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Whole-Rock Stable Isotope Geochemistry for Correlation and Diagenetic Interpretation: Delaware Basin and Monterey Formation Examples

David A. Mucciarone, Dwight M. Trainor, Douglas F. Williams

The stable oxygen and carbon isotope compositions of whole-rock samples from Delaware basin subsurface cores and Monterey Formation outcrop sections were determined to document their use in stratigraphic correlation and diagenetic interpretations. Very few stable isotopic studies have been conducted on deposits dominated by clastics or organic-rich sediments with minor amounts of carbonate. Whole-rock isotopic analyses are not generally utilized due to complications caused by varying mineralogies and the effects of organic matter upon the isotopic signals. In basins where complex depositional mechanisms prevailed or subtle changes in lithology are present, conventional methods such as well logs and core descriptions alone are insufficient in providing detailed regional co relations and depositional models.

The results of this study reveal potential benefits and problems of stable isotope analyses of whole-rock material. The presence of organic matter in whole rocks necessitates the adoption of special precautions to extract the true isotopic signal from the carbonate matrix. Some residual organic fractions react with the phosphoric acid during the carbonate reaction, generating a gas of mass 46 (possibly nitrogen dioxide, NO2). This mass 46 gas contributes to extremely positive "18-O" values and masks the isotope signal from the CO2 derived from the small amount of carbonate present in the whole-rock matrix. Geochemical studies of organic-rich sediments should be aware of this potential mass 46 contribution in whole-rock samples. The results of whole-rock isotopic studies have the potential of providing a unique signal useful for identifying organic-rich zones and assisting with correlations of subsurface wells and outcrop sections.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.