## 4 Discussion
### Influence of Burial Depth on the Development of Pore Structure  
#### 4.2.1 Pore Proportion and OM Pore Size Distribution  
Shale reservoirs develop with a large number of OM pores, inorganic pores, and MF, which represent important places for shale gas storage. Clarifying the proportion of different reservoir space types is conducive to understanding the occurrence, location, and state of shale gas. This study uses image stitching and processing techniques to extract the different types of pores. The specific operation process has been detailed in previous studies [25; 26; 27; 28]. On this basis, we calculated the average values of the different types of pores and MF in shale reservoirs in the different areas. The OM pores account for about 25%, while the inorganic pores account for 60%, and MF account for about 15% of shale reservoirs in the Zhaotong area with a burial depth of less than 2000 m (Figure 9(a)). In the Weiyuan area, the shale reservoirs have a burial depth ranging from 2000 m to 4000 m, and the proportion of OM pores, inorganic pores, and MF is 42.66%, 48.00%, and 9.34%, respectively (Figure 9(b)). In the Luzhou area, in shale reservoirs with a burial depth greater than 4000 m, the proportion of OM pores, inorganic pores, and MF is 61.25%, 33.75%, and 5.00%, respectively (Figure 9(c)).  
It can be seen that as the burial depth increases, the proportion of OM pores gradually increases, while the proportion of inorganic pores and MF gradually decreases. This is because the burial depth is small in the Zhaotong area, the overlying formation pressure of the shale reservoirs is low; the Inter P pores are more fully developed, and the width and length of the MF are larger (Figure 10(a)-d), ultimately forming shale reservoirs with higher porosity (Table 1). Due to the similar TOC contents in shale reservoirs with different burial depths (Table 1) and the small pore size of the OM pores (Figure 8(f)-g), the contribution rate of organic pores to pore volume is not high. In the end, with higher porosity, the proportion of OM pores is 25% (Figure 9(d)). As the burial depth increases, the pressure of the overlying strata increases, and the inorganic pores and MF are gradually compressed or even closed, especially the InterP pores. Due to the compression resistance of minerals and the overpressure of the shale gas, the OM pores in the OM between minerals are well-preserved [65]. This results in the shale reservoirs of the Luzhou area, which have burial depths greater than 4000 m, being dominated by OM pores, the proportion of which can be up to 60%.  
OM pores are an important occurrence place for shale gas, and the size of OM pores is crucial to the occurrence state of shale gas. Histograms of the pore size distribution of OM pores in shale reservoirs with different burial depths were obtained through FE-SEM and image processing techniques (Figure 11). The OM pore size in the Zhaotong area is basically less than 50 nm. The pore size distribution shows two main peaks, namely, 10-15 nm and 30-35 nm (Figure 10(a)). The organic surface rate of the shale reservoirs in the Weiyuan area, which is less than 5 nm, has reached 10%, but the organic surface rate, which is greater than 200 nm, is still the main contributor (Figure 10(b)). For shale reservoirs in the Luzhou area with greater burial depths, the distribution of organic pore size is relatively  
Figure 10: Pore proportion of typical shale gas wells with different burial depths in the Zhaotong, Weiyuan, and Luzhou areas.  
concentrated and is mainly distributed at 50-200 nm, with a pore surface area ratio of over 35% at 50-100 nm (Figure 11c). Overall, the organic pore size of shale reservoirs in the Zhaotong area is less than 50 nm, while shale reservoirs in the Weiyuan area mainly develop organic pores with pore sizes of <5 nm and 200-500 nm. The organic pore size of shale reservoirs in the Luzhou area is mainly distributed between 50 and 200 nm.