## 2 Study Area and Geological Features
### Geologic and Tectonic Characteristics of Depleted Gas Reservoirs
Three sets of faults developed in the directions of north-northwest, nearly north-south, and north-northeast during the Yingcheng formation period, with the north-northwest direction being the main one. Particularly in the fourth layer of the Yingcheng formation, 332 faults were identified, with 63 of them having an extension length greater than 1.5 km, and 269 being less than 1.5 km. Moreover, in the area serving as a depleted gas reservoir-type underground gas storage, there are a total of 12 faults in the Yingcheng formation, mainly in the north-northwest direction. Among them, 3 faults penetrate through the reservoir and cap rock, 6 are internal faults within the reservoir and cap rock, and 3 are source-reservoir faults. The Denglouuk formation has a total of 28 faults that developed from the deposition period and continued until the deposition period of the Quantou formation and Qingshankou formation ended [30; 31]. A total of nine faults developed in the Denglouku formation within the area of the depleted gas reservoir. Among them, there are three faults from the Yingcheng formation to the top of the D2 formation, mainly trending in a north-northwest direction. F2, F5, and F11 are faults from the Yingcheng formation to the top of the D2 formation, with relatively short lengths and gentle displacements. Considering the influence of late tectonic movements, the number of faults in the upper T3 layer is higher than in the lower T4 and T5 layers, and they have more diverse trends, all of which are normal faults. The central-fracture-type volcanic channels of the Yingcheng formation are depicted through planar and cross-sectional methods. The volcanic vents are distributed in a north-northwest direction, generally consistent with the major faults. The seismic reflection characteristics on the profiles are mound-shaped and irregularly cylindrical, often cutting through reflection layers, with medium to high frequency, medium amplitude, and poor continuity. In addition, the boundary of the trap is the actual boundary of the trap for the underground storage in the depleted gas reserve. The overall geological body of Block S in the area extends to 200 km\({}^{2}\). The overall structural shape of the depleted gas reservoir is a dustpan-shaped fault depression, faulting to the west and overlapping to the east, with the main development of the reservoir structure in the downthrown block of the Xuxi fault. Therefore, the Yc3I\({}^{1}\) gas reservoir in the Yingcheng formation is interpreted to have eight sealing layers controlled by the fault trend, with a total area of 31.67 km\({}^{2}\). The trap type of underground storage in depleted gas reserves is mainly dominated by anticlines, faults,  
Figure 3: The drill profile of Block S [28]. and compound traps. The structural formation of the depleted gas reservoir is controlled by multiple volcanic structures distributed along the faults, which overall form a single compound anticline. The high point of the main trap structure is at a burial depth of 2650 m, with a trap line depth of 2820 m, a trap amplitude of 170 m, and a total trap area of 22.62 km\({}^{2}\).