## 5 Discussion
### The Primary Pathways for Hydrocarbon Migration  
The primary migration pathways in source rocks have been extensively investigated across numerous previous studies. A continuous kerogen network or solid bitumen, in  
Figure 14: Vertical section of well Y8 showing lithology, mineral composition, geochemical data, and porosity of Yanchang shale. The yellow color indicates organic-lean sand-silt intervals. combination with kerogen, serve as pathways for primary migration [9; 11; 13; 15; 20; 21; 22; 23]. Other researchers have suggested that micro-fractures or inorganic pores play important roles in primary migration [25; 36; 37; 38; 39; 50; 51]. The interP and intraP pores are well developed in areas where rigid grains with larger grain size and soluble minerals such as feldspar, siderite, calcite are abundant in both the mudstones and sand-silt intervals in the Yanchang shale (Figure 6 HJ-L, Figure 8, Figure 13, and Figure 15A,B,F-K). Dissolution pores can merge with residual primary interP and intraP pores to form interconnected and complex pore networks (Figure 8I, Figure 10F,J,K, Figure 13A,B, and Figure 15A,B,F,G-K). The presence of MSB and liquid hydrocarbons within inorganic pores indicates that these pores serve as one of the important storage spaces and potential migration pathways for hydrocarbons. However, not all portions of the inorganic pore networks are filled with MSB and liquid hydrocarbons (Figure 15E,F), suggesting that the hydrocarbon primary migration within source rocks is heterogeneous. Although OM can locally form interconnected and continuous networks (Figures 5A and 15C,D), its distribution is highly heterogeneous. Moreover, a significant amount of OM is dispersed (Figure 5B,E-G and Figure 15E). OM with abundant organic pores fills inorganic pores, connecting together to form local pore networks. However, the distribution of these networks is also heterogeneous (Figure 10, Figure 13A-D, and Figure 15G). When  
Figure 15: SEM photographs showing organic matter and pore networks in the Yanchang shale. (**A**) Pore networks formed by dominant inorganic pores and a few organic pores, well Y5. (**B**) Inorganic and organic pore networks, located as shown in (**A**). (**C**) Primary organic matters parallel to bedding, well W3. (**D**) Primary organic matters, located as shown in (**C**). (**E**) Some inorganic pores are filled with OM, while others are not filled with OM or liquid hydrocarbons (as indicated by the areas marked with red lines), well Y8. (**F**) The inorganic pore network not filled with OM or liquid hydrocarbons; the location is shown in (**E**). (**G–K**) Local pathway networks composed of organic matter, inorganic pores, and organic pores, well Y8. the TOC is less than 4%, the connectivity of pore networks composed of organic matter and OPs is poor [15]. Therefore, a network composed solely of organic matter and organic pores appears to be insufficient for primary oil migration within source rocks. The heterogeneous distribution of organic and inorganic pores in shale significantly reduces the connectivity of pore networks, resulting in the heterogeneous migration of hydrocarbons. As shown in Figure 15E (red outline), inorganic pores are well developed within the red line and form local inorganic pore networks (Figure 15E,F). However, these inorganic pores are not filled with liquid hydrocarbons and MSB due to their three-dimensional disconnection from other pore networks near kerogens. In other words, hydrocarbons have not migrated to these pore networks. In contrast, the pores in Figure 15H,I appear to have better connectivity in three-dimensional space. Hydrocarbons generated from kerogens migrate into these pores and undergo subsequent thermal metamorphism to form MSBs (Figure 15G-I). The heterogeneous distribution of migrating solid bitumen in shale also indicates that the primary migration of hydrocarbons is selective and non-uniform (Figures 13 and 15). The Yanchang shale contains high-angle fractures, low-angle fractures, and fractures parallel to bedding (Figure 11).