## 4 Discussion
### Influence of Burial Depth on the Development of Pore Structure
#### 4.3.1 Gas Contents of Shale Reservoirs at Different Burial Depths  
In this study, the adsorption gas contents were measured via methane isothermal adsorption experiments, and free gas contents were measured via saturated methane NMR experiments. Due to the limited temperature and pressure ranges of NMR equipment, the free gas content in the Luzhou area cannot be directly tested; therefore, it was calculated via Equations (8) and (9). Thus, the total gas contents of shale reservoirs in the Zhaotong, Weiyuan, and Luzhou areas were obtained (Figure 12). The total gas content of the shale reservoir in the Zhaotong area ranged from 6.91 m\({}^{3}\)/t to 7.93 m\({}^{3}\)/t, with a high proportion of adsorbed gas of up to 60%. The total gas content of the shale reservoir in the Weiyuan area ranged from 9.59 m\({}^{3}\)/t to 10.28 m\({}^{3}\)/t, with an average value of 9.86 m\({}^{3}\)/t. The proportion of free gas was approximately 65.05%. The total gas content of the shale reservoir in the Luzhou area ranged from 11.76 m\({}^{3}\)/t to 18.88 m\({}^{3}\)/t, with an average value of 14.31 m\({}^{3}\)/t, with free gas accounting for 84.47%. It can be seen that as the burial depth increases, both the total gas content and the proportion of free gas increase. There are many factors that affect the gas content of shale reservoirs, including burial depth (temperature and pressure) and reservoir properties (such as porosity, TOC content, mineral composition, and pore structure) [44; 34]. The differences in TOC content and mineral composition of the selected samples in this study are not significant. The following section mainly discusses the influence of burial depths (temperature and pressure conditions) and pore structure on gas content. The impact of burial depth on gas-bearing capacity has a dual impact. As the strata rise, the development degree of inorganic pores and MF in the shale reservoirs increases, especially the scale of the MF, providing channels for the migration of shale gas and reducing the self-sealing ability of shale reservoirs [71]. In the Zhaotong area, the width of MF in shale reservoirs can reach several micrometers (Figure 5d,e), leading to a decrease in the self-sealing ability of the shale reservoir. Therefore, at a burial depth of 1000-2000 m, free gas in the Zhaotong area may escape, resulting in low total gas and free gas contents, as well as a low pressure coefficient of 1.0-1.2 [55]. However, in the Luzhou area, which has a burial depth of more than 4000 m, the reservoir has a strong self-sealing ability and a high pressure coefficient of 1.8-2.2 [55], and the free gas content and total gas content of the reservoir are high (Figure 12). In addition, adsorbed gas mainly exists in the OM pores of shale reservoirs, especially OM pores with a pore size of less than 10 nm and some inorganic pores and MF that are related to clay [72; 73]. Although the porosity of organic pores in the Zhaotong shale reservoir is small, the complex network of shaped small OM pores is extremely well-developed and has a large specific surface area, providing a large number of adsorption sites for shale gas [73]. The adsorbed gas decreases with increasing temperature, and after reaching a certain pressure, the amount of adsorbed gas does not change greatly [74]. The burial depth of the Zhaotong shale reservoir is less than 2000 m, with a formation temperature of about 60 \({}^{\circ}\)C, which is more conducive to adsorption and is much lower than the formation temperature of 130 \({}^{\circ}\)C in the Luzhou area. The shale reservoirs in the Zhaotong area, with a relatively low burial depth, provide a large number of adsorption sites and suitable adsorption temperatures for the storage of adsorbed gas. Therefore, the shale reservoirs in the Zhaotong area have the highest adsorbed gas content. The porosity of inorganic pores in the Luzhou area is smaller than that of OM pores, and OM pores with a pore size of 50-200 nm are in the form of relatively isolated circular pores with fewer adsorption sites.