Abstract: By means of thermal simulation experiment,5 mudstone samples from Palaeogene of the Zhu Ⅰ Depre-ssion have been tested.The studies of methane stable carbon isotope have shown that when R_o is below 1.2%-1.3%,methane stable carbon isotope decreases as R_o increases.When R_o is over 1.2%-1.3%,methane stable carbon isotope increases as R_o increases.The changing principle may be explained by the differences of activation energy between ¹³C methane(heavy methane)and ¹²C methane(light methane).Under experimental and natural conditions,the relationship between methane stable carbon isotope and R_o are different,which might be the results of "effects after hydrocarbon generation".Due to the effects,massive gas migration leads to the weaken-ing of carbon isotope fractionation.The studies may guide gas field exploration.Heavy methane carbon isotope indicates high-mature gas source rocks,large-scale migration and good prospects for gas exploration.