Abstract: Eustatic Control of Tertiary Hydrocarbon Deposits, Central California Borderland Basins

Harold L. Cousminer

In the central California borderland basins, the Vail-Haq Cenozoic Global Eustatic Cycle appears to have influenced depositional patterns that have fundamental significance in the present distribution of hydrocarbon source and reservoir beds. Coupled with tectonic events, traps were created that now control the distribution of hydrocarbon accumulations.

Seismic data combined with subsurface lithostratigraphic, biostratigraphic, and log data from wells drilled on the central California outer continental shelf (OCS) were used to date and correlate Tertiary stratigraphic sequences in the Santa Maria, Bodega-La Honda, and Point Arena basins. Benthic foraminiferal assemblages also served to reconstruct each basin's paleobathymetric history. The west coast benthic foram stages, commercially used to solve subsurface stratigraphic problems for over 60 yr, occur with planktonic microfossil groups that now serve to calibrate the provincial stages with the absolute geologic time scale.

The Miocene through Pliocene stratigraphic sequences in these three depocenters are markedly similar, and record a parallel marine onlap and offlap pattern that correlates closely with the Vail-Haq Cenozoic Global Eustatic Cycle. The highstand depositional cycles are generally dominated by organic-rich sediments of good to excellent source-bed potential. Lowstand regressive to transgressive clastic deposits have good reservoir potential.

The middle Miocene siliceous Monterey Formation was deposited during maximum Tertiary global sea levels and is present in all of these basins. In addition to being a prolific source bed, the Monterey is unique in that when diagenetically altered, it fractures and also becomes an excellent hydrocarbon reservoir.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994