STANLEY, RICHARD G., U.S. Geological Survey, Menlo Park, CA; and PAUL G. LILLIS, U.S. Geological Survey, Denver, CO
Abstract: New Organic Geochemical Results From Surface Occurrences of Oil and Potential Source Rocks of Oil Near Point Reyes, Marin County, California
Natural surface occurrences of petroleum are common along the seacliffs in certain areas of the Point Reyes National Seashore. Much of the petroleum is in oil-saturated sandstone dikes and sills that intrude siliceous fine-grained strata of the Santa Cruz Mudstone (upper Miocene) and Monterey Formation (middle and upper Miocene). Five samples of oil from Wildcat Beach and four samples of oil from Palomarin Beach show stable carbon isotopic compositions (d13CPDB) of -23.17 ‰ to -22.46 ‰ for the saturated hydrocarbon fractions and 22.52 ‰ to -22.20 ‰ for the aromatic fractions. These results, when compared with isotopic data from other California oils, indicate that the oils in our samples were derived from source rocks of Miocene age.
The specific source horizons of the Point Reyes oils have not been identified, but results from Rock-Eval pyrolysis suggest that candidates include organic-rich strata within the Santa Cruz Mudstone and Monterey Formation. Interpretation of our RockEval data is complicated by weathering effects and by evidence of migrated bitumen in some samples. Outcrop samples of rocks from the Santa Cruz Mudstone (10 from Palomarin Beach and 7 from Wildcat Beach) show total organic carbon (TOC) values of 1.51-3.75 weight percent and hydrogen index (HI) values of 149-481, suggesting good oil source potential. Samples from the Monterey Formation (2 from Wildcat Beach) show TOC values of 1.22-5.51 weight percent and HI values of 477-575, suggesting good to excellent oil source potential. Values of Rock-Eval Tmax from our samples of the Santa Cruz Mudstone and Monterey Formation are less than 420 degrees Celsius, indicating that the samples are thermally immature. However, organic-rich strata of the Santa Cruz Mudstone and Monterey Formation may be source rocks of petroleum in nearby areas where these units are more deeply buried and are thermally mature.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California