MAGOON, L. B., T. D. LORENSON, R. J. McLAUGHLIN, R. G. STANLEY, Z. C. VALIN, U.S. Geological Survey, Menlo Park, CA; P. G. LILLIS, U. S. Geological Survey, Denver, CO; S. F. HODGSON, Division of Oil, Gas, and Geothermal Resources, Sacramento, CA; M. L. DAVISSON, Lawrence Livermore National Laboratory, Livermore, CA; and M. B. UNDERWOOD, University of Missouri, Columbia, MO
Abstract: The Geology and Geochemistry of Oil and Gas Seeps in California Indicate up to Tens of Kilometers of Petroleum Migration
Over 500 oil and gas seeps have been mapped in California along the coast and inland. Numerous seeps occur in the Cape Mendocino-Eel River basin areas, Point Arena, Point Reyes, San Mateo-Santa Cruz-Santa Clara counties, western San Joaquin Valley, the coastal and mountainous areas of Santa BarbaraVentura counties, and in the Los Angeles basin (the Rancho La Brea Tar Pit being a famous example). Many natural gas seeps occur with active springs throughout the Coast Ranges. These oil and gas seeps generally occur in the western and southern portion of California where tectonic activity has uplifted petroleum carrier beds and reservoir rocks which are then breached by erosion. On the basis of available thermal maturity data, these oil and gas seeps generally occur in thermally immature host rock.
Using stable carbon isotopes, three oil-types have been geochemically identified that correlate with source rocks of three ages: Cretaceous, Eocene, and Miocene. Similar geochemical analyses on subsurface oil samples from wildcat wells and producing fields in the same area of the seeps provide comparable results. Furthermore, the hydrocarbon gases that emanate at the surface contain not only methane, but ethane and higher homologs indicating a thermogenic origin. Generation of oil and thermogenic gas requires a minimum temperature that has not been attained at the seep sites. Therefore, we conclude that the seep fluids have migrated up to tens of kilometers to their present sites from thermally mature source rocks with correlative geochemical signatures.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California