## 4 Discussion
### Influence of Burial Depth on the Development of Pore Structure
#### 4.2.2 Controlling the Mechanism of Burial Depth and the Development of Pores  
The evolution of OM pores in shale reservoirs is affected by hydrocarbon generation, diagenesis, and tectonism [66], resulting in the different development of OM pores in shale reservoirs with different burial depths. Organic pores have been observed to create \"bubble\" shapes during geological history, and, as the bitumen solidifies, these \"bubbles\" no longer grow and instead solidify into pores of various scales according to certain burial depths, as observed today. From this pattern, we can consider each organic matter particle as a micro-scale gas reservoir. When organic matter undergoes thermal evolution to form liquid hydrocarbons, the generated shale gas will be dissolved in the liquid hydrocarbons, but this process will not form pores. With the continuous generation of shale gas, the crude oil will appear to be supersaturated. The actual dissolved gas in the crude oil is higher than the dissolved gas in the thermodynamic balance. Supersaturation is a necessary condition for bubble nucleation. When ignoring the influence of impurities and the adjacent pores, crude oil saturated with gas belongs to a single homogeneous system [67], which can be expressed as follows [68]:  
\[\Delta G=-4/3\pi r^{3}\Delta P+4\pi r^{2}\sigma_{bp} \tag{10}\]  
where \(\Delta G\) represents free energy, J; \(r\) represents the radius of the bubble pore, m; \(\Delta P\) is the pressure difference inside and outside the bubble pore, MPa; \(\sigma_{\text{bp}}\) is the surface tension of the bubble (the surface tension of liquid OM under geological conditions), in N/m. The thermal evolution degree of the Longmaxi Formation shale reservoirs in the Zhaotong, Weiyuan, and Luzhou areas is in an over-mature stage; they all experienced the maximum ancient burial depth and were then uplifted to the current burial depth [55]. Due to different tectonic processes, the degrees of strata uplift are different in the studied regions, resulting in differences in the current burial depth. Due to the high degree of thermal evolution of the shale reservoirs, at the maximum burial depth, the organic matter has experienced or is currently at a peak of gas generation, and the formation is in an overpressure state [65]. As the burial depth of the formation decreases, the reduction in overlying formation pressure will disrupt the formation balance under overpressure, forming a new pressure balance, thereby affecting the development and maintenance of the pores [5]. From Equation (10), it can be seen that the free energy of the gas in the OM pores of shale is jointly controlled by \(\tau\), \(\Delta P\), and \(\sigma_{\text{bp}}\). The \(\sigma_{\text{bp}}\) is influenced by the pressure of the overlying strata, resulting in differences in the characteristics of organic pores in the shale at different burial depths. Since shale gas reservoirs in the southern Sichuan Basin have experienced the maximum ancient burial depth, these shale reservoirs are mostly overpressured, and the formation pressure coefficient increases with the burial depth increase [65]. Based on the mechanism of bubble pore formation, taking the characteristics  
Figure 11: OM pore size distribution of typical shale gas wells with different burial depths in the Zhaotong, Weiyuan, and Luzhou areas. of OM pores in shale with burial depths larger than 4000 m as a reference, the changes in OM pores during the process of decreasing burial depths were analyzed. In the Luzhou area, the pressure coefficient of the shale reservoirs is greater than 1.8. Due to the large burial depth and high overlying formation pressure, the OM pore size is mainly distributed at 50-200 nm (Figure 11c). With a burial depth decrease to 2000 m, the overlying formation pressure decreases, the overpressured gas-containing OM pores expand, and adjacent OM pores merge with each other to form new organic pores with pore sizes greater than 200 nm (Figure 11b).