## 4 Results and Discussion
### Shale Adsorption Capacity  
In shale reservoirs, gas is preserved in free or adsorbed states, with the adsorbed gas fraction forming a large proportion of the total weight. The adsorbed gas content of a shale reservoir is known to be affected by external factors, such as formation temperature and pressure, as well as internal factors, such as TOC content, organic matter maturity, clay mineral content, and pore structure [51,52]. A quantitative assessment of shale adsorption capacity, typically through isothermal methane adsorption experiments, is thus essential when evaluating shale reservoirs.  
Here, we selected seven shale samples with uniform organic carbon contents from well ED-2 for isothermal methane adsorption tests using the mass balance method to obtain both their absolute and excess methane adsorption isotherms (Figure 15a,b, respectively). We took the absolute methane adsorption at 32 MPa as the maximum adsorption capacity of these samples.  
The samples had maximum methane adsorptions of 2.21-5.41 m\({}^{3}\)/t (averaging 3.53 m\({}^{3}\)/t) at 30 \({}^{\circ}\)C and 32 MPa. The sample TOC content was positively correlated with both excess and absolute methane adsorption (Figures 15 and 16c), and the two samples containing >10 wt.% TOC adsorbed much more methane than the others. The methane adsorption capacity of the siliceous shales was also positively correlated with the volume and specific surface area of micropores and mesopores, but not macropores (Figure 16a,b). Thus, mesopores and micropores provide adsorption sites and storage space for methane  
Figure 14: Correlations between pore structure parameters, mineral contents, and organic matter contents in the Dalong Formation shales: (**a**) Pore volume, (**b**) Pore specific surface area (both separated by pore size class), (**c**) Average pore size, (**d**) Mineral content vs. TOC content.  
Figure 15: (**a**) Excess and (**b**) Absolute methane adsorption isotherms for the Dalong Formation shale samples from well ED-2.  
and are the factors that directly control these shales' adsorption capacities. Methane adsorption was also positively correlated with clay mineral content and weakly negatively correlated with quartz and carbonate contents (Figure 16d).  
Because the volumes and specific surface areas of micropores and mesopores in Dalong siliceous shales are strongly positively correlated with organic matter content (Figure 14a,b), and because most micropores and mesopores are of organic origin, the methane adsorption sites and amounts must be related to the development of organic micropores and mesopores. Indeed, organic matter content is the main factor controlling the adsorption capacity of these shales. Although clay mineral contents are also positively correlated with methane adsorption, we conclude that methane adsorption capacity is dominantly controlled by organic matter content because TOC is also positively correlated with clay mineral content. The high TOC contents of these siliceous shales are conducive to the development of micropores and mesopores, and the associated increase in pore volume and specific surface area results in stronger methane adsorption capacities. These trends have strong implications for exploration in western Hubei and may also contribute to our understanding of methane adsorption in shales in general [53; 54], as well as to our understanding of methane adsorption in less conventional natural gas systems, such as biogenic gas hydrates [55].