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

Abstract

In this study, the geochemistry data of 162 coal measure tight gas samples (77) samples from Ordos Basin, 85 samples from Sichuan Basin) is collected. Then, the carbon isotope evolution of coal measure tight sandstone gas is investigated. The result indicates that three features in carbon isotope evolution of coal measure tight gas appear. (1) The coal measure tight gas has a same feature as shale gas, that the carbon isotope of heavy gas would show a rollover with the decreasing wetness of gases. The corresponding range of gas wetness is about 2.5%~5% at which the carbon isotope of ethane begin to rollover. (2) The carbon isotope series of coal measure tight gas presents successively positive sequence, partial reversal and full reversal with increasing maturity of source rock. (3) The maturity ranges corresponding to of coal measure tight gas with the positive sequence, partial reverse and full reverse of carbon isotope are R o <2.0 %, R o =1.75~3.0% and R o >3.0% of in-situ source rock, respectively. On the bases of pyrolysis experiment of a I type of organic matter, the authors propose that the mixing between primary and cracking gas could just only cause the carbon isotope rollover of heavy hydrocarbon gases, but not cause the carbon isotopic reversal of hydrocarbon. Finally, a conclusion is suggested that carbon isotope reversal of coal measure gas has a close relationship with demethylation happening at the maturity above 3.0% R o according to published data about coal chemical structure measurement. The isotopic fractionation during the process of heavy gases formation by linkage of methyl from demethylation is one of capital factors of carbon isotopic reversal.