Correlation of Source Rocks and Oils in the Sespe Oil Field, Ventura County, California
LILLIS, PAUL G., U.S. Geological Survey, Lakewood, CO, and M. S. CLARK, Colorado School of Mines, Golden, CO
The Sespe oil field in the central Ventura basin produces low sulfur (< 1%), intermediate gravity (23-32 degree API) oil from Middle Eocene to Lower Miocene reservoirs in the upper plate of the San Cayetano thrust. Previous studies proposed the Eocene Cozy Dell, Matilija, and Juncal formations in the San Cayetano overthrust as the source rocks. Alternatively, the oils could be derived from the Miocene Monterey Formation in the subthrust.
Gas chromatography and gas chromatography/mass spectrometry data are used to characterize and correlate the Sespe oils from comparison with bitumen extracts of the possible source rocks. The oils have pristane/phytane ratios of 1.3-1.6, bisnorhopane/hopane ratios of about 0.2, and relatively low amounts of diasteranes, oleanane, and C29 steranes. These data indicate that all of the oils belong to the same family and that variations in the API gravity are due to the degree of biodegradation. In addition, the sterane and triterpane distributions imply that the source organic matter is derived from marine phytoplankton and bacteria with a minor contribution from land plants.
The Sespe oils do not correlate with the upper plate Eocene source rocks but correlate fairly well with the Monterey Formation source rocks. Thus, a subthrust source is proposed. However, the Sespe oils have higher gravity and lower sulfur content than typical Monterey oils. The low sulfur content may result from the higher iron content, due to terrigenous input, of the Sespe field source rocks relative to other Monterey source rocks. Alternatively, the Sespe oils were expelled from the Monterey Formation at relatively high levels of thermal maturity.
AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)