LILLIS, PAUL G., U.S. Geological Survey, Denver, CO; and RICHARD G. STANLEY, U.S. Geological Survey, Menlo Park, CA

Abstract: Petroleum Systems of the La Honda Basin, California

The petroliferous Tertiary La Honda basin is located in the Santa Cruz Mountains west of the San Andreas fault and includes several small oil fields (La Honda, South La Honda, Oil Creek, Half Moon Bay, and Moody Gulch) as well as oil seeps and outcrops of oil-saturated sandstone. Potential source rocks identified on the basis of Rock-Eval pyrolysis include the upper Miocene Santa Cruz Mudstone, the lower and middle Miocene Monterey Formation, the lower Miocene portion of the Lambert Shale, and the Eocene Twobar Shale Member of the San Lorenzo Formation. Oils, immature source-rock extracts, and mature source-rock products of hydrous pyrolysis were analyzed for bulk and molecular organic chemical composition. Stable carbon isotope and biomarker data indicate that all oils analyzed except for Oil Creek are derived from Miocene source rocks, whereas the oil from Oil Creek is most likely derived from the Eocene Twobar Shale. Furthermore, the biomarker data indicate that oils from the Half Moon Bay and La Honda fields represent two different Miocene oil types, respectively, and the oil from middle to upper Miocene oil-saturated sandstone at Majors Creek may be a third Miocene oil type. Miocene source rock correlation is less clear but preliminary results suggest that some of the Half Moon Bay oils are derived from the Lambert Shale, La Honda oils are derived from the Monterey Formation, and the Majors Creek oils are derived from the Santa Cruz Mudstone. Samples from an oil seep along Tarwater Creek and from oilstained sandstone at Point Montara are derived from Miocene source rocks based on the isotope data but are too biodegraded to allow interpretation of the biomarker data.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California