## 4 Results and Discussion
### Characteristics of the Shales' Pore Structures  
We quantitatively characterized the pore structure of the Dalong siliceous shale by low-temperature CO2 and N2 adsorption tests. Sampling depths and measured pore structure parameters are listed in Table 2, and CO2 and N2 adsorption isotherms of selected samples are presented in Figure 10.  
The CO2 adsorption isotherms of all samples rose sharply as the relative pressure (\(P/P_{0}\)) increased from 0 to 0.03 (Figure 10a), indicating the strong CO2 adsorption capacity of the Dalong shales, with maximum adsorption amounts of 1.52-3.53 cm3/g (averaging 2.26 cm3/g). These experiments were primarily conducted to determine the structural parameters of shale micropores with diameters <1.5 nm. Non-local density functional theory (NLDFT) model calculations showed that the micropores (diameter <2 nm) accounted for volumes of 3.58-11.85 \(\times\) 10\({}^{-3}\) cm3/g (averaging 7.31 \(\times\) 10\({}^{-3}\) cm3/g) in the different shale samples. Micropore-specific surface areas were 11.87-37.89 m2/g (averaging 23.4 m2/g) (Table 2). Pore volume increase and cumulative pore volume distribution  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample No. **} & **Depth** & **Average Pore** & \multicolumn{4}{c}{**Pore Volume (cm3/g)**} & \multicolumn{4}{c}{**Specific Surface Area (m2/g)**} \\ \cline{3-8}  & **(m)** & **Size (nm)** & **Micropore** & **Mesopore** & **Macropore** & **Micropore** & **Mesopore** & **Macropore** \\ \hline dl-1 & 1215.9 & 3.697 & 0.00358 & 0.01576 & 0.00247 & 11.873 & 11.085 & 0.106 \\ dl-2 & 1222.5 & 3.787 & 0.00725 & 0.02459 & 0.00171 & 23.827 & 16.426 & 0.073 \\ dl-3 & 1222.9 & 4.441 & 0.00516 & 0.01595 & 0.00185 plots (Figure 11a,b, respectively) demonstrate three dominant intervals of the pore size distribution at 0.3 nm, 0.4-0.7 nm, and 0.8 nm. The N\({}_{2}\) adsorption-desorption isotherms showed that N\({}_{2}\) adsorption gradually increased with increasing pressure. At \(P/P_{0}\) > 0.45, the desorption curves of all samples exhibited desorption hysteresis; the desorption curves were separated from the adsorption curves to form obvious hysteresis loops (Figure 10b). The hysteresis loops of the Dalong Formation shale samples had narrow openings, similar to type-H4 hysteresis loops. They also exhibited type-H3 characteristics because the adsorption curves rose slowly at \(P/P_{0}\) < 0.9 and then rapidly at \(P/P_{0}\) = 0.9-1.0, suggesting that the Dalong Formation shales contain a large proportion of slit pores and few parallel-plate pores. The shale samples had a strong N\({}_{2}\) adsorption capacity, reaching maxima of 14.39-28.91 cm\({}^{3}\)/g (averaging 21.35 cm\({}^{3}\)/g) (Figure 10b). The N\({}_{2}\) adsorption experiments were primarily conducted  
Figure 11: Pore size distributions were obtained by **(a,b)** CO\({}_{2}\) and **(c,d)** N\({}_{2}\) adsorption tests of the Dalong Formation shale samples from well ED-2. Correlations are shown between pore size and **(a,c)** pore volume increase and **(b,d)** Cumulative pore volume. Different curves (colors) represent different samples. Figure 10: (a) CO\({}_{2}\) **(b)** N\({}_{2}\) adsorption-desorption isotherms of Dalong Formation shale samples from well ED-2. to determine the structural parameters of mesopores with diameters >1.5 nm; our BJH model calculations showed that mesopores of 2-50 nm diameter accounted for volumes of 15.8-28.2 \(\times\) 10\({}^{-3}\) cm\({}^{3}\)/g (averaging 21.4 \(\times\) 10\({}^{-3}\) cm\({}^{3}\)/g) and specific surface areas of 10.05-20.83 m\({}^{2}\)/g (averaging 14.75 m\({}^{2}\)/g), whereas large mesopores with diameters >50 nm accounted for volumes of 1.7-4.0 \(\times\) 10\({}^{-3}\) cm\({}^{3}\)/g (averaging 2.4 \(\times\) 10\({}^{-3}\) cm\({}^{3}\)/g) and specific surface areas of 0.07-0.18 m\({}^{2}\)/g (averaging 0.1 m\({}^{2}\)/g) (Table 2).