## 6 Conclusions  
In this study, thin-section observation, X-ray diffraction, Rock-Eval pyrolysis, FE-SEM, and porosity analysis were employed to investigate the petrography, geochemistry, pore system and pore size, and porosity of organic-rich shale and organic-lean sand-silt intervals from samples of the Yanchang Formation in the Ordos Basin, China. The following conclusions can be drawn:  
1. The Yanchang shale exhibits heterogeneity in petrology, mineral composition, geochemistry, OM type, and OM content. Compared to organic-rich mudstones, sand-silt intervals typically have coarser grain sizes, higher quartz and feldspar content, higher OSI and RMSB values, and lower clay mineral content, as well as TOC, S1, and S2 values.
2. There are differences in dominant pore type, pore morphology, pore size, and porosity between mudstones and sand-silt intervals. Compared with mudstones, sand-silt intervals develop more inorganic pores with larger pore sizes, which are often filled with hydrocarbons and MSB with organic pores. The median pore size of inorganic and organic pores in sand-silt intervals is approximately 110 nm and 87 nm, respectively, with porosity ranging from 0.83% to 8.37%, averaging 4.32%. In contrast, the median pore size of inorganic and organic pores in mudstones is 40 nm and 65 nm, respectively, with porosity varying between 0.54% and 7.7%, averaging 3.24%.
3. In the Yanchang Formation, high-angle fractures, low-angle fractures, and fractures parallel to bedding are well-developed. High-angle fractures mainly trend in the east-west direction, with the north-south direction being secondary, and their dip angles are generally greater than 75 degrees. These fractures are commonly filled with calcite, solid bitumen, and liquid hydrocarbons.
4. Sand-silt intervals in the Yanchang shale serve as both \"pathways\" and \"micro reservoirs\" within source rock and play an important role in hydrocarbon storage and migration. Fractures with different trends and dips penetrate and connect the inorganic and organic pore networks, organic matter networks, and sand-silt intervals within different parts of the source rock to form pathways for the hydrocarbon primary migration within Yanchang shale. A speculative model for the primary migration of hydrocarbons in the Yanchang shale is proposed.
5. The sand-silt intervals in the Yanchang shale, characterized by higher brittleness and porosity, enhanced pore connectivity, higher OSI values, and improved oil quality, serve as sweet spots for shale oil and gas exploration.  
Methodology, Y.L. and L.Z.; validation, L.Z. and M.C.; investigation, M.C. and J.Y.; data curation, N.L. and Z.C.; writing--original draft, Y.L. and L.Z.; writing--review and editing, X.W. and J.Y.; visualization, J.Y. and Z.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number 42030808.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data are contained within the article.  
**Acknowledgments:** The study would not have been possible without the support from Shanxi Yanchang Petroleum (Group) Corp., Ltd., Xi'an, China. We thank Shaanxi Yanchang Petroleum (Group) Corp., Ltd., for permission to publish this work. We appreciate the anonymous reviewers very much for valuable comments and suggestions that greatly improved the manuscript.  
**Conflicts of Interest:** Author [PERSON] and [PERSON] were employed by the company Research Institute of Shaanxi Yanchang Petroleum (Group) Co., Ltd. Author [PERSON] was employed by the company Geophysical Laboratory, SINOPEC Geophysical Research Institute Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.