Carbon isotopic composition of organic acids in oil field waters, San Joaquin Basin, California, USA

The carbon isotopic compositions of short-chain organic acids (C₂-C₅) in oilfield waters of the San Joaquin Basin exhibit a wide range of values, from +0.57 to -27.33‰ (PDB). Most, however, are in the range of -15 to -25‰. The bulk isotopic compositions of individual acids generally become isotopically depleted (more negative) with increasing carbon number (chain length) but are consistently more enriched than the isotopic composition of coproduced oils. Relationships between carbon number and isotopic composition of the acids suggest intramolecular isotopic fractionation. Calculated carboxyl carbons (COOH) are isotopically more enriched, typically by 10 to 38‰, than aliphatic carbons (CH). Calculated aliphatic carbon isotopic compositions cluster around two values, -23 and -28‰, consistent with the isotopic composition of coproduced oils derived from two different sources, Miocene Monterey-sourced oils (-22 to -25‰) and Eocene Kreyenhagen-sourced oils (-28 to -30‰). We suggest that the isotopic composition of the aliphatic carbons in the organic acids reflects the carbon isotopic composition of the coproduced oils (or their source kerogen). Isotopic compositions of carboxyl carbons fall within a range of -12.8 to +8.0, consistently heavier than aliphatic carbons. We interpret the enrichment of carboxyl carbons to be inherited from either biological precursors in the source rock kerogen or due to exchange with carbon in dissolved HCO₃. The carbon isotopic enrichment of carboxyl carbons is at odds with theories that attribute organic acids to the inorganic oxidation of hydrocarbons in the subsurface.