## 3 Results and Analysis
### Pore Structure  
#### 3.1.1 Pore Types  
According to the occurrence, the pore types of all the deep-mine sandstone samples collected from the Southwest Shandong coalfield can be divided into four categories: intergranular pores, intragranular pores, intercrystalline pores and microcracks. Intergranular pores are an important pore type that affects the grouting of deep-mine sandstone, and they can be divided into two subcategories--primary pores and secondary pores--according to their genesis. Both subcategories of intergranular pores can be further divided into two subcategories according to their occurrence and origin. The types and contents of intergranular pores in the three layers of sandstone are shown in Table 1, and the types and appearance probabilities of the intergranular pores in the three layers of sandstone are shown in Figure 4. The pores and pore throats of some samples are shown in Figure 5.  
Figure 5: _Cont_. The primary intergranular pores (Figure 5a,c) in the Jurassic sandstone are the most important pore type that affect the grouting performance; they are largely preserved, with a 100% probability of appearance, as shown in the dark blue bar in Figure 4, and comprise 2.5-9.0% of the sandstone visual area, as shown in the bold dark blue numbers in Table 1. The other types of intergranular pores in the Jurassic sandstone are rare, with only a 5.9-29.4% probability of appearance, and they comprise less than 0.5% of the sandstone visual area. In contrast, the primary intergranular pores (Figure 5b,d) have a much lower  
Figure 5: The types of pores and pore throats in the deep-mine sandstone: (**a**) Jurassic sandstone, plane polarized light (PPL). (**b**) Permian sandstone, PPL. (**c**) Jurassic sandstone, SEM. (**d**) Permian sandstone, SEM. (**e**) Permian sandstones, PPL. (**f**) Permian sandstone, PPL. (**g**) Permian sandstone, PPL. (**h**) Permian sandstone, PPL. (**i**) Permian sandstone, PPL. (**j**) Permian sandstone, SEM. (**k**) Jurassic sandstone, PPL. (**l**) Permian sandstone, PPL. (**m**) Permian sandstone, PPL. (**n**) Jurassic sandstone, PPL. content in the Permian sandstone than in Jurassic sandstone; these pores have only a 10-50% probability of appearing in Permian sandstone, comprise 0.1-3.5% of the sandstone visual area, and have a large deviation in size. The other types of intergranular pores, including residual intergranular pores, intergranular dissolution pores and pores in clay aggregates (Figure 5e,f), are relatively well developed in the Permian sandstone; these pores have a 31.2-90% probability of appearance, as shown in Figure 4, and they comprise 0.1-2.0% of the sandstone visual area, as shown in Table 1. Therefore, the large difference in the content of primary intergranular pores between the Jurassic sandstone and Permian sandstone shows that the diagenetic modification of pore structure in the Permian sandstone is much stronger than that in the Jurassic sandstone, which results in the porosity and permeability of the Permian sandstone being much lower than those of the Jurassic sandstone. The intragranular pores are more developed in the Permian sandstone than in the Jurassic sandstone, as shown in Table 2. The most developed intragranular pores in the three layers of sandstone are intragranular dissolution pores of feldspar (Figure 5g), followed by intragranular dissolution pores of rock debris (Figure 5f) and cleavage pores (Figure 5g). Only a small number of moldic pores (Figure 5i) and pores in broken grains (Figure 5h) were found. The intercrystalline pores (Figure 5j) formed during diagenesis and are chiefly associated with diagenetic clay minerals; these pores are mostly found in the intercrystalline areas of calcareous cement and authigenic clay cement in the three layers of sandstone; they are generally micropores with diameters less than 0.5 um and can be identified only under SEM. Because they are too small to grout, they are not the main object of this study.