The Evolution and Preliminary Genesis of the Carbon Isotope for Coal Measure Tight Gas: Cases from Upper Paleozoic Gas Pools of Ordos Basin and Xujiahe Formation Gas Pools of Sichuan Basin, China

In classic theory, the natural gas generated by organic matter (OM) would become dry with the increase of source rock maturity [1]. Meanwhile, the carbon isotope of gas would become heavy during the evolution process of source rock [2-4]. However, some new characteristics of geochemistry appear in shale gas found in shale with over maturity, such as the isotopic rollover, partial reversal and full reversal [5-9]. The isotopic rollover refers to that the isotopic value of one heavy gaseous component (>C2) does not become heavy with the decreasing wetness or increasing maturity of source rock, but become light in a certain range of gas wetness. For example, the carbon isotope of ethane from shale gas found in North America increases with its decreasing wetness initially. Then it decreases with the further decreasing wetness of shale gas [7]. While, the isotopic reversal means that the isotope value do not increase with the increase of carbon number in gas, but become light (δC1 > δC2 > δC3).