Variation in the carbon isotopic composition of alkanes during shale gas desorption process and its geological significance

Taking Chang 7 shale of the Yanchang Formation in the southeastern area of the Yishan slope of the Ordos Basin as our research subject, desorption experiments were performed on more than 30 shale samples that helped this study focus on the variation of carbon isotopic composition of alkanes during the desorption process of shale gas, the possible causes of this phenomenon and its geological significance will also be discussed as follows. It was found that carbon isotopic composition became higher by 9.2‰ (from −50.1‰ to −40.9‰) for methane desorbed gas (δ13C1), and it also became higher by 2.8‰ (from −35.5‰ to −32.7‰) for ethane (δ13C2), but there's barely any differences for propane during the desorption process. At room temperature or constant temperature, carbon isotopic composition for both methane and ethane (δ13C1 & δ13C2) increased continuously. The values of δ13C1 and δ13C2 were low at first and then it gradually became high whilst the desorption process of shale gas that was optimized with the aid of the increasing temperature. The reason for this phenomenon is most likely due to the main adsorption/desorption of shale gas and diffusion migration fractionation of isotope. Additionally, these variation characteristics of methane and ethane carbon isotope composition of shale gas may be a vital reason for the relatively low carbon isotopic composition of methane compared to δ13C calculated by means of the vitrinite reflectance (RO), and this may be applied to evaluate the remaining amount of shale gas resources.