Thermodynamic modeling of water-rock systems to evaluate their generative potential for hydrocarbons

The hydrocarbon generative potential of rocks was evaluated by modeling chemical interactions in the systems carbonate rock—seawater, silty sand rock—seawater, argillaceous rock—seawater, basalt—seawater, and granite—seawater at 25, 100, 200, and 300°C and over a pressure range from saturated vapor pressure to the platform thermal gradient. The term seawater is used here to describe the composition of marine solutions at the halite (SW2), epsomite (SW3), and sylvite (SW4) stages of evaporation. The hydrocarbon generative potential of rocks in these systems increases in the order clays > silty sands > carbonates > mafic rocks, while felsic rocks are nongenerative. It was shown that an increase in the rock—water mass ratio (R/W), which can be considered as a proxy for the duration (grade) of metamorphism, led to an increase in the reduction potential (logfH 2 ) and hydrocarbon generative potential of rocks. At R/W → 1, logfH 2 for carbonates, clays, and silty sands is equal to −2.74, −2.45, and −2.57 at 100°C; −1.2, −1.1, and −1.0 at 200°C; and −0.5, +0.3, and −1.2 at 300°C, which suggests that at higher temperatures (pressures) clays are more capable of reducing chemical elements in the water—rock system.