Thermodynamic modelling of aqueous CH4-bearing fluid inclusions trapped in hydrocarbon-rich environments

This paper investigates the microthermometric properties of aqueous CH4-bearing fluid inclusions entrapped in presence of petroleum. The Daridon equation of state [Daridon, J.L., Lagourette, H., Saint-Guirons, H., Xans, P., 1993. A cubic equation of state model for phase equilibrium calculation of alkane + carbon dioxide + water using a group contribution kij. Fluid Phase Equilib. 91, 31–54] is used to describe the phase equilibria in the H2O–CH4–hydrocarbons systems between aqueous solutions and hydrocarbon phases. Modelling shows that methane solubilities in water vary in a more complex way and at lower levels than predicted by models for H2O–CH4–salts systems. Thus, trapping pressures of these fluid inclusions, determined from measurements of CH4 molalities by Raman spectroscopy [Dubessy, J., Buschaert, S., Lamb, W., Pironon, J., Thiéry, R., 2001. Methane-bearing aqueous fluid inclusions: Raman analysis, thermodynamic modeling and application to petroleum basins. Chem. Geol. 173, 193–205], may be strongly underestimated. The study suggests also that the trapping temperature may be slightly higher than the homogenisation temperature.