Outcrop-aided characterization of a faulted hydrocarbon reservoir: Arroyo Grande Oil Field, California, USA

Abstract

The Arroyo Grande Oil Field in Central California has been in production since 1905 from the Miocene-Pliocene Edna member of the Pismo Formation. The Edna member is a massive, poorly consolidated sandstone with an average porosity of 0.2 and a permeability of 1000-5000 md. The producing levels are shallow, 100 to 500 m from the earth's surface. We mapped the major structures of the oil field in outcrops and determined the distribution and orientation of small faults (deformation bands) both in cores and outcrops. We established the relationship between deformation band density and major faults by detailed mapping, and we used image analysis to determine the petrophysical properties of the sandstone in outcrop and in cores. Outcrop, aerial photo, and core data analysis provided insight into the fault distribution and the petrophysical properties of the reservoir rocks, showing a good correspondence between subsurface and outcrop data. The permeability of faults is on average three orders of magnitude lower than that of the host rock and capillary pressure is 1-2 orders of magnitude larger in faults than in the host rock. Faults with tens of meters offsets are associated with an high density of deformation bands (10 to 250 m-') and with zones of cement precipitation that extend up to 30 m from the fault. This implies that cement precipitation together with cataclasis could form permeability barriers or baffles. We used synthetic logs based on core data and neutron porosity detector characteristics to simulate the log signature of deformation bands in sandstone and we show that the detection of these small faults is very difficult with the conventional logging methods.