## 5 Discussion
### Petroleum Primary Migration Model
Compaction and hydrocarbon generation lead to increased overpressure within the source rock, causing some micro-fractures to form or react (Figure 17C). The hydrocarbons accumulated in sand-silt intervals migrate to thicker, more porous and permeable sandstone intervals or to the sandstone reservoirs and carriers at the top or bottom of the source rock. As hydrocarbons are continuously expelled from the source rock, the total amount of hydrocarbons and the pressure decrease, leading to the closure of some fractures. Consequently, the content of liquid hydrocarbon decreases in the sand-silt intervals and mudstones, while the liquid hydrocarbons remaining in the pores are transformed into MSB with bubble-like and spongy organic pores through subsequent thermal metamorphism (Figure 17C). The content of MSB rises in both the sand-silt intervals and mudstones. Subsequently, as the maturity of organic matter continuously increases, the aforementioned processes of hydrocarbon generation and migration recur. Hydrocarbons generated in the late stages of the organic-rich mudstones migrate once more to the adjacent organic-lean sand-silt interval (Figure 17D). Except for the liquid hydrocarbons adsorbed onto POM, MSB, and clay minerals, the others fill the inorganic and organic pores as free oil. This process is repeated until hydrocarbon generation ceases. Several conceptual models for hydrocarbon migration and expulsion have been proposed by comparing the difference of mineralogy, pore structure, and organic geo-chemical characteristics between meter-scale organic-lean intervals and organic-rich mud rock layers [6; 7; 82]. The organic-clean sand-silt intervals at the millimeter to decimeter scales are more common in shale systems. Compared to models proposed by previous studies [6; 7; 82], the primary hydrocarbon migration and expulsion model proposed in this study not only focuses on pore networks and fractures, but also emphasizes the role of these millimeter- to decimeter-scale organic-clean sand-silt intervals. The model presented in this paper is based on observations of cores from 34 wells and 20 outcrops, a large number of samples' mineralogical, geochemical, and porosity data, as well as SEM observations. These sand-silt intervals show distinct differences in petrology, organic geochemistry, and pore structure compared to the organic-rich mudstone layers, which are similar to the findings of other studies [4; 6; 8; 25; 64]. Therefore, the model proposed in this study is representative of the Ordos Basin. The model presented in this study is primarily based on the differences in pore systems, OSI, and MSB between organic-clean sand-silt intervals and organic-rich mudstone layers. However, the distribution and characteristics of OSI and MSB are not only influenced by hydrocarbon migration but are also affected by other factors such as source organic matter (OM) quality, maturity gradients, and adsorption dynamics. Therefore, the model proposed in this study should be regarded as a preliminary conceptual model. If two-dimension nuclear magnetic resonance analysis and gas chromatography-mass spectrometry analysis are used in future studies to compare the free hydrocarbon content, components, and biomarker characteristics of organic-clean sand-silt intervals and organic-rich mud rock layers, it may help to further understand the hydrocarbon migration and expulsion models in shale systems. The model established in this study can also serve as a reference for shales in other basins with sedimentary environments and characteristics similar to the Yanchang shale in the Ordos Basin.