Controls on the Variation of Crude Oil Quality, Santa Maria Basin, California

LILLIS, P. G., and J. D. KING, U.S. Geological Survey, Denver, CO

Previous studies have documented that early generation of oil from the Miocene Monterey Formation produces low-gravity (6-20 degree API), high-sulfur (4-8 wt.%) crude, although some fields in the Santa Maria basin, California, contain higher gravity (20-39 degree API), lower sulfur (0.5-4 wt.%) oil. Geohistory analysis and geochemical data suggest that the variation in gravity appears to be controlled primarily by the timing of oil generation and migration and the age of the traps. Biodegradation and other secondary processes that alter the oil gravity occasionally are significant.

Early generation began in the late Miocene, with low-gravity oil migrating into existing traps. These older traps were mostly stratigraphic (Santa Maria Valley field), although normal faults and minor folds may also have been effective traps. Anticlines of Pliocene and Quaternary age (Orcutt field) trapped higher gravity oil that was generated and migrated at a later and more mature stage. This younger oil is higher quality because (1) at higher levels of thermal maturity the source kerogen contains reduced amounts of sulfur due to earlier pulses of generation and expulsion; and (2) some of the Monterey Formation kerogen in the basin is lower in sulfur initially and requires higher levels of thermal maturity to generate oil.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)