Based on the fluid phase equilibrium of the C-O-H system,the compositions of various 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=∑Pi 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.
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