Stable carbon isotopes of HCO3- in oil-field waters-implications for the origin of CO2

The δ¹³C values of dissolved HCO₃⁻ in 75 water samples from 15 oil and gas fields (San Joaquin Valley, Calif., and the Houston-Galveston and Corpus Christi areas of Texas) were determined to study the sources of CO₂ of the dissolved species and carbonate cements that modify the porosity and permeability of many petroleum reservoir rocks. The reservoir rocks are sandstones which range in age from Eocene through Miocene. The δ¹³C values of total HCO₃⁻ indicate that the carbon in the dissolved carbonate species and carbonate cements is mainly of organic origin.

The range of δ¹³C values for the HCO₃⁻ of these waters is −20–28 per mil relative to PDB. This wide range of δ¹³C values is explained by three mechanisms. Microbiological degradation of organic matter appears to be the dominant process controlling the extremely low and high δ¹³C values of HCO₃⁻ in the shallow production zones where the subsurface temperatures are less than 80°C. The extremely low δ¹³C values (< −10 per mil) are obtained in waters where concentrations of SO₄²⁻ are more than 25 mg/l and probably result from the degradation of organic acid anions by sulfate-reducing bacteria (SO₄²⁻ + CH₃COO⁻ → 2HCO₃⁻ + HS⁻). The high δ¹³C values probably result from the degradation of these anions by methanogenic bacteria (CH₃COO⁻ + H₂OaiHCO₃⁻ + CH₄).

Thermal decarboxylation of short-chain aliphatic acid anions (principally acetate) to produce CO₂ and CH₄ is probably the major source of CO₂ for production zones with subsurface temperatures greater than 80°C. The δ¹³C 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, δ¹³C values of HCO₃⁻ decrease with increasing temperatures and decreasing concentrations of these acid anions.