Characterization of an Active-margin, Fan-shaped Turbidite Reservoir Complex, Miocene Stevens Sandstone, San Joaquin Basin, California

Clark, M.S., Chevron USA, P.O. Box 1392, Bakersfield, CA 93302

Abstract

Yowlumne is a giant oil field in the San Joaquin basin, CA, that has produced over 100 MMBO from a low-permeability (10-100 md), fan-shaped turbidite complex having left- and basinward-stepping geometries deposited in an active-margin basin. The Yowlumne and other "deep water", upper Miocene sandstones make up a clastic facies of the Monterey Shale called the "Stevens," which has contributed more than 15% of 12 BBO produced from the basin.

The Yowlumne reservoir is a prograding turbidite complex of seven lobes deposited on the basin margin during Miocene orogeny. Although the fan is lens-shaped, it does not significantly incise underlying strata. Apparently, deposition has resulted from con finement of prograding lobes, about 2 km wide by 4 km long, between a faulted paleohigh on the west (left) side of the fan, and another high, associated with overbank deposition and possibly differential compaction, on the east. Basinward-stepping compartments in the reservoir represent deposition during decreasing accommodation, and high sediment flux, whereas left-stepping compartments reflect the influence of Coriolis forces.

More abundant shale-bearing levee facies characterize the east (right) margin of the fan, whereas sand-rich lobe facies characterize the west. Therefore, reservoir quality decreases from the fan axis eastward towards the fan margin. Cost-effective exploitation of bypassed oil trapped against the thinning margin is facilitated by 3D computer modeling to locate effectively highly deviated to horizontal wells, and design completions that maximize productivity from the layered, low-permeability turbidite reservoir that characterizes the distal fan margin.