## 4 Results  
### Petrology and Geochemistry  
In the outcrop sections of the Yanchang shale, sand-silt intervals with even, wavy, vein-like, and lenticular shapes can be observed (Figure 2A-E). The even sand-silt intervals typically consist of a single layer or a group of nearly parallel laminations, which can extend laterally for several meters to tens of meters in the direction parallel to the bedding. The thickness of sand-silt intervals observed in outcrops ranges from 0.1 cm to 20 cm, with a maximum thickness of up to 1.5 m. Sand-silt intervals with thicknesses less than 10 cm account for approximately 87.0% of the total sand-silt intervals (N = 656), while those with thicknesses less than 2 cm account for 67% of the total. The thickness of sand-silt intervals observed in core samples primarily ranges from 0.05 cm to 10 cm, with a maximum thickness up to 1.5 m. Sand-silt intervals with thicknesses less than 3 cm account for approximately 85.5% of the total sand-silt intervals (N = 6336), while those with thicknesses less than 1 cm account for 75.4% of the total (Figure 3A). Sand-silt intervals thicker than 5 cm account for approximately 10.2% of the total. The frequency of these intervals ranges from 4 to 32, with an average of 15, and up to 170 laminae per meter (Figure 3B). There are differences in petrology and mineral composition between sand-silt intervals and mudstones (Figure 2F-N, Table 1). The grain size of sand-silt intervals ranges from 5 to 250 \(\upmu\)m, with an average of 45.1 \(\upmu\)m and a median grain size of 37.2 \(\upmu\)m (Figure 4A). In contrast, the grain size of mudstones is smaller than 20 \(\upmu\)m, with an average grain size of about 8.8 \(\upmu\)m (Figure 4B). Compared to mudstones, sand-silt intervals have higher contents of quartz (13.7% to 67.4%, averaging 32.9%) and feldspar (9.0% to 51.0%, averaging 31.4%)  
Figure 3: Histograms showing the thickness (**A**) and frequency (**B**) of sand-silt intervals. Figure 2: Photographs of sand-silt intervals and organic-rich intervals in Yanchang shale. (**A**) Sand-silt intervals in Yanchang shale of outcrop in Tongchuan. (**B**) The sand-silt intervals (yellow) in section H-H\({}^{\prime}\), with the orange areas representing tuff and the dark grey areas representing mudstone; location is shown in (**A**). (**C–E**) Core photographs displaying even, lens-shaped, and wavy silty laminae and intervals, well Y10. (**F–H**) Thin-section image of sand-silt intervals and organic-rich intervals in Yanchang shale, well Y11. (**J,J**) Thin-section images of organic-rich intervals. (**K–N**). The white arrows indicate the sand-silt intervals. Images (**F,G,J,K,M**) are under transmitted light, while (**H,J,L,N**) are under cross-polarized light. and relatively lower clay mineral content (10.9% to 55.8%, averaging 26.5%). Mudstones contain 7.3% to 45.0% quartz, with an average of 22.4%, and 7.0% to 43.9% of feldspar (averaging 20.5%). Meanwhile, the clay mineral content in mudstones varies from 13.9% to 74.4% (averaging 48.3%). The contents of feldspar, quartz, and clay minerals in mudstones with interlayers are intermediate between those of sand-silt intervals and mudstones (Table 1). Compared to mudstones, sand-silt intervals have relatively lower contents of TOC, S\({}_{1}\), S\({}_{2}\), HI, and extractable organic matter (EOM) but a relatively higher oil saturation index (OSI = [S\({}_{1}\)/TOC] \(\times\) 100; see Table 1). The TOC in sand-silt intervals varies from 0.39% to 7.36%, with an average of 2.32%, which is lower than that of mudstones (ranging from 1.37% to 20.27% and averaging 5.86%). Similarly, the S\({}_{1}\) and S\({}_{2}\) values in sand-silt intervals range from 0.17% to 6.86% (averaging 2.29%) and from 0.25% to 15.22% (averaging 4.44%), respectively. These values are lower than those of mudstones, which range from 1.24% to 13.09% (averaging 4.37%) and 0.39% to 48.29% (averaging 11.52%), respectively.