## 3 Results
### Pore Shape and Connectivity  
Pore shape can be studied by using two approaches: image observation and the N\({}_{2}\) adsorption-desorption curves or mercury injection-withdraw curves (Figure 9) [49]. By observing the SEM images of the sample cross section, specific descriptions of pore shapes of different types can be provided. In shale samples from Shanxi Formation, clay-mineral-related pores are mostly intergranular pores between lamellar clay and have slit-like shapes (Figure 4). OM pores with elliptical shapes or nonspherical shapes are the predominant pore type in coal and an important pore type in shale (Figures 4 and 5). On the contrary, the pores related to minerals present complex and irregular pore morphology in coal and shale (Figures 4 and 5). Thus, pores in coal and shale have some similarities in pore shape, especially the OM pores with similar genetic mechanisms. Reservoir space networks in tight sandstone are greatly different from coal and shale due to the special material compositions and microstructure. Tight sandstone does not contain OM, and material compositions can be significantly divided into two groups of sand-sized mineral particles and clay-sized particles occupying the pore spaces between the sand-sized particles. Pore networks in tight sandstone contain pores and throats. Different types of pores have different predominant shapes. For example, residual interparticle pores (RIPs) usually have a polygonal shape with larger pore sizes, while intercrystalline pores in clay interstitial substance (ICPs) show silt-like shapes with nanoscale pore sizes (Figure 6). Figure 9 shows the corresponding pore shapes of mercury injection-withdraw curves and N\({}_{2}\) adsorption-desorption curves. The mercury injection and withdrawal curves of the samples studied are shown in Figure 10. According to Figure 9a, the shapes of mercury injection and withdrawal curves are influenced by the spatial development characteristics of pores, including the shape and connectivity. The curves of coal, shale, and tight sandstone show huge differences in accumulative pore volume and morphology (Figure 10). Injection and withdrawal curves of shale are more open than coal samples, and therefore, flask pores are well-developed in shale considering there is a correspondence between curve shapes and pore morphology (Figure 10a). The coincidence degree between the mercury injection curve and the mercury withdrawal curve is relatively high in coal samples, showing that open pores are the predominant pores in high-rank coal (Figure 10b). Pore shapes in tight sandstone are complex, and open pores, flask pores, and semi-open pores are all developed there (Figure 10c). It should be noted that coal samples have the highest accumulative pore  
Figure 9: Mercury injection–withdraw curves, N\({}_{2}\) adsorption–desorption curves with their corresponding pore shapes. (**a**) Corresponding pore shapes of mercury injection–withdraw curves. (**b**) Corresponding pore shapes of N\({}_{2}\) adsorption–desorption curves (Red line: adsorption curve; Green line: desorption curve). volume, which is followed by tight sandstone samples. Shale samples have the lowest accumulative pore volume. The microreservoir space in unconventional reservoirs is mainly contributed by transitional pores (10 nm-100 nm) and micropores (<10 nm). Therefore, it is of significance to study the pore shape and connectivity in this size range. N\({}_{2}\) adsorption-desorption curves of shale, coal, and tight sandstone samples are shown in Figure 11. According to Figure 8b, considering the corresponding relationship between adsorption-desorption loop morphology and pore morphology, transitional and micropores can be divided into four groups: cylindrical pores, fracture-shape pores, conical pores, and inkbottle-shape pores.