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 aretwo criticalparameters for determining shale gas resources and devising developmentplans. Existing traditional methods have their limitations in solvingthe above issues, while a novel approach for evaluating the aforementionedkey parameters was provided by the carbon isotope fractionation (CIF)model. To investigate the stable carbon isotope fractionation andshale gas content characteristics during the degassing process, 10shale samples from Well JY A in the Jiaoshiba area of the SichuanBasin were chosen for on-site degassing tests. The on-site degassingpatterns were recorded, and the methane, ethane, and carbon dioxideisotope values were measured. The CIF model was used to evaluate thegas degassing patterns and isotope fractionation characteristics ofthe complete degassing process and accurately assess the in situ gas-bearingparameters of the Well JY A shale. Research has shown that duringa 20 h degassing process, the methane fractionation magnitude canreach 10 & PTSTHOUSND;, and the ethane fractionation magnitude is about2 & PTSTHOUSND;. There are no significant fractionation characteristicsfor carbon dioxide. The complete cumulative degassing curve exhibitsa "downward convex" shape followed by an "upwardconvex" shape, while the complete isotope fractionation curveshows a three-stage pattern of "initially stable, then lighter,and finally heavier". Free gas diffuses first, followed bythe desorption of adsorbed gas, which lags behind significantly, andthe adsorbed gas ratio controls the apparent carbon isotope fractionation.Initially, diffusion fractionation dominates, while later, adsorption-desorptionfractionation gradually becomes dominant. Using the CIF model, theGIP 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, theGIP content ranges 1.49-4.14 mL/g, averaging 2.69 mL/g, whichis significantly underestimated in this area.