## 3 Results  
### Pore Types and Genesis  
The genetic types and shape of nanoscale pores were qualitatively studied using FESEM in combination with EDS data. Coal, shale, and tight reservoirs have significantly different pore genesis, especially in the nanoscale. The genetic types of nanopores were identified according to the differences in material compositions, morphological characteristics, and spatial locations of pores. The genetic types of nanoscale pores in high-rank coal are mainly organic matter (OM) pores. Mineral pores are not well-developed because of their low mineral content. However, mineral pores have an important influence on pore networks and micropermeability [21]. At the microscale, the predominate genetic types of OM pores in coal are gas pores, which are similar to OM pores of shale/mudstone in shape and pore size because of the similar thermal process (Figure 4a), but some gas pores were filled with autogenous minerals during the reservoir diagenesis (Figure 4b). Few residual structure pores were observed at the microscale (Figure 4c). At the nanoscale, phyteral residual nanopores and microscopic organic constituents interparticle nanopores with smaller pore sizes (usually \(<\)100 nm) and various shapes, including triangular, slit-shaped, irregular, or quadrilateral shapes, predominate among OM pores in coal (Figure 4d-f). On a molecular scale, OM pores were proved to be metamorphic pores and intermolecular pores using an atomic force microscope (AFM) [21; 48]. Thus, OM pores in coal have different types and formation mechanisms in different scales. Gas pores developed at the microscale have similar characteristics as OM pores in shale. It altered the understanding gained from the previous comparison between coal and shale pores under the insufficient resolution of coal nanoscale pores [35]. Mineral-related pores have relatively larger pore sizes (Figure 4g). Due to the special structure and mechanical properties, cleats and fractures are well-developed in high-rank coals (Figure 4h,i). The size of pores in shale reservoirs was concentrated at the nanoscale. Complex genetic types of nanoscale pores were identified and divided into four major groups including OM pores, clay mineral pores, granular mineral pores, and (micro)fractures (Figure 5). The diameter of gas pores in OM ranges from 10 nm to 500 nm according to image processing results (Figure 5a,b). However, the development degree of OM pores is highly controlled by the OM type and thermal evolution process. In the observation, OM pores are absent in some samples (Figure 5c), showing the strong heterogeneity of pore development degree. The limited development of OM pores in shale increases the proportion of mineral-related pores in the reservoir space. Intergranular pores between lamellar clay crystals, pores between clay-mineral aggregates (Figure 5d,e), and pores on mineralogical edge (pores formed by dissolution or microfractures, Figure 5f,h) are well-developed and make a higher contribution to microscopic reservoir space compared with high-rank coals. Shale is characterized by low porosity and permeability. Thus, microfractures on the mechanical weak surface between different material compositions have exerted an important influence on improving the reservoir's physical characteristics, serving as the seepage channels from nanoscale pores to fractures (Figure 5i). Figure 4: Microreservoir space of nano–micro pores and small-scale fractures in high-rank coal matrix. (**a**) Gas pores in microscale; (**b**) gas pores with autogenous minerals; (**c**) residual structure pores in microscale; (**d,e**) organic constituents interparticle nanopores; (**f**) pore size processing of nanopores; (**g**) mineral pores in coal; (**h**) fractures; (**i**) cleats. The genetic types of pores in tight sandstone are almost all mineral-related pores. There is significant heterogeneity in the physical characteristics of tight sandstone because of the differences in sedimentary structure and material compositions.