Petroleum, oil field waters, and authigenic mineral assemblages Are they in metastable equilibrium in hydrocarbon reservoirs

Although the presence of carboxylic acids and carboxylate anions in oil field waters is commonly attributed to the thermal maturation of kerogen or bacterial degractation of hydrocarbons during water-washing of petroleum in relatively shallow reservoirs, they may have also been produced in deeper reservoirs by the hydrolysis of hydrocarbons in petroleum at the oil-water interface. † † The term oil field waters is used in the present communication as a generic designation of saline formation waters in hydrocarbon reservoirs, regardless of their salinity. To test this hypothesis, calculations were carried out to determine the distribution of species with the minimum Gibbs free energy in overpressured oil field waters in the Texas Gulf Coast assuming metastable equilibrium among calcite albite, and a representative spectrum of organic and inorganic aqueous species at reservoir temperatures and pressures. The cohort of waters chosen for this purpose was restricted to include only those for which analyses reported in the literature list separately analytical concentrations of both organic and inorganic carbon. These values were specified in the Gibbs free energy minimization calculations to constrain the fugacity of oxygen (ƒ O 2(g) ) . ‡ This constraint is predicated on the hypothesis that the oxidation of carboxylic acids to CO 2 is rapid in the context of geologic time, but slow in terms of the time span of laboratory studies. The calculations resulted in credible solution pHs and activities of aqueous CO 2 ( a CO 2( aq) ). The values of log ƒ O 2(g) generated by the calculations exhibit a remarkably smooth distribution with temperature which is similar to, and within the range of those characteristic of common mineral assemblages. Similar variation with temperature is exhibited by values of log ƒ O 2(g) resulting from calculation of the distribution of species with the minimum Gibbs free energy in oil field waters recovered from the San Joaquin basin of southern California. These observations strongly support the hypothesis that homogeneous equilibrium obtains among carboxylate and carbonate species in oil field waters. To determine the extent to which these species may also be in metastable equilibrium with hydrocarbon species in petroleum at the oil-water interface, representative values of the computed fugacities of oxygen in hydrocarbon reservoirs in the Texas Gulf Coast were used together with corresponding values of a CO 2( aq) in the waters, to calcu