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Monterey Source Rock Facies and Petroleum Formation - A Synthesis of Results of the Cooperative Monterey Organic Geochemistry Study

Jurgen Rullkotter and Previous HitCarolineTop M. Isaacs

Based on the analysis of two carefully selected coastal outcrop series of Monterey rocks and a suite of eleven crude oils from the onshore and offshore Santa Maria and Santa Barbara-Ventura basins, an interlaboratory effort was undertaken (i) to characterize organic-matter-rich sections of the source rock geochemically in terms of changes in depositional environment and (ii) to assess the effect of these variations on crude oil composition. Despite intra- and interbasinal variations in organic carbon enrichment and hydrogen richness, the organic matter appears to be largely derived from a common planktonic source. Differences in organic matter quality are primarily attributed to different extents of early diagenetic alteration, most probably by sulfate-reducing bacteria i the pore water, while differences organic matter enrichment are suggested to essentially indicate varying levels of dilution and dissolution of biogenic mineral matter. Prolific source rock horizons with hydrogen-rich organic matter were found at the Lions Head section, whereas the Naples Beach section in general represents more highly microbially altered organic matter with lower hydrogen contents according to Rock-Eval pyrolysis. Although petroleum generation in more cases occurred at low levels of thermal stress, distinct molecular indicators suggest a significant range of maturity values among the oils investigated. Maturation behavior is clearly related to sulfur content. Maturity evolution South Elwood oils differing from those of other oils may be organofacies related but possibl also related to local heating rate anomalies.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California