--> Abstract: C<7> Chemistry and Origin of Monterey Oils and Source Rocks from the Santa Maria Basin, California, by A. S. Kornacki; #90987 (1993).

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KORNACKI, ALAN S., Shell Offshore, Inc., New Orleans, LA

ABSTRACT: C<7> Chemistry and Origin of Monterey Oils and Source Rocks from the Santa Maria Basin, California

Standard C<7> source parameters (e.g., the X<2> selectivity ratio) can be used to identify two classes of primary Monterey oil in the Santa Maria Basin. One type of oil, which is enriched in mono- and polybranched isoheptanes, comprises a single homologous set of very heavy crudes (<15 degrees API). The other class of oil, which is more enriched in cyclic C<7> compounds, is comprised of three homologous sets. Each homologous set in the more naphthenic class consists only of low-, medium-, or high-gravity oil. A C<7> maturity parameter (the 2,4-DMP/2,3-DMP ratio) indicates that naphthenic Monterey oils were generated at a higher temperature than more paraffinic crudes.

The C<7> chemistry of core samples of phosphatic and siliceous Monterey shales over a wide range of thermal maturity was compared to the C<7> chemistry of the oils they generated in the Santa Maria Basin. The values of C<7> source parameters in these source-rock samples demonstrate that: (1) phosphatic shales generated the very heavy, paraffinic oils; (2) siliceous shales generated the other class of lighter, naphthenic crudes; and (3) C<7> selectivity ratios are not influenced significantly by thermal maturity effects or mineral matrix effects (e.g., clay catalysis). Furthermore, the in situ thermal decomposition of heavy oil retained by Monterey source rocks does not adequately explain these observations. It is concluded that phosphatic Monterey shales genera e commercial quantities of only very heavy oil, while siliceous Monterey shales generate low-, medium-. or high-gravity oil (depending upon their clay content).

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.