The micro-reservoir space in coal, shale, and tight sandstone reservoirs--mainly pores and micro-fractures--is developed at the nanoscale with complex pore structures and poor connectivity [17; 18; 19; 20; 21]. The differences in pore systems, including the type, structure, size distribution, and connectivity, are the direct influencing factors of reservoir permeability and gas-bearing property [21; 22]. Despite the similarity of being unconventional reservoirs with low porosity and permeability, these types of reservoirs differ greatly from each other (Figure 1). Influenced by the sedimentary environment, they vary in material component types and characteristics. This leads to differential diagenesis, which further produces variations in the rock structure, pore system, physical properties, and gas-bearing properties. For such fine-grained sediments and nanoscale pore-microfracture systems, many nanotechnologies have been applied to the physical property testing and stimulation of reservoirs [23]. The difference in material composition and pore structure of different reservoirs in strata is the basis of forming multi-superimposed gas systems. Therefore, it is necessary to improve the mining technology to avoid interaction between systems in order to improve the production of the strata. In view of this, it is of great significance to study and make a comparison of the nanoscale characteristics of different reservoirs in coal-bearing strata. Research on the characterization of nanoscale pores in unconventional reservoirs has increased dramatically since the successfully commercialized exploration of unconventional oil and gas in the North American area [18; 24; 25]. Many achievements have been realized with the progress of nano-scale characterization technologies, including high-pressure mercury injection porosimetry (MIP), gas adsorption methods (low-pressure nitrogen gas adsorption and carbon dioxide adsorption), argon ion polishing-field emission scanning electron microscope (AIP-FESEM), etc. The pore size distributions (PSDs) and pore structures are commonly quantitatively studied by using MIP, NMR, and gas adsorption methods [20; 26; 27]. The pore types, pore shape, pore genesis, spatial distribution, and connectivity are qualitatively observed using high-resolution images or three-dimensional models using FESEM, CT, FIB-SEM, AFM, and other observation methods [26; 27; 28; 29; 30; 31]. Similar methods were also applied to study the pore structure characteristics of coal and tight sandstone reservoirs [19; 32; 33]. Simulation and modeling methods are applied based on experimental data to further reveal the relationship between pore structure and gas behavior [21; 33]. For example, the fractal model, which can be established to study the relationship between pore structure, surface irregularities, and migration of gas, is commonly used as an effective tool to investigate the non-intuitive pore structure in nanoscale using data from MIP, LN\({}_{2}\)GA, or digital image information [34; 35]. As revealed by previous studies, strong fractal characteristics of pores in coal, shale, and tight reservoirs provide a quantitative approach to describing structural features and their influence on gas accumulation and migration [36; 37]. The interconnected reservoirs developed in coal-bearing strata and their influence on fluid pressure systems have raised scholars' attention, but very few studies have been focused on the comparative study of reservoir characteristics. Recently, a few studies have focused on the comparison of different unconventional reservoirs in the aspects of geological conditions [38], reservoir characteristics and gas accumulation [35], flow unit and pore structure [39; 40], and adsorption characteristics [41]. As reservoir pore research has progressed, the evaluation of pores has shifted from focusing on several specific structural characteristics (e.g., porosity, permeability, PSDs, etc.) to the comprehensive characterization of pore systems.