## 3 Samples and Methods  
The samples selected in this study are from well Maye-1, Mahu Sag, Junggar Basin, with a depth of 4587-4858 m, belonging to the Fengcheng Formation interval. Well Maye-1 was drilled in 2019, with a maximum oil yield of 50.6 m\({}^{3}\) and an average daily oil output of 22.5 m\({}^{3}\), achieving a major breakthrough in the Fengcheng Formation shale oil [26]. Therefore, it is of great significance to study the Fengcheng Formation shale oil in Mahu Sag, Junggar Basin by selecting samples from the Fengcheng Formation of well Maye-1. In this study, rock samples from different depths of well Maye-1 were first subjected to X-ray diffraction (XRD) experiments to analyze the mineral composition of the study area. The lithofacies were then classified based on grain size and mineral content. Two samples were selected from each lithofacies based on the lithofacies classification results, and were prepared separately for scanning electron microscopy (SEM) observation and nuclear magnetic resonance (NMR) experiments to investigate the development characteristics of pore-fracture systems in the Fengcheng Formation [27]. The specific procedures for each experiment are outlined below:  
Figure 1: Schematic diagram of structural location of Mahu Sag in Junggar Basin. (**a**) The Junggar Basin and its location; (**b**) Location of the study area in the Junggar Basin; (**c**) Division of geological structural units in the research area; (**d**) MY1 well fengcheng Formation Lithological Column. 1. The sample for the X-ray diffraction experiment needs to be ground into a powder sample; the particle size is 320 mesh, the total weight of the powder is about 5 g, and then the powder sample is used for the X-ray diffraction experiment;
2. The samples used for SEM observation first need to be cut into a square with a side length of about 1 cm and a thin slice with a thickness of 2-3 mm. Next, the observation surface is polished by argon ion, and then, the pore and crack characteristics of the samples are observed by using field emission scanning electron microscopy (FE-SEM);
3. The sample for the NMR experiment needs to be cut into a cylinder with a diameter of 2.5 cm and a height of 5 cm, and then the sample is saturated with water. The NMR experiment is carried out on the sample with a saturated water content. Large-field splicing technology is a technology that first obtains multiple consecutive photos with large magnification and small field of view through FE-SEM, and then splices these high-precision small-field photos and assembles them together to obtain large-field high-precision images [28]. In this experiment, a GeminiSEM 300 FE-SEM instrument (Carl Zeiss AG, Oberkochen, Germany) was utilized to observe the pore and crack features on the surface of the sample. Subsequently, suitable regions were selected to obtain wide-field stitched images with dimensions of 500 \(\upmu\)m \(\times\) 500 \(\upmu\)m and a maximum resolution of 10 nm. These images were then used to analyze the characteristics of pores and cracks in the corresponding samples. The theoretical basis of the NMR technique is the NMR relaxation behavior of spin hydrogen nuclei (\({}^{1}\)H) in the fluid contained in rocks under the action of the uniform distribution of a static magnetic field and radio frequency field. Based on this principle, the data of the full aperture inside the sample can be obtained from the measured results when the sample is saturated with water [29]. The experimental equipment used in this NMR analysis test is the RecCore-04 low magnetic field NMR rock sample analyzer (PetroChina Exploration & Development Research Institute, Beijing, China). The full aperture distribution characteristics of the samples were obtained and analyzed by using this instrument. X-ray diffraction (XRD) is a commonly used material structure characterization method.