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Department of Geophysics, Stanford University, Stanford, CA 94305, USA
1 BP-Amoco Exploration, 501 Westlake Park Boulevard, Houston, TX 77479, USA
2 GeoMechanics International, Inc., 250 Cambridge Avenue, Palo Alto, CA 94306, USA
Recent field studies show that critically stressed faults, that is, faults that are close to frictional failure in the current stress field, serve as conduits for fluid flow. Similarly, geological field studies of the low permeability siliceous shales in the Monterey Formation, California, clearly indicate that faults influence hydrocarbon transport. We report here a study of the Antelope Shale, a low permeability siliceous shale hydrocarbon reservoir in the Buena Vista Hills field in the southern San Joaquin Valley to determine the influence of the stress state on the relative hydraulic conductivity of the fractures and faults present in the subsurface. Because production has both lowered reservoir pressure and the horizontal stresses, it was necessary to "restore" the reservoir stress state to initial conditions in order to identify correctly the most highly productive intervals. This analysis demonstrates that prior to production, faults in the reservoir were active in a transitional reverse/strike-slip faulting stress state, consistent with regional tectonics. Initial production rates in the field were 2000 barrels of oil per day, principally from intervals where critically stressed faults were encountered.
