PETROLEUM, OIL-FIELD WATERS, AND AUTHIGENIC MINERAL ASSEMBLAGES - ARETHEY IN METASTABLE EQUILIBRIUM IN HYDROCARBON RESERVOIRS

Although the presence of carboxylic acids and carboxylate anions in oi l field waters is commonly attributed to the thermal maturation of ker ogen or bacterial degradation of hydrocarbons during water-washing of petroleum in relatively shallow reservoirs, they may have also been pr oduced in deeper reservoirs by the hydrolysis of hydrocarbons in petro leum at the oil-water interface.dagger To test this hypothesis, calcul ations 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, albit e, and a representative spectrum of organic and inorganic aqueous spec ies at reservoir temperatures and pressures. The cohort of waters chos en for this purpose was restricted to include only those for which ana lyses reported in the literature list separately analytical concentrat ions of both organic and inorganic carbon. These values were specified in the Gibbs free energy minimization calculations to constrain the f ugacity of oxygen (f(O2(g)).double dagger This constraint is predicate d on the hypothesis that the oxidation of carboxylic acids to CO2 is r apid in the context of geologic time, but slow in terms of the time sp an of laboratory studies. The calculations resulted in credible soluti on pHs and activities of aqueous CO2 (a(CO2(aq)). The values of log f( O2(g)) generated by the calculations exhibit a remarkably smooth distr ibution 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(O2(g)) resulting from calculation of the distribution of species with the minimum Gibbs fre e energy in oil field waters recovered from the San Joaquin basin of s outhern 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 sp ecies may also be in metastable equilibrium with hydrocarbon species i n petroleum at the oil-water interface, representative values of the c omputed fugacities of oxygen in hydrocarbon reservoirs in the Texas Gu lf Coast were used together with corresponding values of a(CO2(aq)) in the waters, to calculate equilibrium activities of various hydrocarbo n species in crude oil. The calculations resulted in reasonable activi ties of n-alkanes with carbon numbers greater than or similar to appro ximately 6-15, depending on the activity of aqueous CO2. However, it a ppears that n-alkanes with lower carbon numbers in crude oil cannot ac hieve heterogeneous metastable equilibrium with oxidized carbon-bearin g species in the crust of the Earth. The calculations also indicate th at Ca 2- , H-, CO2, CH3COOH, CH3COO-, and other aqueous species in oil field waters may be in metastable equilibrium at the oil-water interf ace with hydrocarbons other than the light paraffins in crude oil, as well as with calcite and other minerals in hydrocarbon reservoirs.sect ional sign If this is indeed the case, the compositions of formation w aters can be used together with Gibbs free energy minimization calcula tions to guide sequential exploration drilling for hydrocarbon accumul ations in sedimentary basins. Both thermodynamic and compositional con siderations suggest that the fugacity of oxygen in calcite-bearing res ervoirs may be controlled at the oil-water interface by metastable equ ilibrium states among the heavier hydrocarbons in crude oil and/or cal cite and the oxidized carbon-bearing species in the aqueous phase. Irr eversible reaction of the light paraffins in petroleum with H2O at the oil-water interface to form lighter paraffins and CO2(aq), CH3COOH(aq ), and other oxidized carbon-bearing aqueous species is strongly favor ed by the large chemical affinities of the reactions. Because these ir reversible hydrolytic disproportionation reactions are both exergonic and endothermic, they may be mediated at high temperatures and pressur es by hyperthermobarophilic archea or bacteria.II 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. Alth ough analytical data reported in the literature indicate that maturati on of crude oil does not occur to an appreciable degree in static hydr ocarbon reservoirs, irreversible hydrolytic disproportionation of the light paraffins in petroleum favors maturation of crude oil in flow ch annels and reservoirs in young dynamic basins in which fluid flow is e xtensive and oil, water, and gas are in pervasive contact. It appears that irreversible production of carbonic acid during the hydrolytic di sproportionation of the light paraffins in petroleum at the oil-water interface may drive much of the diagenetic process in such basins by l owering the pH of the oil field waters. At near-neutral pHs, the react ions favor precipitation of carbonates, but at lower pH values, they f avor carbonate dissolution, albitization of plagioclase, illitization of smectite, and other diagenetic reactions. These observations have f ar-reaching implications with respect to the development and fate of s econdary porosity in hydrocarbon reservoirs.