Comparing the Pore Networks of Coal, Shale, and Tight Sandstone Reservoirs of Shanxi Formation, Qinshui Basin: Inspirations for Multi-Superimposed Gas Systems in Coal-Bearing Strata  
[PERSON]  
1 Artificial Intelligence Research Institute, China University of Mining and Technology, Xuzhou 221000, China 11 Artificial Intelligence Research Institute, China University of Mining and Technology, Xuzhou 221100, China 11 Artificial Intelligence Research Institute, China University of Mining and Technology, Xuzhou 221100, China 11 Artificial Intelligence Research Institute, China University of Mining and Technology, Xuzhou 221000, China 1 Artificial Intelligence Research Institute, China University of Mining and Technology, Xuzhou 221100, China  
developed in China's major energy basins, which mainly includes coal seams, organic-rich mudstones, shales, and tight sandstones. Hence, a comprehensive gas-bearing system with the joint preservation of several types of unconventional gases is discovered, referred to as a multi-superimposed gas system [2]. CBM and shale gas are the major types of natural gas resources in transitional coal-bearing strata and are stored in a mixed reservoir of a coal-mud-sand structure system [6; 7]. As revealed by [PERSON] (2016), the gas resource in coal-bearing strata of China (without the Northeast Area) is mainly CBM about 16.8 \(\times\) 10\({}^{12}\) m\({}^{3}\), shale gas about 32 \(\times\) 10\({}^{12}\) m\({}^{3}\), and tight gas about 2032 \(\times\) 10\({}^{12}\) m\({}^{3}\)[7]. Now, it is well acknowledged that multi-superimposed gas systems are widely present in coal-bearing strata and that co-exploration and co-exploitation are crucial to increasing the efficiency of gas production [8; 9; 10]. Coal, organic-rich shale, and tight sandstone reservoirs of the terrestrial sedimentary environment are interbedded and deposited in the vertical direction to form different gas-bearing systems, which have a significant influence on the evaluation and co-exploration of gases in coal-bearing strata [9]. The transitional upper carboniferous Shanxi Formation in Qinshui Basin is a typical transitional coal-bearing stratum with multiple unconventional gases stored in coal-mud-sand reservoirs that interbedded and deposited in the stratum [11; 12]. Scholars have tried to use simulation and modeling methods to explore the feasibility of the co-exploration and co-exploitation potential of unconventional natural gas at a macro scale [3; 4; 5; 11; 13; 14]. However, the significantly strong heterogeneity in coal-bearing strata increases the necessity of having a deeper understanding of the gas-bearing system of the reservoir and its complex influencing factors. Previous literature has revealed the particularity of multi-superimposed gas systems in coal-bearing strata, showing the existence of different independent fluid pressure systems due to the combination of interbedded reservoirs with other physical and gas-bearing properties [15; 16]. In coal-bearing strata, the coal is not the only source of gas, and the shale plays multiple roles of being source rock, reservoir, and cap in the process of gas accumulation and preservation [11]. Extensively developed shale with low permeability generally grew as lithological boundaries of different fluid pressure systems in strata, affecting the comprehensive evaluation of gas reservoirs and the design of co-exploration technologies [8]. Further research on the development characteristics of different types of unconventional reservoirs may provide new insights to help better understand the geological mechanism of gas accumulation and to assist reservoir evaluation and prediction for the complex gas-bearing system. Unconventional reservoirs are characterized by low porosity, low permeability, and strong microheterogeneity. The characterization of micro-reservoir space has been a hot topic and focus among related investigations of unconventional natural gas.