Abstract: Pliocene Facies Trends and Controls on Deposition of Lower Gusher Shallow Gas Reservoirs, North Coles Levee Field, San Joaquin Basin, California

Daniel C. Steward, Janice M. Gillespie

Net sand isochore maps of three Pliocene-age Lower Gusher sands in the Etchegoin Formation at North Coles Levee field, southern San Joaquin basin, California display geometries suggestive of deposition in delta front settings. The north-south depositional strike of these sands approximately parallels the orientation of the paleoshoreline. The sands thicken and display greater lateral continuity near distributary channel sands, which are oriented east-northeast approximately perpendicular to the shoreline.

A comparison of the isochore maps of each of the three sand bodies show that they are stacked vertically above each other, indicating that the position of the shoreline remained stationary during deposition of the Gusher interval. This apparent stillstand of the shoreline is superimposed on an overall regression of the sea from the San Joaquin basin during the Pliocene. Therefore, we believe that the Lower Gusher sands were deposited during a period of relatively rapid basin subsidence, which negated the effects of the marine regression and caused vertical aggradation of shoreline facies in the North Coles Levee area.

The Lower Gusher interval at North and South Coles Levee contains the most prolific shallow gas reservoirs on the Bakersfield Arch. A thorough knowledge of depositional trends in the Lower Gusher interval increases the probability of encountering reservoir-quality facies in exploration programs focusing on Pliocene gas.

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