--> Abstract: Origin of Petroleum, Neogene Accretionary Margin, Cape Mendocino-Eel River Basin, Northwestern California, by R. J. McLaughlin, R. G. Stanley, P. G. Lillis, L. B. Magoon, Z. C. Valin, M. B. Underwood, K. R. Aalto, and P. C. van de Kamp; #90920 (1999).

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McLAUGHLIN, R. J., R. G. STANLEY, P. G. LILLIS, L. B. MAGOON, Z. C. VALIN, U.S. Geological Survey, Menlo Park, CA; M. B. UNDERWOOD, University of Missouri, Columbia, MO; K. R. AALTO, Humboldt State University, Arcata, CA; and P. C. VAN de KAMP, Scio, OR

Abstract: Origin of Petroleum, Neogene Accretionary Margin, Cape Mendocino-Eel River Basin, Northwestern California

Oil and gas reservoirs and seeps in the Cape MendocinoEel River basin region are associated with Neogene melange and forearc deposits in an onshore extension of the accretionary margin. This seismically active region experiences high uplift rates, moderately high heat flow, and high fluid pressures.

Oil from seeps and wells in the Cape Mendocino-Petrolia oil field area, and condensate from the Tompkins Hill gas field, have stable carbon isotopic compositions similar to petroleum derived from Miocene source rocks elsewhere in California. Exposed potential source rocks of Miocene or younger age in the area include (1) melange and folded strata of the King Range and False Cape terranes of the Franciscan Complex; and (2) Miocene and younger forearc and marginal basin strata (including the Wildcat Group and Bear River beds) structurally interleaved with the Franciscan Complex.

Radiometric (K-Ar) and apatite fission track dating and vitrinite reflectance (Ro>or =1.0), indicate that the King Range terrane lost any petroleum generation potential during heating in the middle Miocene. Low total organic carbon (TOC) values and hydrogen indices (HI) from the False Cape terrane indicate that these rocks also are not viable sources of petroleum. Fair to good petroleum generative potential is indicated, however, for thermally immature Miocene shale and mudstone in the upper Bear River and near Briceland, with TOC values of 1.1-1.8 wt %, HI >200, and Tmax values of about 420<deg>C. These data and regional structure suggest petroleum could have been generated from similar forearc source rocks interleaved with the Franciscan Complex, that reached thermal maturity during thrust burial to several kilometers.

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