## 5 Discussion  
### The Role of Sand-Silt Intervals in Primary Migration  
As mentioned above, compared with mudstones, sand-silt intervals have higher contents of quartz and feldspar, as well as greater OSI and RMSB values. They also exhibit larger detrital grain sizes and pore diameters, higher porosity, and relatively lower values of TOC, S\({}_{1}\), S\({}_{2}\), and EOM. As shown in Figure 13A, the sand-silt interval is characterized by high contents of feldspar and quartz, as well as large grain sizes. The interP pores between rigid particles and the dissolution pores of feldspar are very well developed, with pore diameters ranging from tens of nanometers to tens of micrometers (Figure 13A,G,H). Additionally, there are a few interP and intraP pores of clay minerals, as well as intercrystalline pores of pyrite. The surface porosity of inorganic pores is approximately 4.3% (Figure 13B). As shown in Figure 13C,D, most of the pores in the sand-silt interval are filled with MSB that hosts organic pores, and most of the MSB is soluble in organic solvents (Figure 13G,H). Additionally, some pores are filled with liquid hydrocarbons (Figure 13E,F). The sand-silt interval has a low content of POM, while the RMSB reaches up to 98.8% (Figure 12B). The MSB forms when the oil generated from kerogen is released, migrates into adjacent pores, and undergoes metamorphism [21]. Therefore, the distribution and abundance of MSB can be used as an indicator for studying the primary migration of hydrocarbons. In Figure 13A, the MSBs and liquid hydrocarbons are oils generated from  
Figure 12: Histograms of porosity. (**A**) Sand-silt intervals. (**B**) Mudstone with siltstone interlayers. (**C**) Mudstones. kerogen in adjacent organic-rich mudstones that have migrated into the inorganic pores of the sand-silt interval, with some undergoing metamorphism to form solid bitumen. The oil saturation index (OSI = [S\({}_{1}\)/TOC] \(\times\) 100) defined by [PERSON] et al. [77] can be used to evaluate the relative content of \"free\" oil, which can overcome the sorption threshold exerted by the organic matters. The OSI serves as an indication of migrated and retained oil in shales [24,77]. As shown in Figure 14, in the Yanchang shale of well Y8, although the S\({}_{1}\), S\({}_{2}\), EOM, and TOC values in the sand-silt intervals are significantly lower than those in the mudstones, the OSI and porosity are relatively higher. This indicates that the \"free\" oil content in the sand-silt intervals is higher than that in the mudstones. In the Yanchang shales, the OSI and R\({}_{\text{MSB}}\) values in the sand-silt intervals are also significantly higher than those in the mudstones (Table 1, Figure 6). Similar characteristics have also been observed in the Kimmeridge Clay Formation in the North Sea [24]. Figure 13: Photographs of pores, organic matter, and liquid hydrocarbons in the sand-silt interval from well Y2. (**A**) SEM image showing inorganic pores and organic matters. (**B**) Image showing POM (red color) and MSB (black color). (**C,D**) SEM images showing MSBs filling inorganic pores; the location of (**C,D**) is shown in (**A**). (**E**) Thin-section image under UV light showing liquid hydrocarbons. (**F**) SEM image showing liquid hydrocarbon filling interP pores. (**G**) SEM image showing inorganic pores and OMs before extraction. (**H**) SEM image showing inorganic pores after extraction. The mudstones adjacent to sand-silt intervals contain more open pores and micro-fractures, which are interconnected to form a pore network [45; 53; 54]. Furthermore, the EOM of the sand-silt intervals have lower sorption affinities and contain more light oil, with fewer polar compounds and asphaltenes [10; 24; 77]. This is attributed to the compositional fractionation by adsorption or migration [10; 78; 79; 80]. Therefore, the sand-silt intervals function as \"micro reservoirs\" within source rocks. Hydrocarbons generated and expelled from kerogen migrate to these sand-silt intervals and accumulate.