## 1 Introduction  
Due to the rapid growth in energy demand and progress in exploration technologies, shale reservoirs have become a valuable target for the production of natural gas and oil around the world [1; 2; 3]. The matrix of shale reservoirs is tight, with extremely low porosity and permeability, and a large number of nanopores are developed [4; 5]. These nanopores (especially organic matter-associated pores) are the main storage spaces of shale gas [6; 7]. Currently, the burial depth of the commercially developed shale gas reservoirsof the Sichuan Basin (China) is between 500 m and 5000 m, mainly concentrated in areas such as Zhaotong, Weiyuan, Fuling, Luzhou, and Yuxi [8]. After the hydraulic fracturing development of shale gas, a large amount of fracturing fluids will remain in the shale reservoir, causing changes to the pore structure of the shale reservoir and affecting shale gas seepage and development [9; 10; 11]. Therefore, it is of great significance to clarify the pore structure and gas-bearing characteristics of shale reservoirs at different burial depths to understand the occurrence mechanism, flow behavior, and exploitation after the fracturing of shale gas areas. The rapid development of technologies for enhanced petroleum production has led to the continuous need for and improvement of pore structure testing methods for shale reservoirs. Overall, shale pore structure testing methods can be divided into two categories, comprising direct observation methods, such as field emission-scanning electron microscopy (FE-SEM) [12; 13], computed tomography (CT) [14; 15], broad ion-beam scanning electron microscopy (BIB-SEM), and focused ion-beam SEM (FIB-SEM) techniques to analyze the pore networks [16; 17], and fluid introduction techniques such as low-temperature gas (N\({}_{2}\) or CO\({}_{2}\)) physisorption, mercury intrusion porosimetry (MIP) [18; 19], nuclear magnetic resonance (NMR) [20], small-angle neutron scattering (SANS) [21; 22], and synchrotron-sourced small-angle X-ray scattering (SAXS) [23]. Each characterization method can obtain pore structure information within a specific size of scale, due to differences in its principles or data-interpretation models. For example, MIP can obtain connected pore throats that are larger than 3 nm, but the elastic compression of the minerals and organic matter reduces the reliability of test results at high intrusion pressures. Low-pressure gas (N\({}_{2}\) and CO\({}_{2}\)) physisorption mainly obtains information regarding microporous (<2 nm in diameter) and mesoporous (2-50 nm) pores but is greatly influenced by data-interpretation models. FE-SEM imaging has a high resolution but is greatly affected by sample heterogeneity and experimental operators [24]. To obtain objective pore structure characteristics, a splicing technique employing FE-SEM images is gradually being developed, which not only allows for intuitive observations of pore morphology but also combines image-processing techniques to ascertain the porosity and pore size distribution of different pore types [25; 26]. Shale gas has typical in-situ reservoir characteristics and mainly exists in organic-rich matrix pores and fractures, appearing in the form of adsorbed gas and free gas [27]. Gas content is an important indicator for reservoir evaluation and the selection of favorable areas for shale gas exploration, in which the ratio of free gas to adsorbed gas directly affects the productivity, cumulative production, and lifespan of a single gas well [28; 29]. The accurate measurement of free gas and adsorbed gas in shale gas reservoirs is of great significance for the cost-effective exploration and development of shale gas. Nowadays, the mainstream measurement methods for shale gas content can be divided into two categories. The first type is laboratory testing, including isothermal methane adsorption experiments [30] and saturated methane with low temperature (<60 \({}^{\circ}\)C) and pressure (<15 MPa) NMR experiments [31; 32].