## 3 Samples and Methods  
Sixteen lacustrine shale samples were obtained from K\({}_{2}\)n\({}^{1+2}\) with a depth range of 972.26-1128.17 m in the south borehole of SK-1 (Figure 1), and scanning electron micro-scopy, nitrogen adsorption, X-ray diffraction (XRD), and rock pyrolysis analysis were conducted. Field emission scanning electron microscope (FE-SEM) was used to observe the 2D structure of the pores. Before the samples were observed under the microscope, argon ion polishing technology (Ar-BIB) was used to etch the surface of the samples (Leica RES102 Ion Milling System of Leica company, Wetzlar, Germany). The cross-sections of the samples were bombarded by three independent argon ion beams under a voltage of 7.0 kV and current of 2.6 mA. The polishing time was 5-7 h for each sample. The polished surface was crescent-shaped with an area of about 2 mm\({}^{2}\). For SEM analysis (Apreo FE-SEM of FEI company, Hillsboro, USA, 5.0 kV, 0.40 nA), the samples were treated by carbon plating to make the surface conductive. Then the samples were placed in the electron microscope instrument, the distance between the SEM lens and the top of the sample table was adjusted to about 4 mm, and the backscattering mode (BSE) was used to observe the rock surface. Using a high-resolution electron microscope, the distribution of the organic matter, pore size, shape, contact relationship between the pores and organic matter was clearly observed. At the same time, the plane distribution of inorganic minerals, such as pyrite, and their contact relationship with the organic matter and pores were also observed [54; 55; 56] (Figure 2). Low-pressure N\({}_{2}\) adsorption is an effective method to analyze the nanopore of shale [57; 58; 59; 60; 61; 62]. The samples were ground to a 40-80 mesh powder (~178-425 um particle size) and dried at 110 \({}^{\circ}\)C for 8 h to remove any adsorbed moisture and volatiles. An  
Figure 2: Pore types of shales by SEM in K\({}_{2}\)n\({}^{1+2}\), Songliao Basin. (**a**) Mudstone sample at 1082.4 m; (**b**) shale sample at 978.2 m; (**c**) mudstone sample at 1052.1 m; (**d**) mudstone sample at 1127.8 m.  
N\({}_{2}\) adsorption experiment was carried out on the ultra-high performance automatic gas adsorption system ASAP 2020 produced by the Micromeritics Company (Georgia State, USA). The range of the pore size distribution was 1.7 to 300 nm. The equivalent surface areas were determined with the Brunauer-Emmett-Teller (BET) method. The pore volumes were calculated using the density functional theory (DFT) model. The average pore diameters were calculated using the adsorption branch of the Barrette-Joyner-Halenda (BJH) model [54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66]. The whole rock mineral composition of shale was analyzed by an X-ray diffractometer produced by Rigaku Motor Co., Ltd., Tokyo, Japan. The test conditions were carried out with reference to industry standard SY/T 5163-2010. The whole rock mineral analysis was carried out on samples of 300 mesh powder. With reference to the K value of the international standard sample, the mass percentage of each mineral was calculated using MDI JADE software (6.5, Central South University, Hunan, China). The analysis of the total organic carbon content (TOC) was carried out in accordance with the national standard GB/T 19145-2003. The Leco CS230 carbon and sulfur instrument was used to analyze the TOC. In the first step, a sample powder of \(\sim\)10 mg was weighed by electronic balance and put into the porous porcelain crucible (which was heated in a muffle furnace to 1000 \({}^{\circ}\)C for 2 h). In the second step, a sufficient amount of 12.5% HCl was added to the electric heating plate at 60 \({}^{\circ}\)C for 2 h until the reaction was complete. In the third step, the crucible was put into the filter container and washed with distilled water every half an hour to one hour for three days. In the fourth step, the crucible was dried in a furnace at 60 \({}^{\circ}\)C and cooled, and the TOC was determined.