## 4 Discussion
### Inspirations for Multi-Superimposed Gas System  
The physical properties of rocks determine their role as a reservoir, interlayers, or source rock in complex gas-bearing systems. The microscopic pore structure controls the physical properties of the sample, and the heterogeneity of the sample affects its physical properties. Therefore, pore structure data can provide important information for exploring the formation mechanism of multi-superposed gas systems. The following information can be obtained from the study on the pore structure characteristics of coal, shale, and tight sandstone in coal measure strata (Figure 14):  
1. In terms of pore development degree, tight sandstone samples have the most developed pores and more reservoir space than shale and coal at nano- and micrometer scales. The pores of coal samples are more developed than those of shale samples. 2. In terms of the pore development scale, the pores of shale and coal are relatively developed in the nanoscale, and the pores of tight sandstone are relatively developed in the macropore and mesopore size ranges. It should be noted that the pores of shale samples are poorly developed in the range of mesopores, which shows the connectivity of intersecting pore systems. 3. In terms of pore development type, the nanopores developed in coal are mainly composed of organic matter pores with strong adsorption and storage capacity. The shale pore system consists of organic matter pores and mineral pores, but the pore development is poor and the structure is complex. The pores of tight sandstone are mainly composed of mineral pores, and the reservoir space is better developed, but the adsorption is relatively poor. 4. In terms of pore connectivity, the shale pore system has the worst connectivity, followed by coal, and the tight sandstone pore system has the best connectivity. 5. In terms of stability and heterogeneity of pore system structure, shale samples from different intervals have the most similar overall pore characteristics, with good stability and relatively weak heterogeneity between intervals and samples. Coal samples take second place, and the heterogeneity between the dense sandstone samples is the strongest. In transitional coal-bearing strata, the coal seam, organic-rich mudstone or shale, and tight sandstone reservoirs are interbedded, forming an unconventional gas-bearing system [16; 51; 52]. Pore development and network characteristics greatly influence the roles of different reservoirs in strata. OM pores are proven to be the best microreservoir space in unconventional reservoirs, with strong aerophilic and adsorption capacity. Therefore, the predominant pore type of OM pores and well-developed nanoscale reservoir space in high-rank coal reservoirs are beneficial to the gas adsorption and storage in strata. It is notable that there are special mechanical properties in high-rank coal reservoirs, and its pore-fracture-cleat structure with mesopores and micropores is not developed as well as nanopores and cleats. Shale reservoirs with more complex material compositions have similar pore network characteristics in the aspect that they have well-developed nanopores and are undeveloped in the pore size range of mesopores. However, the fracture scale reservoir space is observed less developed than coal, which limits the flow capacity through shale. Because of the undeveloped mesopores in coal and shale, the reservoirs lack seepage channels between nanopores and fractures. Thus, high-rank coal is the major source rock  
Figure 14: The systematic comparison of reservoir space of high-rank coal, shale, and tight reservoirs at the nanoscale to fracture scale. (**a–e**) Representative pore types of pores with different size range. with strong adsorption and storage capacity and well-developed shale is the separating layer of different pressure systems in strata with relatively lower storage capacity. Tight sandstone is different from coal and shale. Due to the lack of OM, tight sandstone is not the source rock in strata.