## 3 Results and Analysis
### Factors Influencing the Pore Structure
#### 3.2.1 Sedimentation  
Table 5 summarizes the statistics of the main compositional and textural properties of all the sandstone samples. All the Jurassic sandstone samples are well-sorted fine-grained sandstone, and their grain size frequency curve has a single peak and nearly normal distribution (Figure 7a). In contrast, the Permian sandstone samples are heterogranular and are a moderately to well-sorted sandstone, and their grain size frequency curve has a multipeak and nonconcentrated distribution (Figure 7b). Recent studies have shown that the grain size, sorting, shape, and matrix content determine the initial pore space among the sand grains, i.e., the intergranular pore volume of the sediment [33; 34; 35]. Researchers have performed a considerable amount of research on this topic. [PERSON] and [PERSON] [36] studied the relationships among the grain size, sorting and accumulation modes of grains, and their study results show that the porosity of sand depends on the sorting coefficient but is independent of grain size. [PERSON] and [PERSON] [37]  
Figure 6: The comparisons and correlations of porosity and permeability: (**a**) The porosity comparisons of the three layers of sandstone. (**b**) Permeability comparisons of the three layers of sandstone. (**c**) The correlation between porosity and permeability in the Jurassic sandstone. (**d**) The correlation between porosity and permeability in the Permian sandstone. and [PERSON] [38] established a statistical formula that encompasses porosity and the sorting coefficient, as shown in Equation (1). Many researchers [39; 40] have used the formula to estimate the initial porosity of sandstone in a diagenetic study. In this paper, the initial porosity of deep-mine sandstone in the study area was estimated based on the formula given by [PERSON] (Equation (1)). \[\Phi_{\text{o}}=20.91+22.9/S_{o} \tag{1}\]  
where \(\Phi_{\text{o}}\) is the initial porosity and \(S_{o}\) is the Trask sorting coefficient. The result of the calculation is illustrated in Figure 8. Figure 8a shows that the original porosities of sandstone in the Shanxi and Shihezi Formations are similar but lower than those of the Jurassic sandstone, which is related to the good sorting and low matrix content in the Jurassic sandstone and shows that sedimentation directly controls the development of the original pore structure of sandstone. Figure 8b-d shows that the permeability has a moderately positive correlation with the content of grains (diameters greater than 0.125 mm) and a moderately negative correlation with the sorting coefficient and the matrix content, which indicates that sedimentation has an obvious influence on the permeability of sandstone; however, it is not the only influential factor because the R\({}^{2}\) value fitted by the three curves is not very high. In summary, sedimentation is an innate factor affecting the sandstone pore structure and permeability, and the influence of the sorting coefficient, which is the most important sedimentation indicator, has been consistently recognized by many researchers. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline
**Composition and Texture** & **Jurassic Sandstone** & **Shihezi Sandstone** & **Shanxi Sandstone** \\ \hline Grain size & Fine sandstone & Heterogranular sandstone & Heterogranular sandstone \\ \hline Sorting coefficient (S\({}_{\text{o}}\)) & Well & Moderately to well sorted & Moderately to well sorted \\ \hline Clay matrix content (\%) & Mostly 2–5, Seldom 5–12 & 5–12 & 5–12 \\ \hline Cement types & Porous & Porous, Contact & Porous, Contact \\ \hline Grains contact types & Point, Line & Line & Line, Mosaic \\ \hline Textural maturity & Secondary immaturity & Immaturity & Immaturity \\ \hline \end{tabular}
\end{table} 
Table 5: The compositional and textural properties of the three layers of sandstone.