## 2 Geological Background and Experimental Methods
### Experimental Methods  
The samples for this study were selected from the Long-1 sub layer, and the core samples were prepared in different forms (plug sample, cube sample, powder sample). The plug sample is used for rock electric experiments, the cube sample is used for high-pressure mercury injection and porosity testing, and the powder sample is used for TOC content testing, nitrogen adsorption, X-ray diffraction, methane isothermal adsorption, and other experiments. The basic experimental analysis of TOC content testing, porosity testing, X-ray diffraction, nitrogen adsorption, and high-pressure mercury injection can be found in reference [19], which will not be repeated in this article. The following mainly introduces the resistivity testing experiment, field emission scanning electron microscopy maps observation, and methane isothermal adsorption experiment analysis. Figure 1: Structural location map and comprehensive stratigraphic column chart of the study area. In order to analyze the reasons for the decrease in shale resistivity, this study tested the resistivity under dry state, initial state, and different water saturation states. The resistivity was tested by the LSR-3 Seebeck & Electric Resistance Unit produced by the German company Lindsey, which can test cylindrical samples with a diameter of 5 mm and a length of 25 mm. The static DC method was used for resistivity testing at \(-\)100-500 \({}^{\circ}\)C, with a conductivity range of 0.01 to 2 \(\times\) 10\({}^{5}\) s/cm and an accuracy of \(\pm\)5%. After taking the sample on site, the plug sample (25 \(\times\) 25 mm) immediately marked and the resistivity of the original state under laboratory conditions was measured. Subsequently, the rock sample was saturated under pressure of 25 MPa with a sodium chloride solution with a mineralization degree of 20,000 mg/L, and then subjected to resistivity testing in the saturated state. Finally, samples with different water saturation were obtained by gradually drying at 60 \({}^{\circ}\)C, and electrical resistivity testing was completed. Shale micro-image acquisition used FEI Quanta 650 FEG (Thermo Fisher Scientific Inc., Waltham, MA, USA) field emission scanning electron microscopy. The specific steps are as follows: first, make the tested sample into rectangular blocks of 20 mm \(\times\) 20 mm \(\times\) 10 mm, and then use sandpaper with grit ranging from 600 to 4000 for mechanical polishing; on this basis, argon ion polishing is performed to obtain a smoother surface and improve the observation efficiency of nanopores; Finally, a brief spray coating treatment is applied to the polished sample to increase its conductivity and improve the resolution of scanning electron microscopy image observation. During the observation process, Phenom MAPS module was used to set the image storage quantity to 1024, which can achieve large-area imaging stitching without any trace, facilitating the observation of the microscopic distribution of mineral particles in the shale sample. The methane isothermal adsorption experiment was conducted using the ISOSORP-GAS SC magnetic levitation balance high-pressure isothermal adsorption instrument manufactured by the German company Rubotherm (Herten, Germany). The maximum single-phase testing temperature and pressure of this instrument reach 200 \({}^{\circ}\)C and 70 Mpa, respectively, which can meet the demand for deep shale gas. The isothermal adsorption test temperature for this study is 120 \({}^{\circ}\)C, with 21 pressure points designed at 2.0, 4.0, 6.0, 8.0, 12.0, 14.0, 16.0, 18.0, 20.0, 22.0, 23.9, 26.0, 28.0, 30.0, 34.9, 40.0, 44.9, 49.9, 55.0, and 60.0 MPa, respectively. Using this instrument for isothermal adsorption testing includes four steps: blank testing, sample pretreatment, buoyancy testing, and adsorption testing. Blank testing, sample pretreatment, and buoyancy testing use helium gas with a purity of 99.999%, while adsorption testing uses methane with a purity of 99.999%.