## 2 Study Area and Geological Features
### Reservoir Characterization of Depleted Gas Reservoirs  
Due to the differences in reservoir spatial morphology formed by different volcanic rock lithological conditions, a statistical analysis of the reservoir spatial morphology in different lithological areas of the depleted gas reservoir was conducted. The results show that for the rhyolite, primary pores are the main type, accounting for 69.25%, followed by devitrified pores at 22.89%, and a small amount of microfracture pores. For the rhyolitic tuff and rhyolitic welded tuff, volcanic ash dissolution pores are the main type, accounting for 72.22% and 88.89%, respectively, followed by primary pores. Andesitic breccia has intra-boulder and inter-boulder pores and volcanic ash dissolution pores as the main types, followed by primary pores, feldspar dissolution pores, and a small amount of microfracture pores [32]. Considering that the main reservoir spatial distribution in the underground storage in the depleted gas reserve is in the Yingcheng formation, based on the geological characteristics data, the majority of the lithology is rhyolite, accounting for 73.40%. It can be assumed that the reservoir spatial morphology of the underground gas storage facility for depleted gas reservoirs is primarily dominated by primary pores [33].  
The spatial morphology of the reservoir determines various physical parameters of the reservoir. The porosity and permeability parameters of the depleted gas reservoir are crucial for gas reservoir exploitation and processes such as cyclic injection and production in underground gas storage [34]. Therefore, physical property analysis was also conducted on 262 core samples from 9 core wells encountered in the Yingcheng formation within the underground gas storage for depleted gas reservoirs. The statistical results of the core physical property analysis are shown in Figure 4. The porosity of the reservoir ranges from 4.0% to 27.5%, with an average of 8.4%. Samples with porosity greater than 12% account for 37.79%, while those with porosity less than 6% account for 25.19%. The permeability of the reservoir ranges from 0.006 mD to 319 mD, with an average of 1.19 mD, mainly distributed between 0.01 mD and 1 mD. Samples with permeability greater than 1.0 mD account for 27.39%, and those with permeability between 0.1 mD and 1 mD account for 40%.  
Figure 4: Statistical results of core physical property data analysis should be listed as: (**a**) Core porosity frequency distribution; (**b**) Frequency distribution of core permeability.  
In addition, as geological environmental conditions, the temperature and pressure system of the formation also affect the storage and sealing of gas in the reservoir and therefore require analysis [35; 36]. As shown in Figure 5, the formation temperature of the depleted gas reservoir is around 120 \({}^{\circ}\)C, with a temperature gradient of 3.29 \({}^{\circ}\)C/100 m. The original formation pressure of the depleted gas reservoir is 31.78 MPa, and the pressure gradient ranges from 0.15 to 0.2 MPa/100 m, all within the normal range for the temperature-pressure system.  
The overall gas-water distribution characteristics of the depleted gas reservoir are complex, and controlled by both structural and volcanic factors. In the vertical direction, it follows the law of gravity differentiation, showing a general distribution pattern of gas above and water below. The depleted gas reservoir has a roughly unified gas-water interface, with the depth of the gas-water interface for most wells located around 3000 m. In the northern region, some gas-water interfaces are deeper, exceeding 3150 m. In most areas, the gas layers are mainly distributed in the Yc\({}_{3}\)I\({}^{1}\) gas reservoir group, which is also the basis for the selection of the Yc\({}_{3}\)I\({}^{1}\) segment of the Yingcheng formation as the main sealing and storage space for the depleted gas reservoir.