Spellacy reservoir sandstones, Midway-Sunset Field

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 degrees 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.