Abstract
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 (SO42- + 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. End_of_Article - Last_Page 479------------
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