## 4 Discussion  
### Micro Pore Structure Parameters of Various Types of Sandstone  
The microscopic pore parameters of various groups of sandstone differ considerably due to the differences in their sedimentary environment, tectonic stress and burial depth. Figure 7 displays the diagram of the pore structure of the sample. The graph provides an intuitive comparison of the differences and trends of the same pore structure parameters of various samples. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{2}{c}{**Sandstone in Luo’he**} & \multicolumn{3}{c}{**Sandstone in Zhi’luo**} & \multicolumn{3}{c}{**Sandstone in Yan’an**} \\  & **Formation** & \multicolumn{3}{c}{**Formation**} & \multicolumn{3}{c}{**Formation**} \\ \hline ID & L & Average & ID & L & Average & ID & L & Average \\
1 & 0.986 & & 7 & 0.407 & & 13 & 0.097 & \\
2 & 0.895 & & 8 & 0.467 & & 14 & 0.131 & \\
3 & 0.895 & & 9 & 0.397 & & 15 & 0.055 & \\
4 & 0.909 & & 10 & 0.436 & & 16 & 0.110 & \\
5 & 0.904 & & 11 & 0.408 & & 17 & 0.011 & \\
6 & 0.915 & & 12 & 0.425 & & 18 & 0.052 & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Statistics of the calculation results of the sandstone water-bearing capacity index. Figure 7: Point-line plot and radar chart of characteristic parameters of the pore structure test. (**a**) Pore structure parameters of _PP_; (**b**) Pore structure parameters of _APD_; (**c**) Pore structure parameters of _ASS_; (**d**) Pore structure parameters of _ASC_; (**e**) Pore structure parameters of _THC_; (**f**) The radar chart of pore structure characteristic parameters. Figure 7a reveals that the PP of various formation samples exhibits obvious step changes, which indicates that the sedimentary environments of the Luo'he, Zhi'luo and Yan'an formations differ considerably, and their pore content is different. In all the formations, the coefficient of variation of PP of different samples is 0.1599 at most, indicating that PP of different samples is stable in the same stratum. As described in Figure 7b, the APD of Luo'he sandstone is considerably larger than that of the Zhi'luo and Yan'an sandstone, whereas the APD of Zhi'luo and Yan'an sandstone is similar. Thus, the pore connectivity of the Luo'he formation is considerably superior to that of the Zhi'luo and Yan'an formations. In all the formations, the coefficient of variation of APD of different samples is 0.074 at most; APD fluctuations in the same formation do not exceed 8 \(\upmu\)m.  
Figure 7c illustrates that the ASS of the sandstone in the Luo'he formation differs less from that of the sandstone in the Zhi'luo formation, whereas the ASS of Yan'an sandstone is considerably smaller than Luo'he and Zhi'luo sandstone. The particles of the Luo'he and Zhi'luo formations are rough, with poor roundness and strong adsorption capacity. In all formations, the coefficient of variation of ASS of different samples is 0.093 at most; ASS fluctuations in the same formation do not exceed 0.13 \(\upmu\)m\({}^{-1}\). As displayed in Figure 7d, the PSC of the sandstone in the Luo'he formation is large. The PSC between Zhi'luo and Yan'an sandstone has a few differences. The sandstone of the Luo'he formation exhibits superior seepage capacity compared with that of the Zhi'luo and Yan'an formations. In the same formation, the coefficient of variation of PSC of different samples is no more than 0.0456, indicating that the PSC of the same layer is similar. Figure 7e displays the variation trend of the THC, which is similar to the variation trend of Figure 7a. The THC of various formation samples exhibits considerable step change. In the same formation, the coefficient of variation of THC of different samples is no more than 0.0682, indicating that the THC of the same layer is similar. The microscopic pore structure parameters of different samples in the same formation are similar, and the microscopic pore parameters of the samples in different formations show large differences.