## 4 Result and Discussion
### The Calculation Result of Diffused Gas Volume  
By arranging the various tectonic stages of the stratum in a time sequence and converting the generation time into geological time, the cumulative history of free gas diffusion per unit area of the Longmaxi shale in the Weiyuan region is determined (Figure 9). The Weiyuan area, located in southern Sichuan, is characterized by shallow burial in the early phases, profound burial in the middle stages, and fast uplift in the late stages. As the Ro value of the Longmaxi Formation reached 0.5% in the Permian period, the organic matter accumulated to levels that may generate hydrocarbons. The pyrolysis and gas capacity of organic matter improved in the middle and late Jurassic, leading to a rapid increase in the gradient of diffusion concentration and a cumulative diffusion of 2.51 \(\times\) 10\({}^{4}\) m\({}^{3}\)/m\({}^{2}\). By the middle of the Cretaceous period, during the final stage of uplift, the content of free gas reached its maximum level, and the cumulative diffusion increased to 10.89 \(\times\) 10\({}^{4}\) m\({}^{3}\)/m\({}^{2}\). During the final stage of formation uplift, the generation of hydrocarbons from organic matter stopped, and the gas content of the formation decreased rapidly, but gas diffusion continued. About 29 Ma years ago, when the formation was affected by differential uplift, the evolution of the gas diffusion history differed between nearby positive and negative construction. When the positive construction reached the depth of the formation rupture about 23 Ma years ago, the gas loss increased significantly. Ultimately, the cumulative diffused gas volume in the forward section was approximately 15.13 \(\times\) 10\({}^{4}\) m\({}^{3}\)/m\({}^{2}\), while in the negative section, it was about 13.75 \(\times\) 10\({}^{4}\) m\({}^{3}\)/m\({}^{2}\). According to the statistics of shale gas diffusion in different geological eras, it can be concluded that the Jurassic and Cretaceous periods were the peaks of shale gas diffusion. The cumulative diffused gas in the positive part accounted for 32.40% and 47.78%, whereas in the negative part, it accounted for 35.84% and 48.05%, respectively (Figure 10). During the early and middle Cretaceous periods, the Longmaxi Formation was deeply buried for a considerable amount of time, resulting in the complete pyrolysis of organic matter, leading to the generation of large quantities of shale gas. During this process, the gas reservoir also underwent significant diffusion loss. Although hydrocarbon production stopped due to stratigraphic uplift during the later part of the Cretaceous period, diffusion remained high due to the high gas content in the reservoir. Figure 9: Evolution of cumulative diffused gas volume of Longmaxi shale in different tectonic locations in Weiyuan area. Since the Neogene, the significant upward movement in the positive construction has resulted in a rapid decrease in gas content due to the increase in diffusion, which in turn leads to a relatively smaller diffusion concentration gradient; therefore the free gas diffusion in the negative construction slightly exceeded that in the positive construction only during the Neogene and Quaternary period, while the free gas diffusion in the positive construction has always been greater than that in the negative construction in other periods (Figure 10). The composition and isotopic characteristics of shale gas are important parameters that can be used to characterize gas diffusion and migration in shale formations [43]. As shale gas diffuses and migrates, rock components have a relatively high adsorption and retention capacity for heavy hydrocarbon components, resulting in isotope fractionation. Consequently, the diffusion direction of CH\({}_{4}\) component enhances its concentration, while the heavy hydrocarbon component of C\({}_{2}^{+}\) decreases proportionally. Meanwhile, \(\delta^{13}\)C and \(\delta^{13}\)DC\({}_{1}\) values gradually become lighter.