Evaluation of gas-in-place content and gas-adsorbed ratio using carbon isotope fractionation model: A case study from Longmaxi shales in Sichuan Basin, China

Gas-in-place (GIP) content and gas-adsorbed ratio are crucial parameters for resource potential assessment, sweet spot prediction, and production strategy optimization. Various methods have been proposed from different perspectives to evaluate these parameters; however, none have been widely accepted. The carbon isotope fractionation (CIF) model provides a powerful tool for clarifying the mechanism of isotopic fractionation during natural gas transport and evaluating key parameters of unconventional gas resources. In this study, five shale samples from the Longmaxi Formation in the Jiaoshiba area, Sichuan Basin, China, were subjected to canister degassing experiments to investigate degassing behaviors and their variations in the isotopic compositions of methane (δ13C1), ethane (δ13C2), and carbon dioxide (δ13CCO2). The results showed that the degassing volume (rate) and isotopic fractionation amplitude (slope) of methane positively correlate with the TOC content after controlling the lost time. The released ethane was more enriched in 12C than methane and did not fractionate during degassing, resulting in an increased isotope reversal. The δ13CCO2 exhibited complex fractionation features corresponding to later stages (stages III and IV) in the general pattern. Furthermore, we evaluated the GIP content, gas-adsorbed ratio, and in-situ Langmuir parameters (VL and PL) using the CIF model. The calculated GIP content ranges from 3.48 cm3/g to 7.29 cm3/g with an average of 5.32 cm3/g, and the gas-adsorbed ratio is between 13.87% ~ 43.75% (average 30.79%). The optimized VL and PL show an excellent correlation with TOC content, demonstrating the great advantage of the CIF model in evaluating Langmuir parameters. Compared with the CIF model, the traditional USBM method underestimated shale gas resources in the Jiaoshiba area by 2% ~ 22%, with an average of 11.5%.