## 4 Results
### Characteristics of Organic Matter (OM)  
Based on its source and origin, the organic matter in source rocks can be categorized into primary and secondary organic matter [27; 68; 69]. Primary organic matter (POM) is the product of the decomposition of depositional organic matter during diagenesis [27; 68; 69]. Secondary solid organic matter includes migrated solid bitumen, which forms through the metamorphism of hydrocarbons generated from primary organic matter and subsequently migrates to adjacent pores. It also includes pyrobitumen, which is insoluble in organic solvents and results from the secondary cracking of oil [68; 69]. In the Yanchang shale, primary organic matter (POM) has various occurrences, including isolated patches, elongated patches distributed along the bedding planes, and organic-clay mineral complexes (Figure 5). The distribution of POM is highly heterogeneous. Elongated POM patches along bedding planes typically contain detrital quartz, feldspar, and clay minerals. Compaction causes the POM to deform and intrude into the interP pores between adjacent rigid particles. Plastic materials such as mica and clay platelets, which surround the organic matter, are also deformed by compaction and are distributed around the OM and rigid particles (Figure 5A,F). Some terrestrial POM exists as discrete particles, with distinct boundaries separating them from detrital minerals (Figure 5B). Additionally, within some POM particles, detrital quartz, feldspar, and clay minerals can also be observed (Figure 5C,F). POM coexisting with clay minerals forms an organic-clay complex formed during sedimentation. The organic matter in these complexes has an amorphous boundary and is dispersed among flocculent, acicular, and hair-like clay platelets (Figure 5D,E). The organic-clay complex primarily occurs in mudstones. Figure 5: Histograms showing the quartz (**A**), feldspar (**B**), clay mineral (**C**), TOC (**D**), \(S_{1}\) (**E**), \(S_{2}\) (**F**), EOM (**G**), HI (**H**) and OSI (**I**) of mudstones, sand-silt intervals, and mudstones with interlayers. Given that the organic maturity of the Yanchang shales is generally less than 1.3% [36; 37; 58; 59; 60; 61; 62; 63; 64; 65], the solid bitumen present in these Yanchang shales is predominantly migrated solid bitumen (MSB), with no significant development of pyrobitumen. The MSBs are primarily found filling interP pores between detrital particles, in dissolution pores of soluble feldspar and siderite, and in mineral cleavage seams. Additionally, MSBs can be observed in intercrystalline pores of clay and pyrite aggregates (Figure 6H-L). MSBs do not have a specific shape. Instead, their morphology is usually consistent with that of the pores that they fill. For example, the boundaries of MSBs filling the interP pores between rigid particles such as quartz and feldspar are mostly straight (Figure 6L,K,L). In contrast, those filling the dissolution pores of feldspar and siderite are mostly dendritic and embayed (Figure 6J,K,L). There are significant differences in the proportion of POM and MSB to the total solid organic matter content between sand-silt intervals and mudstones (Figures 5 and 6). In  
Figure 6: SEM photographs showing the types and occurrences of solid organic matter. (**A**) Primary solid organic matter distributed along the bedding plane, well W3. (**B**) Dispersed POM with no organic pores, well Y5. (**C**) Dispersed POM containing deformed detrital clay minerals, well C3. (**D**) Clay-organic matter complex, well Y5. (**E**) Dispersed POM and clay–organic complex preserved in compaction-resistant pressure shadows, well Y8. (**F**) Dispersed POM with spongy organic pores, well Y8. (**G**) Dispersed POM without organic pores, well W3. (**H**) MSB with spongy OPs filling inter-particle and dissolution pores, well Y12. (**I**) MSB with spongy, bubble OPs, well Y8. (**J**) MSB with spongy OPs filling dissolution pores, well Y8. (**K,L**) MSB with spongy OPs filling inter-particle and dissolution pores, well W3.