## 2 Study Area and Geological Features  
### Geologic and Tectonic Characteristics of Depleted Gas Reservoirs  
The structural location of Block S is on the northern segment of the Xingshan fault depression in the middle of the Xuijaweizi fault depression zone in the deep Songliao basin and its geological map, see Figure 2[23; 24]. The Xuijaweizi fault depression is one of the seven fault depression structural units in the northern Songliao basin, and it is composed of a graben basin controlled by two main fault zones, Xuxi and Songxi [25]. It can be further divided into three second-level structural units and sixteen third-level structural belts [26]. According to drilling data, the deep strata in the northern part of the Songliao basin where Block S is located developed from bottom to top as follows: basement (C-P series) metamorphic rocks, Hercynian intrusive rocks, the Huoshiling formation of Upper Jurassic system, the Shahezi formation, the Yingcheng formation, the Denglouku formation, and the Quantou formation of the lower Cretaceous and the above strata [27]. The specific sequence of deep strata can be seen in Table 1. Among them, the Yingcheng formation is divided into two members of \(\mathrm{Yc_{3}}\) and \(\mathrm{Yc_{4}}\) based on the typical lithology differences, and the \(\mathrm{Yc_{3}}\) can be further divided into three gas layer formations: \(\mathrm{Yc_{3}I^{1}}\), \(\mathrm{Yc_{3}I^{2}}\), and \(\mathrm{Yc_{3}I^{3}}\), with the gas bearing layer mainly located in the upper \(\mathrm{Yc_{3}I^{1}}\). Analysis reveals that the main reservoir layer for underground storage in depleted gas reserves is the Yingcheng formation, with the gas-bearing layer mainly distributed in the \(\mathrm{Yc_{3}}\)[24]. The specific profile of Block S can be seen in Figure 3. \begin{table}
\begin{tabular}{c c c c c c} \hline \multicolumn{2}{c}{**Stratigraphical Sequence**} & \multicolumn{2}{c}{**Typical Lithology**} & \multicolumn{2}{c}{**Top Surface**} & \multicolumn{2}{c}{**Isotope Age**} \\ \multicolumn{2}{c}{} & \multicolumn{1}{c}{} & **Earthquake** & \multicolumn{1}{c}{**and Paleomagnetic**} \\ \multicolumn{2}{c}{} & \multicolumn{1}{c}{} & **Reflection** & \multicolumn{1}{c}{**Polarity**} \\ \hline \multirow{6}{*}{K1} & Quantou & \multicolumn{2}{c}{Interbedded light gray sandstone with dark purple} & \multirow{2}{*}{T2} & \multirow{2}{*}{} \\  & K1q & & & mudstone and sandy mudstone. & \\ \cline{2-2} \cline{5-6}  & \multirow{2}{*}{Denglouku} & Fourth & \multicolumn{2}{c}{Interbedded light gray sandstone with dark purple} & \multirow{2}{*}{} \\  & K1d & Third & & mudstone and sandy mudstone. & \\ \cline{2-2} \cline{5-6}  & \multirow{2}{*}{K1d} & Third & Grey-white, light grey sandstone, pink sandstone, & \\  & Second & & dark purple shale, and pink sandy shale interlayer. & \\ \cline{2-2} \cline{5-6}  & \multirow{2}{*}{Yingcheng} & Fourth & \multicolumn{2}{c}{Tuffaceous coarse sandstone and mudstone} & \multirow{2}{*}{} \\  & Kly & Third & & Intermediate-acidic volcanic rocks and trifaceous & \\ \cline{2-2} \cline{5-6}  & \multirow{2}{*}{Shahezi} & Upper & \multicolumn{1}{c}{} & \multirow{2}{*}{} \\  & K1 sh & Lower & & & \\ \hline \multirow{2}{*}{J2} & Huoshiling & \multicolumn{2}{c}{} & \multirow{2}{*}{} \\  & J2h & & & & \\ \hline \multicolumn{2}{c}{} & \multicolumn{1}{c}{} & \multirow{2}{*}{} & \multirow{2}{*}{} \\ \end{tabular}
\end{table} 
Table 1: Deep stratigraphic sequence of the block S.  
Figure 2: The geological maps of Block S.  
Regarding the development of faults, a total of four major faults have developed in the underground storage in the depleted gas reserve, starting from the deposition period of the Huoshiling formation and continuing until the end of the deposition of the Yingcheng formation [29]. These four faults control the formation and evolution of the strata deposition and the development of uplifts in this area. During the Yingcheng formation period, the activity of the fault weakened, and the faults have the characteristics of being fewer in number and shorter in extension.