Subsurface Characterization of the Potrero-Ryer Island Thrust System, Western Sacramento-San Joaquin Delta, Northern California

UNRUH, JEFFERY; SCOTT HECTOR, Gary Drilling Co., Davis, CA

Drill hole and seismic reflection data have been used to interpret the geological structure in the area between the Concord-Green Valley fault system and the Pittsburg-Kirby Hills fault zone. This low-relief area just east of San Francisco Bay is characterized by east-west to west northwest-east northeast trending anticlines, most of which contain natural gas fields. Fields include Los Medanos, Ryer Island, Honker, Van Sickle, Suisun Bay, and Kirby Hills. Subsurface structural contour maps, primarily on the top of the Eocene Domengine sandstone, a common gas reservoir, were constructed over each field.

The study concludes that several of the Western Delta region anticlines are underlain by blind thrust faults. They formed as fault-bend or fault-propagation folds. South of the Sacramento River the folds are asymmetric and the underlying thrusts dip northeast. North of the Sacramento River the folds are underlain by southwest dipping thrust faults. The anticlines are part of a contractional belt that extends southward to the Northern Diablo Range and Mt. Diablo.

Based on the orientation of the fold-and-thrust structures and their spatial association with strike-slip faults, we interpret that shortening in the Western Delta region primarily is driven by distributed northwest-directed dextral shear between the Concord-Green Valley fault system on the west and the Pittsburg-Kirby Hills fault zone on the east.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California