## 4 Discussion
### Classification and Quantitative Evaluation of Pore Permeability
#### 4.1.4 Comprehensive Classification and Quantitative Evaluation
Figure 11: _Cont._  
Figure 10: Comparisons of pores, pore throats and pore connectivities among the four subcategories of sandstone in casting thin slices under a polarizing microscope (note that the deep blue colors are mostly intergranular pores, the light blue colors are mostly intragranular pores and micropores). (**a**) Subcategory I\({}_{1}\) sandstone (Z23). (**b**) Subcategory I\({}_{2}\) sandstone (Z11). (**c**) Subcategory II\({}_{1}\) sandstone (X9). (**d**) Subcategory II\({}_{2}\) sandstone (T13). Figure 11: Comparison of pores, pore throats and pore connectivities among the four subcategories of sandstone under SEM (note that the images (**a–d**) correspond to the images a–d in Figure 10, respectively). (**a**) Subcategory I\({}_{1}\) sandstone (Z23). (**b**) Subcategory I\({}_{2}\) sandstone (Z11). (**c**) Subcategory II\({}_{1}\) sandstone (X9). (**d**) Subcategory II\({}_{2}\) sandstone (T13). Figure 12: _Cont._  
**Subcategory I2 Jurassic sandstone** was originally deposited in a crevasse splay or channel levee environment; it has the characteristics of good sorting and a low clay content. By undergoing moderate compaction and cementation, it became a weakly consolidated sandstone with medium-small pores and fine pore throats; this sandstone has a high primary intergranular pore content (2-7%), contains mainly necking pore throats and secondary laminated shape pore throats, and has a good pore connectivity (Figures 10b and 11b). It has a high porosity and high permeability. The shapes of the capillary pressure curves show slight coarse skewness and well- to moderately sorted pore throats, and the sandstone has a large average pore throat radius, low threshold pressure and low pressure at 50% mercury saturation (Figure 12b). **Subcategory II1 Permian Shihezi and Shanxi Formation sandstones** were originally deposited in stream channel or delta distributary channel environments; they have the characteristics of good sorting and a high clay content. By undergoing strong compaction and strong cementation, these sandstones became a tight sandstone with medium-small pores and fine pore throats or small pores and microthroats; these sandstones have irregularly shaped pores and large differences in pore sizes and thus a large variability in the  
Figure 12: Comparison of the capillary pressure curves of four subcategories of sandstone, showing the differences in the pores, pore throats and their permeability contributions among the four subcategories of sandstone: (**a**) Subcategory I1 sandstone shows a lower left distribution, well-sorted pore throats, coarse throat peaks at 4 to 6 \(\upmu\)m, and a higher peak; nearly all of the pore throats contribute to the permeability. (**b**) Subcategory I2 sandstone shows a slight lower left distribution, well- to moderately sorted pore throats, coarse throat peaks at 1.6 to 4 \(\upmu\)m, and a high peak; most of the pore throats contribute to the permeability. (**c**) Subcategory II1 sandstone shows a slight upper right distribution, moderately to poorly sorted pore throats, coarse throat peaks at 1 to 1.6 \(\upmu\)m, and a moderate to low peak; approximately half of the pore throats contribute to the permeability, but with large differences. (**d**) Subcategory II2 sandstone shows an upper right distribution, poorly sorted pore throats, coarse throat peaks at 0.0063–0.1 \(\upmu\)m, and a lower peak; minute amounts of pore throats contribute to the permeability. distributions of primary intergranular pores (0.5-2%) and laminated shape and curved lamellar pore throats; this results in a poor pore connectivity (Figures 10c and 11c). They have a low porosity and low permeability. The shapes of the capillary pressure curves show slight fine skewness and moderately to poorly sorted pore throats; these sandstones have a large deviation in the distribution of the average pore throat radius, high threshold pressure and high pressure at 50% mercury saturation (Figure 12c).