Abstract: A Second Petroleum System(?) in the Cuyama Basin, California

Paul G. Lillis

The Cuyama basin formed by extension during the early Miocene and was filled with a variety of nonmarine and marine sediments. Previous studies have documented the Soda Lake-Painted Rock (!) petroleum system. The lower Miocene Soda Lake Shale Member of the Valqueros Formation is the source rock for the high quality oil (avg. 33° API; 0.4% S) produced from the lower Miocene Painted Rock Sandstone Member of the Vaqueros Formation. Oil generation began in the middle to late Miocene, and most of the oil migrated into the existing traps before 3 Ma.

Some oil and gas leaked out of the lower Miocene reservoirs through the Russell fault into sands of the upper Miocene Santa Margarita and middle Miocene Branch Canyon Formations. Oil trapped in the upper Miocene sands has undergone mild to moderate biodegration that has lowered the oil gravity (13-26° API). The saturated hydrocarbon composition of oil from the Santa Margarita sands indicates that a second pulse of light oil has mixed with the biodegraded oil. Carbon isotope data suggest that the Soda Lake Shale is not the source of the light oil; thus, a second petroleum system may be present in the basin. The generation and migration of this oil clearly occurred after the migration and biodegradation of the Soda Lake oil and may have been a fairly recent event. The light oil (46 #176; API) produced from the Branch Canyon Formation in the Central Cuyama field may also be part of this second petroleum system. An important exploration implication of this petroleum system is that other upper Miocene sands may be charged with light oil and younger traps should be considered viable prospects.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994