Stable carbon isotopes of HCO 3− in oil-field waters—implications for the origin of CO 2

Abstract

The δ 13C values of dissolved HCO 3 − 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 CO 2 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 δ 13C values of total HCO 3 − indicate that the carbon in the dissolved carbonate species and carbonate cements is mainly of organic origin. The range of δ 13C values for the HCO 3 − of these waters is −20–28 per mil relative to PDB. This wide range of δ 13C values is explained by three mechanisms. Microbiological degradation of organic matter appears to be the dominant process controlling the extremely low and high δ 13C values of HCO 3 − in the shallow production zones where the subsurface temperatures are less than 80°C. The extremely low δ 13C values (< −10 per mil) are obtained in waters where concentrations of SO 4 2− are more than 25 mg/l and probably result from the degradation of organic acid anions by sulfate-reducing bacteria ( SO 4 2− + CH 3 COO − → 2 HCO 3 − + HS −). The high δ 13C values probably result from the degradation of these anions by methanogenic bacteria ( CH 3 COO − + H 2 O ai HCO 3 − + CH 4). Thermal decarboxylation of short-chain aliphatic acid anions (principally acetate) to produce CO 2 and CH 4 is probably the major source of CO 2 for production zones with subsurface temperatures greater than 80°C. The δ 13C values of HCO 3 − for waters from zones with temperatures greater than 100°C result from isotopic equilibration between CO 2 and CH 4. At these high temperatures, δ 13C values of HCO 3 − decrease with increasing temperatures and decreasing concentrations of these acid anions.