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Five model compounds with representative chemical structures were selected for use in simulation experiments of pyrolytic gas production. The gas production and isotopic fractionation characteristics were observed and analyzed. Then, the factors affecting carbon isotope fractionation during natural gas generation were discussed, and a fractionation model was established and calibrated. We concluded that the final hydrocarbon gas (C1–5) yield of octadecane, octadecylamine, octadecanoic acid, decahydronaphthalene, and 9‐phenylanthracene decreased in turn with the effective hydrogen content. Compared with linear alkanes or alkyl compounds, cycloalkanes have higher thermal stability and generate gas later. The variation in the carbon isotopic composition of natural gas is primarily controlled by the following three factors: (a) the thermal evolution of organic matter results in a gradually heavier isotopic composition for the main gas production stage. (b) Gas inherits the isotopic composition of its parent material, and this effect is evident when the chemical structure and gas generation mechanism between parent materials are similar. (c) The structure of organic matter determines the reaction mechanism of gas generation, which has a significant influence on the range and trend of carbon isotope fractionation in the process of methane generation. An improved chemical kinetic model can accurately characterize carbon isotope fractionation during gas generation. Key Points: The factors affecting carbon isotope fractionation during natural gas generation are revealedA chemical kinetic model describing isotope fractionation is establishedThe structure and gas‐generation mechanisms of organic matter controlling isotopic composition of natural gas are explained [ABSTRACT FROM AUTHOR]

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 (δ 13C 1), ethane (δ 13C 2), and carbon dioxide (δ 13C CO2). 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 δ 13C CO2 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 (V L and P L) 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 V L and P L 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%. • Degassing volume and fractionation amplitude/slope are controlled by TOC content. • CIF model is a powerful tool for evaluating GIP content and gas-adsorbed ratio. • V L and P L calculated by the CIF model positively correlated with TOC content. • The contribution ratio of adsorbed/free gas dominates the apparent fractionation. • USBM method underestimates the shale gas resource potential in the Jiaoshiba area. [ABSTRACT FROM AUTHOR]