 kilometers below the surface (340 meters above the volcaniclastic Obispo Formation) oil generation in the Monterey Formation has ceased due to high temperatures.