Combining isotopic geochemical data and logging data to predict the range of the total gas content in shale: A case study from the Wufeng and Longmaxi shales in the middle Yangtze area, South China

The marine shales of the lower Silurian Longmaxi formation (S1l) and the upper Ordovician Wufeng formation (O3w) in the middle Yangtze area of southern China have been recognized as effective gas shales with an enormous potential. To estimate the total gas content in the shales, shale gas samples were collected at 20 °C, 48 °C/65 °C, and 90 °C in the gas release process of each core sample during desorption experiments. Based on the isotope fractionation data during the desorption of gas in shale, an adsorbed gas content(mixture gas)/free gas content(mixture gas) to δ13C1(mixture gas) model was established to estimate the proportions of free and adsorbed gas in shale by using the δ13C1 data of shale gas. The proportion of adsorbed gas(mixture gas) in the Wufeng shales is higher than that in the Longmaxi gas shales. The adsorption capacity of the Wufeng shales with relatively high total organic carbon (TOC) is greater than that of the Longmaxi shales with a relatively low TOC. Subsequently, the free gas co...