Reservoir and Source-Bed History of Great Valley, California

Abstract

The application of geochemical concepts and relations of porosity-permeability-depth helps focus exploratory efforts on the favorable parts of trends in partially explored basins. Porosity data from 165 producing ranging in age from Late Cretaceous to Pleistocene show that the best reservoirs lose porosity at a rate of approximately 1.52% per 1,000 ft (0.46% per 100 m) of burial. Reservoirs on the large-amplitude folds on the west side of the San Joaquin Valley have a more rapid porosity loss with depth. A crossplot of porosity-permeability indicates a best reservoir relation of a tenfold decrease in permeability for each decrease of seven porosity units. Within the Great Valley, four major depocenters are definable by use of isopach data. Each has had a different source-bed history. Continental margin sedimentary rocks of Late Cretaceous age contain organic material that generally is structured and is believed to be the source of gas in the Sacramento Valley. Although a Tertiary depocenter exists in the Delta area, subsidence has failed to place Paleocene and Eocene source beds into the thermal zone thought to be required for oil and gas generation. Gas trapped in Paleocene and Eocene reservoirs, therefore, must have migrated from more deeply buried Cretaceous source beds. Tertiary beds in the Buttonwillow and Tejon depocenters in the southern San Joaquin Valley contain large amounts of sapropelic organic material which is believed to be the source of the oil and gas found there. Source beds in the Buttonwillow depocenter have been in the thermal zone for generation for only about 5 m.y. In marked contrast, source beds in the Tejon depocenter started subsiding into the thermal zone more than 15 m.y. ago. Explorationists who recognize the best reservoirs and relate them to source, migration, and trap parameters in undrilled areas will be successful in finding future reserves of oil and gas and may avoid some unprofitable geologic successes that are economic failures.