The ^dgr¹³C values of dissolved HCO₃^- in 75 water samples from 15 oil and gas fields were determined in a study of the source of carbon dioxide of the dissolved species and the carbonate cements that modify the porosity and permeability of many petroleum reservoir rocks. The fields are located in the San Joaquin Valley, California, and the Houston-Galveston and Corpus Christi areas of Texas. The reservoir rocks are sandstones ranging in age from Eocene through Miocene. The ^dgr¹³C values of total HCO₃^- indicate that the carbon in the dissolved carbonate species and carbonate cements is mainly of organic origin.

The range of ^dgr¹³C values for the HCO₃^- of these waters is -20 to 28 permit relative to the PDB standard. This wide range of values is explained by three mechanisms. Microbiologic degradation of organic matter appears to be the dominant process controlling the extremely low and high ^dgr¹³C values (-20 to 28 per-mil) of HCO₃^- in the shallow production zones where the subsurface temperatures are less than 80°C. The extremely low ^dgr¹³C values are obtained in waters where the concentration of SO₄ is more than 25 mg/L and probably result from the degradation of organic acid anions by sulfate-reducing bacteria (SO₄^2- + CH₃COO^- ^rarr 2HCO₃- + HS^-). The high ^dgr¹³C values probably result from the degradation of acetate by methanogenic bacteria (CH₃COO^- + H₂O^rlarrHCO₃^- + CH₄).

For samples from production zones with subsurface temperatures greater than 80°C, thermal decarboxylation of short-chain aliphatic acid anions (principally acetate) to produce CO₂ and CH₄ is probably the major source of CO₂. The ^dgr³C values of HCO₃^- for waters from zones with temperatures greater than 100°C result from isotopic equilibration between CO₂ and CH₄. At these high temperatures, ^dgr¹³C values of HCO₃^- decrease with increasing temperatures and decreasing concentrations of these acid anions.

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