Abstract: Considering the equilibrium system of CH₄-H₂O-CO₂ and the reaction of methane with CaSO₄(gypsum)in water for example,with the viewpoint of statistical thermodynamics and chemical thermodynamic equilibrium,the conditions and direction of the reaction are studied,as a result,the rela-tion of equilibrium constant and temperature-pressure are inferred.Besides,the neccessary conditions for the preservation and destruction of methane gas under different burial depths and for the generation of carbon dioxide are discussed.It is proposed that the acidity of sedimentary environment,or the pH value,is an important factor which acts in conjunction with temperature in the hydrolysis of carbonate rocks.