In Sichuan basin the Lower Palaeozoic source rock has reached high to over-mature stage, but it is still considered that it has obvious genetic link with some gas reservoirs which were formed later. Therefore, the accumulation process between ancient high to over-mature source rock and the late formation of gas reservoirs has become one of the key factors to recognize the law of natural gas accumulation in Sichuan Basin. Based on some characteristics of Carboniferous gas reservoir in eastern basin, such as the gas compositions, stable carbon isotope, it is found that the gas reservoir is oil-cracking gas. The result reveals that the formation and evolution of present gas reservoir is that an ancient oil pool experienced the processes of forming, accumulating, destructing, and thermal maturating and cracking into gas reservoir.
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