Determination of Shale Gas-Bearing Properties Based on Carbon Isotope Fractionation Model: A Case Study from Longmaxi Formation Shales in Jiaoshiba Area, Sichuan Basin, China

Adsorbed gas ratio and gas-in-place (GIP) content are two critical parameters for determining shale gas resources and devising development plans. Existing traditional methods have their limitations in solving the above issues, while a novel approach for evaluating the aforementioned key parameters was provided by the carbon isotope fractionation (CIF) model. To investigate the stable carbon isotope fractionation and shale gas content characteristics during the degassing process, 10 shale samples from Well JY A in the Jiaoshiba area of the Sichuan Basin were chosen for on-site degassing tests. The on-site degassing patterns were recorded, and the methane, ethane, and carbon dioxide isotope values were measured. The CIF model was used to evaluate the gas degassing patterns and isotope fractionation characteristics of the complete degassing process and accurately assess the in situ gas-bearing parameters of the Well JY A shale. Research has shown that during a 20 h degassing process, the methane fractionation magnitude can reach 10‰, and the ethane fractionation magnitude is about 2‰. There are no significant fractionation characteristics for carbon dioxide. The complete cumulative degassing curve exhibits a "downward convex" shape followed by an "upward convex" shape, while the complete isotope fractionation curve shows a three-stage pattern of "initially stable, then lighter, and finally heavier". Free gas diffuses first, followed by the desorption of adsorbed gas, which lags behind significantly, and the adsorbed gas ratio controls the apparent carbon isotope fractionation. Initially, diffusion fractionation dominates, while later, adsorption–desorption fractionation gradually becomes dominant. Using the CIF model, the GIP content of shale in the Well JY A ranges 2.52–5.11 mL/g, averaging 3.56 mL/g, and the adsorbed gas ratio accounts for 4.39%–30.28%, averaging 14.64%. Using the U.S. Bureau of Mines (USBM) method, the GIP content ranges 1.49–4.14 mL/g, averaging 2.69 mL/g, which is significantly underestimated in this area.