Section II Reservoir Description
Yowlumne is a giant oil field in the San Joaquin Basin, California, that has produced over 16.7 million m3 (105 million bbl) of oil from the Stevens Sandstone, a clastic facies of the Miocene Monterey Shale. Most Yowlumne production is from the Yowlumne Sandstone, a layered, fan-shaped, prograding Stevens turbidite complex deposited in a slope-basin setting. Well log, seismic, and pressure data indicate seven depositional lobes with left-stepping and basinward-stepping geometries.
Log-derived petrophysical data, constrained by core analyses, indicate trends in reservoir quality. Concentration of channel and lobe facies along the axis and western (left) margin of the Yowlumne fan results in average net/gross sandstone ratios of 80%, porosity (cj>) of 16%, and liquid permeability (KUquid) °f 10-20 md. By contrast, more abundant levee and distal margin facies along the eastern margin result in shale-bounded reservoir layers with higher clay contents and lower net/gross sandstone ratio (65%), porosity (12%), and permeability (2 md). Although a waterflood will enable recovery of 45% of original oil in place along the fan axis, reservoir simulation indicates 480,000 m3 (3 million bbl) of oil trapped at the thinning fan margins will be abandoned with the current well distribution. Economic recovery of this bypassed oil will require high-angle wells with multiple hydraulic fracture stimulations to provide connectivity between the reservoir layers.