## 5 Discussion
### Petroleum Primary Migration Model  
According to the heterogeneous characteristics of sand-silt intervals in the Yanchang shales, and the differences between organic-lean sand-silt intervals and organic-rich mudstones in terms of detrital composition, grain size, organic geochemistry, pore systems and pore size, and porosity, a speculative model for the primary migration of hydrocarbons in the Yanchang shale is proposed (Figure 17). Figure 17: Schematic illustration of hydrocarbon generation, expulsion, and migration model in the Yanchang shale of the Ordos Basin, China. (**A**) Hydrocarbons adsorbed onto kerogen are discharged  
from organic matter and then migrate into the adjacent inorganic pores and micro-fractures. (**B**) As more hydrocarbons are generated and released from the kerogen, they migrate into pores and micro-fractures farther away from the kerogen. Subsequently, the hydrocarbons migrate vertically and laterally into the inorganic pores of sand-silt intervals. (**C**) High-angle fractures are reactivated or newly formed due to overpressure. As a result, hydrocarbons in the pores of mudstones and sand-silt intervals migrate vertically through these fractures into thicker sand intervals, as well as into overlying and underlying reservoirs and carrier beds. (**D**) Episodic hydrocarbon generation and expulsion from kerogen occur, with the hydrocarbons migrating into the pores and micro-fractures of mudstones and sand-silt intervals until hydrocarbon generation and expulsion cease. The Yanchang shale contains heterogeneously distributed organic-lean sand-silt intervals (Figures 2 and 13). These intervals, which have relatively lower POM content, are characterized by well-developed interP, intraP, and dissolution pores. They typically exhibit larger pore diameters and higher porosity. In contrast, organic-rich shales have higher POM content, smaller pore diameters, and lower porosity. Low-angle and high-angle fractures penetrate both the organic-lean sand-silt intervals and the organic-rich mudstones, interconnecting the inorganic pore network, organic pore network, and organic matter network to form migration pathways (Figures 16 and 17). Before organic matter reaches the threshold for hydrocarbon generation, as burial depth increases and temperature rises, the POM forms some derivatives during its transformation into kerogen [81]. These derivatives migrate over ultra-short distances to inorganic pores adjacent to the kerogen and form pre-oil solid bitumen [21], albeit in limited quantities (Figure 17A). Once organic matter reaches the threshold for hydrocarbon generation, the organic matter in organic-rich mudstone begins to generate oil and forms some organic pores. When the amount of oil and gas exceeds the adsorption capacity of kerogen, the excess hydrocarbons are released from the kerogen and migrate into adjacent inorganic pores (Figure 17A). Similarly, a small amount of hydrocarbons generated from kerogen in organic-lean sand-silt intervals also migrates into adjacent inorganic pores. Some liquid hydrocarbons are adsorbed onto POM, MSB, and clay minerals, while others fill the pore space and micro-fractures as free oil. As maturity increases, more hydrocarbons are generated and released from kerogen, while the content of POM decreases due to hydrocarbon generation in the organic-rich mudstones. These hydrocarbons continuously migrate away from the kerogen into inorganic pores through a network composed of inorganic pores, organic matter, and fractures (Figure 17B). Eventually, they migrate through these pathways to the pores and micro-fractures of adjacent sand-silt intervals, which have larger pore diameters and greater porosity and permeability, forming micro-accumulations (Figure 17B). Consequently, the content of liquid hydrocarbons increases in the sand-silt intervals. The sand-silt intervals serve as \"pipelines\" and \"micro reservoirs\" during hydrocarbon primary migration. As the maturity of organic matter further increases, kerogen generates an increasing amount of hydrocarbons.