--> ABSTRACT: Carbon Isotopic Composition of Organic Acids in Oilfield Waters, San Joaquin Basin, California, by S. G. Franks, J. R. Boles, A. L. Fincannon, K. H. Freeman, A. G. Holba, and E. D. Jordan; #91021 (2010)

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Carbon Isotopic Composition of Organic Acids in Oilfield Waters, San Joaquin Basin, California

FRANKS,STEPHEN G., J. R. BOLES,  A. L. FINCANNON, K. H. FREEMAN,  A. G. HOLBA, and E. D. JORDAN

We report what we believe to be the first published measurements of carbon isotopes of organic acids from oilfield waters. Short-chain organic acids (C[1]-C[5]) in oilfield waters of the San Joaquin Basin exhibit a wide range of values, from +0.57 to -27.33 per mil (PDB). Most, however, are in the range of -15 to -25 per mil. The delta{13}C of individual acids generally decreases with increasing carbon number but is consistently heavier than delta{13}C of coproduced oils. Relations between chain length and delta{13}C of the acids indicate intramolecular isotopic ordering. The carboxyl carbon (COOH) is consistently heavier (10 to 38 per mil) than alkyl carbons (CH{3}, CH{2}). Extrapolated delta{13}C of alkyl carbons cluster tightly around two values, -23 and -28 per mil, respectively, for acids in waters co-produced with Miocene-sourced oil (delta{13}C[sat] = -22 to -25 per mil) and Eocene-sourced oil (delta{13}C[sat] = -28 to -30 per mil). In contrast, extrapolated carboxyl delta{13}C ranges from -0.8 to -12.8 (waters with Miocene oils) to +8.2 to -18 (waters with Eocene oils), and can vary significantly within a field.

Because of the large variation of delta{13}C in carboxyl carbons, carbonate cements containing carbon released by decarboxylation will also exhibit large variations. The delta{13}C of these cements may not always be negative and thus may not appear to have an organic "signature".

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.