## 3 Results and Analysis
### Factors Influencing the Pore Structure
#### 3.2.2. Diagenesis  
A large number of casting thin slice observations in this study indicate that the main diagenesis types are compaction, cementation and dissolution (Figure 5). Current studies show that the strength of diagenesis can be characterized quantitatively by three diagenesis indicators: the visual compaction rate, visual cementation rate and visual dissolution rate. The three diagenesis indicators are calculated from the casting thin slice observation data following established formulas [35; 39; 41; 42; 43; 44], as shown in Figure 9.  
According to the classification standard of the visual compaction rate, visual cementation rate and visual dissolution rate used by most scholars [43; 45], as shown in Figure 9 (green letters), the Jurassic sandstone has a moderate degree of compaction, a moderate degree of cementation and a weak degree of dissolution, while the Permian Shihezi and Shanxi Formation sandstones have a strong degree of compaction, a strong degree of cementation and a moderate degree of dissolution (Figure 9). These results indicate that the pore structure of the Permian sandstone has been modified more heavily by compaction, cementation and dissolution compared to that of the Jurassic sandstone.  
Figure 8: The effects of sedimentation on the pore structure and permeability of the deep-mine sandstone: (**a**) Initial porosity comparisons of the three layers of sandstone. (**b**) Correlation between the permeability and the content of grains larger than 0.125 mm. (**c**) The correlation between the permeability and sorting coefficient. (**d**) The correlation between the permeability and clay matrix contents.  
Because the visual compaction rate, visual cementation rate and visual dissolution rate represent only one aspect of diagenetic intensity, many scholars have introduced the concept of a comprehensive diagenetic coefficient to characterize diagenetic intensity [35, 43, 44], as shown in Equation (2).  
\[C=\left[\frac{\Phi}{C_{1}+C_{2}+\Phi m}\right]\times 100\% \tag{2}\]  
where \(C\) is the comprehensive diagenetic coefficient, \(\Phi\) is the porosity, \(C_{1}\) is the visual compaction rate, \(C_{2}\) is the visual cementation rate, and \(\Phi_{m}\) is the microporosity rate.  
The effective porosity, compaction strength and cement strength are integrated in Equation (2) to quantitatively characterize the diagenetic intensity. Therefore, the comprehensive diagenetic coefficient can fully and quantitatively reflect the intensity of various diagenetic processes experienced by the sandstone, and the larger the value of the comprehensive diagenetic coefficient, the lower the degree of diagenesis.  
As shown in Figure 9 (orange bars), the comprehensive diagenetic coefficient of the Shihezi Formation sandstone is close to that of the Shanxi Formation sandstone but far smaller than that of the Jurassic sandstone. In other words, the three layers of deep-mine sandstone in the Southwest Shandong coalfield can be clearly divided into two categories by the comprehensive diagenetic coefficient: category I is the Jurassic sandstone and category II is the Permian sandstone.