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 (f{hook}_O2(g)).^‡ This constraint is predicated on the hypothesis that the oxidation of carboxylic acids to CO₂ 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 CO₂ (a_CO2(aq)). The values of log f{hook}_O2(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 f{hook}_O2(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_CO2(aq) in the waters, to calculate equilibrium activities of various hydrocarbon species in crude oil. The calculations resulted in reasonable activities of n-alkanes with carbon numbers ≳~6-15, depending on the activity of aqueous CO₂. However, it appears that n-alkanes with lower carbon numbers in crude oil cannot achieve heterogeneous metastable equilibrium with oxidized carbon-bearing species in the crust of the Earth. The calculations also indicate that Ca²⁺, H⁺, CO₂, CH₃COOH, CH₃COO^-, and other aqueous species in oil field waters may be in metastable equilibrium at the oil-water interface with hydrocarbons other than the light paraffins in crude oil, as well as with calcite and other minerals in hydrocarbon reservoirs.^§ § Note in this regard that mere recognition of a given equilibrium state carries no necessary causal implication with respect to mass transfer processes that may have led to the state. If this is indeed the case, the compositions of formation waters can be used together with Gibbs free energy minimization calculations to guide sequential exploration drilling for hydrocarbon accumulations in sedimentary basins. Both thermodynamic and compositional considerations suggest that the fugacity of oxygen in calcite-bearing reservoirs may be controlled at the oil-water interface by metastable equilibrium states among the heavier hydrocarbons in crude oil and/or calcite and the oxidized carbon-bearing species in the aqueous phase. Irreversible reaction of the light paraffins in petroleum with H₂O at the oil-water interface to form lighter paraffins and CO_2(aq), CH₃COOH_(aq), and other oxidized carbon-bearing aqueous species is strongly favored by the large chemical affinities of the reactions. Because these irreversible hydrolytic disproportionation reactions are both exergonic and endothermic, they may be mediated at high temperatures and pressures by hyperthermobarophilic archea or bacteria.^∥ ∥ As indicated above, the term hydrolytic disproportionation is used in the present communication to refer to the reaction of a given hydrocarbon in crude oil with H₂O to form a lighter hydrocarbon and oxidized carbon-bearing aqueous species. In contrast, the term disproportionation is used by Tissot and Welte (1984) to refer to production of low and high molecular weight hydrocarbons from those of intermediate molecular weight with accompanying "decarboxylation dehydration, and desulfurization [of the crude oil to yield] carbon dioxide, water, and hydrogen sulfide." However, the extent to which this occurs at the oil-water interface in any given reservoir may depend on whether or not methane can escape from the system. Although analytical data reported in the literature indicate that maturation of crude oil does not occur to an appreciable degree in static hydrocarbon reservoirs, irreversible hydrolytic disproportionation of the light paraffins in petroleum favors maturation of crude oil in flow channels and reservoirs in young dynamic basins in which fluid flow is extensive and oil, water, and gas are in pervasive contact. It appears that irreversible production of carbonic acid during the hydrolytic disproportionation of the light paraffins in petroleum at the oil-water interface may drive much of the diagenetic process in such basins by lowering the pH of the oil field waters. At near-neutral pHs, the reactions favor precipitation of carbonates, but at lower pH values, they favor carbonate dissolution, albitization of plagioclase, illitization of smectite, and other diagenetic reactions. These observations have far-reaching implications with respect to the development and fate of secondary porosity in hydrocarbon reservoirs.