Thermodynamic study on the compositions of fluid phases in the C-O-H system under high temperature and pressure

Based on the fluid phase equilibrium and the C-O-H system, the compositions of carious fluid phases under high temperatures and pressures have been calculated in terms of the available thermodynamic data and new P-V-T data and on the assumption of PT = ΣPi in this paper. The results indicate that in this system there are 5 major fluid phases in different proportions at various T and P. CH4 is the dominant phase (about 70%) under relatively lower T and P. Its proportion obviously decreases with increasing T, P and fO2. The results provide sufficient theoretical grounds for discussing the possibility of CH4 formation and the physical-chemical conditions of its stable occurrence and proportion in the geological environment.