Although thermochemical sulfate reduction (TSR) is an irreversible process, it can be demonstrated that many reactions involved in TSR occur at or near equilibrium. This applies not only to minerals and formation waters, but also to hydrocarbons and organic sulfur compounds (OSC) in crude oils and condensates. The metastable equilibrium approach pioneered by Helgeson et al. (1993) is here extended to construct a thermodynamic model of TSR which allows to calculate compositional changes in the water-rock-hydrocarbon-gas system as a function of reaction progress. Using oxygen fugacity as a redox variable to describe the stability relations among minerals, formation waters, and hydrocarbon and organic sulfur species in petroleum, it is shown that the redox state of the water-rock-hydrocarbon-gas system progressively shifts with increasing reaction progress from reduced conditions (under which petroleum is stable), towards more oxidizing conditions imposed by the anhydrite-calcite-sulfur assemblage (and under which pyrobitumen and elemental sulfur may form). This change in redox conditions during TSR appears to be consistent with the observed increase in the aromatic fraction of crude oils during TSR. The results of the speciation calculations compare favorably with compositional data reported in the literature for reservoired oils affected by TSR. Geochemical parameters which have been compared include saturate/aromatic ratios, organic sulfur contents, and concentrations of dibenzothiophenes of crude oils, as well as the CO2/H2S ratio of the gas phase.