## 4 Result and Discussion
### The Calculation Result of Diffused Gas Volume
Conversely, diffusion of the CH\({}_{4}\) component from the source in the shale reservoir reduces its concentration, leading to an increase in the C\({}_{2}^{+}\) component and heavier \(\delta^{13}\)C and \(\delta^{13}\)DC\({}_{1}\) values [44]. The average CH\({}_{4}\) content, \(\delta^{13}\)C\({}_{1}\), \(\delta^{13}\)C\({}_{2}\) and \(\delta^{13}\)DC\({}_{1}\), of two horizontal wells in the Wei201 well area are 96.02%, \(-\)35.3%, \(-\)39.8% and \(-\)144%, respectively. The average CH\({}_{4}\) content, \(\delta^{13}\)C\({}_{1}\), \(\delta^{13}\)C\({}_{2}\) and \(\delta^{13}\)DC\({}_{1}\), of five horizontal wells in the Wei204 well area are 97.6%, \(-\)36.5%, \(-\)41.0% and \(-\)146.6%, respectively. (Figure 11). The geochemical indicators reveal that the levels of light hydrocarbon components and lighter isotopes are lower in the Wei201 well area compared to those in the Wei204 well area. The reason behind this difference is the greater cumulative shale gas diffusion in the former than the latter, which strengthens the diffusion of light hydrocarbon components and results in a more noticeable isotope fractionation effect. The geochemical features of the gas reservoir confirm the validity of the shale gas diffusion calculation. The diffusion rate per unit area of Longmaxi Formation shale in Weiyuan region ranges from 14.10 \(\times\) 10\({}^{4}\) to 16.50 \(\times\) 10\({}^{4}\) m\({}^{3}\)/m\({}^{2}\). The basic trend is that diffusion is more prominent in the northwest and less significant in the southeast. The high values are mainly distributed near Weiyuan and Ziliujing anticlines, while the low values are primarily found near the Baimazhen syncline (Figure 12). According to exploration results [43], shale gas in the Longmaxi Formation of Weiyuan region is predominantly enriched in the southeast, indicating that the overall loss of gas reservoirs in this direction is minimal, which is consistent with the numerical simulation results. The Longmaxi Formation's shale plane area in the Weiyuan area is about 2800 km\({}^{2}\) with recoverable resources totaling 0.25 \(\times\) 10\({}^{12}\) m\({}^{3}\)[44]. According to the calculations in this paper, an average loss of 0.13 \(\times\) 10\({}^{12}\) m\({}^{3}\) gas in the positive part and 0.30 \(\times\) 10\({}^{12}\) m\({}^{3}\) gas in the negative part is observed, resulting in a total gas loss of approximately 0.43 \(\times\) 10\({}^{12}\) m\({}^{3}\). The total gas loss is about 1.72 times greater than the current recoverable resources, which  
Figure 10: Comparison of diffusivity of Longmaxi Formation in different tectonic positions in Weiyuan area during different geological periods. is obviously very unfavorable for the accumulation of shale gas in the study area; therefore, when evaluating the prospects of shale gas in other areas, it is necessary to consider the diffusion loss of shale gas in order to make an accurate assessment of gas content prospects. Otherwise, prospect gas content will be overestimated. Figure 11: Composition and isotopic composition of shale gas in Longmaxi Formation, Weiyuan area. (**a**) Longmaxi Formation shale gas components in Wei 201 and Wei 204 well areas; (**b**) isotopic composition of Longmaxi Formation in Wei201 and Wei204 well areas. Figure 12: Diffusion gas contour map per unit area of Longmaxi shale in Weiyuan area. The shale gas reservoir has lost a significant amount of methane gas due to diffusion over the long geological history. It is extremely difficult to accurately simulate the amount of gas diffusion therefore, this study can only retroactively estimate the validity of the simulation results based on current exploration and development data. In future research, some experimental methods are planned to restore the evolution history of ancient pressure and gas content in strata. Through the comparison of experimental results and numerical simulation results, the accuracy of the results in this article can be better demonstrated.