STANLEY, RICHARD G., and ZENON C. VALIN, U.S. Geological Survey, Menlo Park, CA, and MARK J. PAWLEWICZ, U.S. Geological Survey, Denver, CO

ABSTRACT: Rock-Eval Pyrolysis and Vitrinite Reflectance Results from Lower Miocene Strata in the Onshore Santa Maria Basin and Santa Barbara Coastal Area, California

Much petroleum in California coastal basins was derived from organic-rich source rocks within the Miocene Monterey Formation.

However, strata beneath the Monterey also may have generated hydrocarbons. We examined the petroleum source potential of two pre-Monterey, lower Miocene units (the Lospe Formation of the onshore Santa Maria basin and the Rincon Shale of the Santa Barbara area) using Rock-Eval pyrolysis and vitrinite reflectance.

The Lospe Formation is as much as 830 m thick and consists of sedimentary and minor volcanic rocks that record initial tectonic subsidence of the Santa Maria basin. Rock-Eval results from 14 outcrop samples collected near Point Sal show that lacustrine and shallow-marine mudstones of the Lospe are low in total organic carbon (range 0.18-0.68 wt.%, mean 0.35 wt.%), suggesting poor hydrocarbon source potential. Vitrinite reflectance values (range 0.68-1.56% Ro, mean 1.29% Ro) from the Lospe and overlying Miocene Point Sal Formation are unusually high, and may have resulted from heating associated with local hydrothermal activity and emplacement of a gabbro sill.

The Rincon Shale of the Santa Barbara area is as much as 760 m thick and consists mainly of mudstone, shale, and dolomite that record bathyal marine deposition. Rock-Eval results from 51 samples collected in fresh excavations at the Tajiguas landfill (40 km west of Santa Barbara) show high values of total organic carbon (range 0.21-5.71 wt.%, mean 2.66 wt.%), suggesting good hydrocarbon source potential. A modified van Krevelen diagram shows that the kerogens are oil-and gas-prone types II and III. Vitrinite is sparse, but values of Rock-Eval Tmax are less than 432 degrees C, indicating that the samples are thermally immature. Organic-rich strata within the Rincon Shale are likely sources of petroleum in areas of the Santa Barbara Channel and onshore Ventura basin, where the Rincon ha been buried as deep as the oil window.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.