Gas composition and carbon isotopic variation during shale gas desorption: Implication from the Ordovician Wufeng Formation - Silurian Longmaxi Formation in west Hubei, China

This paper focuses on the gas composition and carbon isotopic variation during gas desorption from the shale in Ordovician Wufeng Formation-Silurian Longmaxi Formation. The results show that the desorbed shale gases are predominately comprised of methane (content: 98.36–98.76%) and thus are typical dry gas. As the desorption proceeded, the content of methane gradually decreased, and the drying coefficient (C1/C1-5) of the desorbed shale gases also gradually decreased. The drying coefficient decreased from 98.58 to 99.04% (98.81% on average) of the initial desorbed samples to 88.72–96.78% (94.15% on average) of the final desorbed samples. Based on the variation patterns of gas component ratios during the shale gas desorption, the order of the shale's absorption capacity for different gas components is determined as CO2 > H2 > C3H8 > C2H6 > CH4 > He. The drying coefficient of the shale gas is negatively correlated with the total shale gas content due to the different adsorption capability of shale for gaseous hydrocarbons. The δ13C values tended to become heavier during desorption in general. The δ13C1 values of the initial desorbed samples were the lowest, ranging from −30.71‰ to −25.63‰. In detail, the δ13C values of methane increased by 5.41–25.93‰, while those of ethane increased within a relatively smaller range. The carbon isotopic fractionation is mainly ascribed to diffusion and adsorption/desorption. Moreover, the carbon isotopic fractionation degree of methane was correlated with the shale gas content during the field desorption, suggesting that the carbon isotopic fractionation degree of methane is a potential tool for the identification and selection of favorable sweet spots of shale gas. In addition, the shale gas from Wufeng-Longmaxi Formation in west Hubei consists of highly mature oil-cracking gas and kerogen-cracking gas. All of the desorbed shale gas is characterized by isotopic reversal and the extent of the isotopic reversal increased as desorption continued. This is primarily caused by the mixing of the gas from thermal degradation of kerogen with the gas from residual oil cracking and wet gas as well as the diffusion of the shale gas. The carbon isotope reversal of methane and ethane is not directly related to shale gas content.