## 4 Discussion
### Classification and Quantitative Evaluation of Pore Permeability
#### 4.1.4 Comprehensive Classification and Quantitative Evaluation  
Summarizing the classification results of the above categories and subcategories in this section, the comprehensive classification and evaluation of the deep-mine sandstone pore structure in the Southwest Shandong coalfield is as follows (Table 9 and Appendix D Tables A5-A7). **Subcategory I\({}_{1}\) Jurassic sandstone** was originally deposited in a stream channel environment and has the characteristics of very good sorting and low clay content. By undergoing moderate compaction and moderate cementation, this sandstone became a weakly consolidated sandstone with medium pores and pore throats; it has the highest primary intergranular pore content (>7%), contains mainly necking pore throats and has a better pore connectivity (Figures 10a and 11a). The sandstone has the largest porosity and the highest permeability. The shapes of the capillary pressure curves show coarse skewness and well-sorted pore throats, and the sandstone has the largest average pore throat radius, the smallest threshold pressure and the smallest pressure at 50% mercury saturation (Figure 12a). \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Cluster Variables**} & \multicolumn{2}{c}{**Subcategory I\({}_{1}\)**} & \multicolumn{2}{c}{**Subcategory I\({}_{2}\)**} & \multirow{2}{*}{**Significance Level of Variables**} \\ \cline{2-2} \cline{4-6}  & **Cluster** & **Cluster** & **Cluster** & **Cluster** & **Cluster** \\  & **Center** & **Range** & **Center** & **Range** & **Variables** \\ \hline K (mD) & 72.12 & \textgreater{}40 & 3.71 & \textless{}40 & 100\% \\ \(\Phi\) (\%) & 20.66 & \textgreater{}20 & 16.82 & \textless{}20 & 100\% \\ \(\text{Rave}\) (\(\upmu\)m) & 4.24 & \textgreater{}3.7 & 1.59 & \textless{}3.7 & 100\% \\ C (\%) & 5.77 & \textgreater{}5 & 4.05 & \textless{}5 & 99.2\% \\ \(\text{S}_{0}\) & 0.78 & \textless{}1.00 & 1.20 & \textgreater{}1.00 & 100\% \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Final cluster center and cluster range of subcategories I\({}_{1}\) and I\({}_{2}\). \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Cluster Variables**} & \multicolumn{2}{c}{**Subcategory II\({}_{1}\)**} & \multicolumn{2}{c}{**Subcategory II\({}_{2}\)**} & \multirow{2}{*}{**Significance Level of Variables**} \\ \cline{2-2} \cline{4-6}  & **Cluster** & **Cluster** & **Cluster** & **Cluster** & **Cluster** \\  & **Center** & **Range** & **Center** & **Range** & **Variables** \\ \hline K (mD) & 0.15 & \textgreater{}0.025 & 0.010 & \textless{}0.025 & 97.8\% \\ \(\Phi\) (\%) & 9.28 & \textgreater{}7 & 5.07 & \textless{}7 & 100\% \\ \(\text{Rave}\) (\(\upmu\)m) & 0.29 & \textgreater{}0.138 & 0.09 & \textless{}0.138 & 99.9\% \\ C (\%) & 1.79 & \textgreater{}1.5 & 0.99 & \textless{}1.5 & 100\% \\ \(\text{S}_{0}\) & 1.55 & \textless{}1.60 & 1.98 & \textgreater{}1.60 & 100\% \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Final cluster center and cluster range of subcategories II\({}_{1}\) and II\({}_{2}\). \begin{table}
\begin{tabular}{l l l l l l} \hline \hline \multirow{2}{*}{**The Representativeness of Indicator**} & \multirow{2}{*}{**Indicators**} & \multicolumn{2}{c}{**Category I**} & \multicolumn{2}{c}{**Category II**} \\ \cline{3-5}  & & **I\({}_{1}\)** & **I\({}_{2}\)** & **II\({}_{1}\)** & **II\({}_{2}\)** \\ \hline Principal indicator & K (mD) & \textgreater{}40 & 0.7–40 & 0.025–0.7 & \textless{}0.025 \\ Comprehensiveness & \(\Phi\) (\%) & \textgreater{}20 & 15–20 & 7–15 & \textless{}7 \\ Similar test & \(\text{Rave}\) (\(\upmu\)m) & \textgreater{}3.7 & 0.7–3.7 & 0.138–0.7 & \textless{}0.138 \\ Diagenesis & C (\%) & \textgreater{}5 & 2.5–5 & 1.5–2.5 & \textless{}1.5 \\ Sedimentation & \(\text{S}_{0}\) & \textless{}1.00 & 1.00–1.40 & 1.40–1.60 & \textgreater{}1.60 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Comprehensive classification of the pore structure of the deep-mine sandstone.