## 3 Samples and Analytical Methods  
In this study, a total of 54 WF-LMX shale samples were collected from two drilling wells, well DA and well DB (Figure 1B). All the 54 samples were first used to measure TOC content. Samples were crushed into powder (-200 mesh), and approximately 0.1-0.3 g was selected to react with 12.5% HCL in order to remove the inorganic carbon, and finally the residual samples were dried at 60-80 \({}^{\circ}\)C for -12 h. Then, the powder samples were placed in the Elemental Rapid CS analytical instrument and burned with oxygen with a purity of 99.9% at a flow rate of 800 mL/min (\(\pm\) 25). The generated CO\({}_{2}\) volume was used to calculate the TOC content. Thirty representative samples were selected for X-ray diffraction (XRD) bulk mineral composition analysis using the Bruker AXS D8 Advance X-ray diffraction instrument produced by the German company Bruker (Berlin, German). X'Pert Pro DY2198 X-ray diffraction instrument. Samples were crushed into 200 mesh powder, and then placed in the groove of glass sheets. The flatted glass sheet was scanned from 5\({}^{\circ}\) to 80\({}^{\circ}\) (29) at a  
Figure 2: Stratigraphic column of the Ordovician Wufeng–Lower Silurian Longmaxi formations showing lithology, gamma ray (GR) log, density (DEN) log, acoustic transit time (AC) log, and TOC content of wells (**A**) DA and (**B**) DB. Abbreviations: Orodo. = Ordovician, Sy. = System, Se. = Series, Fm. = Formation, Me. = Member, Lith. = Lithology. rate of 2\({}^{\circ}\)/min. The mineral content was semi-quantitatively determined by the difference in the intensity of characteristic diffraction peaks in the X-ray diffraction patterns. Pore structure parameters, including PV, SSA, and pore size distribution, were calculated from low-temperature nitrogen and carbon dioxide gas adsorption experiments that were performed on the Quantachrome Autosorb-iQ3 instrument. Samples were crushed into small particles (60-80 mesh) and then degassed at 110 \({}^{\circ}\)C for 8 h to remove the water and volatile substances. For the low-temperature carbon dioxide adsorption experiments, the temperature was kept at 273.15 k, and the relative pressure was set in the range of 0.0004 to 0.03. For the low-temperature nitrogen adsorption experiments, the temperature was kept at 77.3 k, and the relative pressure was from 0.001 to 0.998. The CO\({}_{2}\) adsorption-desorption isotherms were used to calculate pore volume and pore size distribution using the density functional theory (DFT) model [29]. The N\({}_{2}\) adsorption-desorption isotherms were used to calculate pore volume, pore size distribution, and SSA using the BJH model and BET equation, respectively [30]. The shale sample required for high-resolution field emission scanning electron microscope (FE-SEM) observation was first cut into a plug with the size of 1 cm \(\times\) 1 cm \(\times\) 0.5 cm, and then polished by argon ion-milling techniques to reduce the surface roughness that was created during mechanical polish. Then, the polished samples were carbon-coated and placed under the Zeiss Gemini SEM 500 field emission scanning electron microscope instrument for observation with an accelerating voltage of 2-15 kV and a working distance of 5-10 mm. The Back-scattered electron (BSE) and secondary electron (SE) images were used to describe pores and minerals. The energy dispersive X-ray (EDS) was used to identify minerals, and the cathodoluminescence (CL) technique was used to differentiate detrtial quartz from quartz cements. In this study, the combustion method combined with low-temperature gas adsorption experiments were used to characterize organic pores and inorganic pore, respectively. The samples of 60-80 mesh were combusted in a muffle furnace to remove organic matter. After the removal of organic matter, we assumed that organic pores were removed from the samples, and the remaining pores were mostly inorganic pores. The combustion temperature and time were set to 350 \({}^{\circ}\)C and 30 h, respectively.