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The AAPG/Datapages Combined Publications Database

Pacific Section of AAPG


Geology of the Midway-Sunset Oil Field and Adjacent Temblor Range, San Joaquin Basin, California, 2001
Pages 155-164
Miocene Sandstone Units

Spellacy Reservoir Sandstones, Midway-Sunset Field

David H. Sturm


The Midway-Sunset Oil Field lies along the tectonically active western margin of the southern San Joaquin basin, only 6-7 mi (9.6-11.2 km) east of the western edge of the North American plate. Upper Miocene Spellacy coarse clastics were derived from a granitic source located nearby to the southwest. Poorly sorted conglomerates and arkosic sands were transported through canyons cutting the upper slope and were deposited by sediment gravity flows in a deep water, intra-slope environment that was actively folding. The flows typically deposited normally graded and amalgamated beds that form hydrocarbon-bearing sandstone bodies with high porosities (23-34%) and high horizontal permeabilities (500-4,000 md). Spellacy reservoirs at Midway-Sunset field produce significant quantities of typically heavy (11-14° API) oil from shallow depths. After more than 90 years of production, many Spellacy reservoirs are still in the process of being developed by delineation and infill drilling. Well spacings of 5/8 acre (165 ft / 50 m apart) are common in spite of the high reservoir quality because of both high-viscosity oil and low reservoir pressures. Thermal stimulation has been used extensively since the mid-1960’s to increase production rates.

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