## 3 Samples and Experiments
### Experimental Methods  
The LECO CS-200 sulfur-carbon analyzer was used to analyze the TOC of shale samples. The sample with a particle size of 0.075 mm, weighing 0.1 g, was taken. The inorganic carbon component in the sample was removed by diluting hydrochloric acid with a mass fraction of 5%. The treated sample was washed with deionized water and then placed in an oven at 80 \({}^{\circ}\)C for continuous drying for 48 h. Finally, the TOC determination experiment was carried out after the sample had been dried. The macroals and Ro of shale samples were measured using the LEICA DM 4500 P micro-spectrophotometer. The macroals were identified and quantitatively analyzed following the China Petroleum and Natural Gas Industry Standard (SY/T 6414-2014) [32]. The Ro was measured following the China Petroleum and Natural Gas Industry Standard (SY/T 5124-2012) [33].  
The mineral composition was analyzed using a Bruker D8 ADVANCE DaVinci X-ray diffractometer. A sample weighing more than 0.5 g and with a particle size of less than 0.075 mm was taken. The test instrument utilized the Cu target, and the rotation angle ranged from 3\({}^{\circ}\) to 70\({}^{\circ}\). Quantitative analysis was conducted through step scanning at a speed of 4\({}^{\circ}\)/min and an interval of 0.02\({}^{\circ}\). The database for mineral identification comes from the China Petroleum and Natural Gas Industry Standard (SY/T 5163-2018) [34].  
The shale was prepared into a block measuring approximately 1 cm \(\times\) 1 cm \(\times\) 0.3 cm, and the sample surface was polished using the Gatan693 ion polishing instrument. The  
Figure 1: (a) Location map of the study area in the SNCB (modified after [30]); (b) Synthesis histogram of Lower Permian in z well of SNCB (modified after [31]).  
polished samples were then placed in a Quanta 250 FEG high-resolution SEM to observe the pore morphology. The SEM has a resolution of up to 0.8 nm, magnification ranging from 6 to 1 \(\times\) 10\({}^{6}\) times, and an adjustable accelerating voltage range of 0.2 to 30 kV.  
First, approximately 0.5 g of the sample was subjected to high-temperature calcination to eliminate organic matter. Subsequently, around 0.5 mg of the resulting residue was weighed and dissolved in a mixture of HNO\({}_{3}\), HF, and HClO\({}_{4}\) with a ratio of 2:2:1, using a volume of 7.5 mL. After being evaporated to dryness at high temperature, a mixed acid of HNO\({}_{3}\) and HF with a ratio of 1:1 was added and kept at 190 \({}^{\circ}\)C for 48 h. After cooling, the solution was evaporated to obtain a wet salt state and then slowly added with 4 mL of HNO\({}_{3}\) and kept at 170 \({}^{\circ}\)C for 4 h. The dissolved sample was diluted by a factor of 2000 using 2% HNO\({}_{3}\). Finally, the dissolved and diluted sample was tested on an ICP-MS instrument, and the relative error of the analysis data was less than 5%.  
The LTNA experiment utilized the Micromeritics ASAP2460 automatic SSA and pore size analyzer. The shale sample was crushed to 80 mesh, and 1.5 g of the shale sample underwent vacuum degassing at 250 \({}^{\circ}\)C for 12 h to eliminate free water and volatile substances in the sample. The relative pressure range for the experiment was set between 0.0001 and 0.995. The PV, pore size distribution (PSD), and SSA of the samples were calculated using BJH and BET models, respectively.  
Methane adsorption experiments were conducted using the H-Sorb 2600 high-pressure gas isothermal adsorption instrument. The shale samples were crushed to a size of 60-80 mesh, and 1.5 g of samples were vacuumed at 105 \({}^{\circ}\)C for more than 12 h. The experiment utilized the static volume method with a test temperature of 90 \({}^{\circ}\)C and a maximum experimental equilibrium pressure of 12 MPa.