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Abstract: Field Aided Characterization of a Sandstone Reservoir: Arroyo Grande Oil Field, California, USA

Marco Antonellini, Atilla Aydin

The Arroyo Grande Oil Field in Central California has been productive since 1905 from the miopliocene Edna member of the Pismo formation. The Edna member is a massive poorly consolidated sandstone unit 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 ground surface.

Excellent surface exposures of the same formation along road cuts across the field and above the reservoir provide an opportunity to study reservoir rocks at the surface and to relate fracture and permeability distribution obtained from cores to folds and faults observed in outcrops.

We mapped in outcrops the major structures of the oil field and determine the statistical distribution and orientation of small faults (deformation bands) that have been observed both in cores and outcrop. The relation between deformation bands and major structures has also been characterized with detailed mapping. By using synthetic logs it is possible to determine the log signature of structural heterogeneities such as deformation bands in sandstone; these faults cause a neutron porosity drop respect to the host rock in the order of 1-4%. Image analysis has been used to determine the petrophysical properties of the sandstone in outcrop and in cores; permeability is three orders of magnitude lower in faults than in the host rock and capillary pressure is 1-2 orders of magnitude large 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-1) and with zones of cement precipitation up to 30 rn from the fault By combining well and field data, we propose a structural model for the oil field in which high angle reverse faults with localized deformation bands control the distribution of the hydrocarbons on the limb of a syncline, thereby explaining the seemingly unexpected direction of slope of the top surface of the reservoir which was inferred by well data only.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France