Thermodynamic Modeling of the Rock–Water–Organic Matter Systems and Origin of Hydrocarbons

Abstract

Chemical interactions in carbonate rock–seawater, silty sand–seawater, clay rock–seawater, basalt–seawater, and granite–seawater systems, where the term “seawater” denotes the compositions of halite, epsomite, and sylvinite stages of seawater densification, were simulated at 25, 100, 200, and 300°С and various pressures in order to assess the ability of rocks to generate hydrocarbons. Based on the efficiency of hydrocarbon generation, the rocks are arranged in the following series: clay > silty sandstone > carbonate > mafic rocks; felsic rocks are unproductive. It is shown that with an increase in the weight rock/water ratio (R/W), which may be taken as the conditional time (degree) of metamorphism, the reduction potential (lgfH2) of rocks, i.e., their ability to generate hydrocarbons, increases. At R/W → 1, the reduction potentials (lgfH2) for carbonate, clay, and silty sandstone are –2.74, –2.45, –2.57 at 100°С, –1.2, –1.1, –1.0 at 200°С, and –0.5, +0.3, –1.2 at 300°С, respectively, which shows a clear advantage of clay at high temperatures (pressures) in terms of its ability to reduce chemical elements and generate hydrocarbons. Thermodynamic modeling of interactions in a closed water–mineral precipitate–natural organic matter system at the T–P parameters of diagenesis was performed. A mature type of kerogen and associated substances (water-dissolved hydrocarbons, nitrogen compounds) are formed in the system in the course of the reactions. It is shown that the removal of CO2 (g) and N2 (g) from the system promotes hydrocarbon and kerogen formation reactions. It was found that the water phase changes insignificantly during kerogen formation. In general, the effect of desalination and changes in pH and Eh, as well as the increase in the content of CO2 dissolved in the water, is significant.