Abstract: Reservoir and Source-Bed History in Great Valley, California

D. L. Zieglar, J. H. Spotts

The application of geochemical concepts together with reservoir relations (porosity-permeability-depth) helps focus exploratory efforts on the favorable parts of geologic trends. Porosity data from 165 producing reservoirs ranging in age from Late Cretaceous to Pleistocene show that the "best reservoirs" lose porosity at a rate of about 1.5% per 1,000 ft (30 m) of burial. Reservoirs on the large-amplitude folds on the west side of the San Joaquin Valley have a more rapid loss with depth. A cross plot of porosity-permeability indicates a "best reservoir" relation of a 10-fold increase in permeability for each increase of 7 porosity units.

In the Great Valley four major depocenters are definable with isopach data, each with a different source-bed history. Upper Cretaceous continental-margin sedimentary rocks contain mostly structured organic material believed to be the source of gas in the Sacramento Valley. Although a Tertiary depocenter is present in the delta area, it did not subside enough to place Paleocene and Eocene source beds in the zone for thermal generation of oil and gas. Gas trapped in Paleocene and Eocene reservoirs probably migrated from more deeply buried Cretaceous source beds.

Tertiary beds in the Buttonwillow and Tejon depocenters in the San Joaquin Valley contain large amounts of sapropelic organic material believed to be the source of oil and gas in the San Joaquin Valley. Source beds in the Buttonwillow depocenter have been in the zone of thermal generation for only about the last 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 reduce the risks in exploratory ventures.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC