Abstract: Do Little Faults Matter? Seismic Imaging of Distributed Faults in the Cymric Foldbelt, San Joaquin Valley, California

MYERS, GARY, DONALD MILLER, and ED VEITH, Aera Energy LLC, Bakersfield, CA

Folds in transpressive structural settings provide important trapping mechanisms for oil accumulations. Faults within folded sedimentary strata provide important reservoir barriers that control fluid flow that is critical to successful oil production. This study uses high-resolution (10 ft x 15 ft. trace spacing) 3D seismic data to document complex patterns of small-scale faults within the foldbelt at Cymric Field, adjacent to the San Andreas fault system in the San Joaquin Valley of California.

The seismic data indicate that at least three predominant fault patterns cut the Pliocene-Pleistocene Tulare reservoirs. (1) Steeply dipping fold-parallel fault sets are characterized by both normal and reverse displacement in closely spaced sets of discontinuous, en echelon faults. These faults generally converge at depth. (2) Fold-oblique cross faults are characterized by variable amounts of vertical and horizontal displacement. These are typically steeply dipping and occur as groups of en echelon fault sets which cross folds at a high angle. (3) Low-angle reverse faults are present but are less prevalent then the other fault patterns, have limited lateral extent, and represent small scale compensation features.

These observed fault geometries compared with field production illustrate the difficult task of determining which faults, within a highly faulted fold, are important reservoir seals and which are not. Clusters of discontinuous faults can create very tortuous paths that baffle fluid flow but may not totally isolate portions of the reservoir with respect to fluid pressure. High-resolution data, like those used in this study, allows one to determine the likelihood of cross flow and pressure communication between fault blocks that is important to reservoir management.

The patterns of numerous faults provide evidence of distributed shear strain during folding of the Cymric anticlines. No single fault is recognized as fundamental to fold formation in the shallow portion of these folds. This study suggests that folds in this structural setting form by complex fault patterns. The degree of structural complexity increases with higher resolution data as evidenced through direct comparison of regional 2D seismic data versus high resolution 3D seismic used in this study.

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