## 3 Results and Analysis
### Pore Structure
#### 3.1.4 Pore Connectivity and Permeability  
To summarize the previous three sections (Sections 3.1.1-3.1.3): primary intergranular pores are common in the Jurassic sandstone (Figure 5a,c); the pores have a 100% probability of appearance in sandstone and comprise 2.5-9.0% of the visual area of sandstone. The Jurassic sandstone is dominated by necking pore throats and has average pore throat radii of 0.74-4.93 \(\upmu\)m; thus, this sandstone has a high connectivity, porosity and permeability (Figure 6a,b), and the correlation between porosity and permeability is strong (Figure 6c). In general, the Jurassic sandstone is a medium- to low-permeability sandstone.  
However, the Permian sandstone, including the Shihezi and Shanxi Formations, has irregular pore shapes and large pore size differences. The primary intergranular pores in the Permian sandstone are much lower in number (Figure 5b,d), have a 10-50% probability of appearance in sandstone, comprise 0.1-3.5% of the visual area of sandstone, and have a large deviation in size; these are some of the reasons for the low porosity and permeability of Permian sandstone (Figure 6a,b). The pore throats in the Permian sandstone are small and fine, with average throat radii of 0.04-0.62 \(\upmu\)m; laminated shape pore throats and curved lamellar pore throats are the main shapes, which is another reason for the poor pore connectivity and low permeability. This is similar to the pore throat characteristics of the Lower Cretaceous Quantou Formation sandstone [30]. An increase in the three types of pores, including intergranular dissolution pores, intragranular dissolution pores and intercrystalline pores, can improve the porosity of sandstone, but most of these pores are dead-end pores (Figure 5e-j), which have a limited contribution to the pore connectivity of the Permian sandstone, so the correlation between porosity and permeability is poor (Figure 6d). Generally, the Permian sandstone is a low- to ultralow-permeability sandstone, with the pore structure characterization of tight sandstones [31; 32].  
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**Sizes of Pore (\(\upmu\)m)** & \multicolumn{3}{c}{**Sizes of Throat (\(\upmu\)m)**} \\ \hline
**Grades of Pore Size** & **Diameter** & **Grades of Throat Size** & **Main Range of Radius of Pore Throat** \\ \hline Large & \textgreater{}50 & Coarse & \textgreater{}5 \\ Medium & 20–50 & Medium & 2–5 \\ Small & 5–20 & Fine & 0.5–2 \\ Micro & \textless{}5 & Micro & \textless{}0.5 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Classification standard of the sizes of pores and pore throats in deep-mine sandstones.  
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**Sizes of Pores and Pore Throats** & **Jurassic** & **Shihezi** & **Shanxi** \\  & **Sandstone** & **Sandstone** & **Sandstone** \\ \hline Max. visual diameter of pores D (\(\upmu\)m) & 150 & 150 & 250 \\ Main distribution range of pores size D (\(\upmu\)m) & 10–50 & 1–50 & 1–50 \\ Max. radius of throats R\({}_{\text{max}}\) (\(\upmu\)m) & 10.21 & 1.17 & 3.55 \\ Aver. radius of throats R\({}_{\text{ave}}\) (\(\upmu\)m) & 0.74–4.93 & 0.13–0.55 & 0.04–0.62 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Sizes of pores and pore throats in deep-mine sandstone.