###### Abstract
When the solid organic matter content is between 2% and 6%, a connected network of organic matter and organic pores can form in three-dimensional space, which serves as the main pathway for primary migration within source rocks [9; 11; 13; 15; 20; 21; 22; 23]. However, because kerogen is often dispersed within source rocks or coexists with clay minerals, the organic matter is generally non-connected in three-dimensional space when the TOC is less than 6%. Instead, it forms interconnected networks in two-dimensional space when the TOC exceeds 4% [15]. Therefore, it is challenging to explain primary migration in source rocks with a TOC lower than 4% solely through the solid organic matter network. Mature and over-mature organic-rich shales contain not only kerogen but also a significant amount of solid bitumen [20; 21; 22; 23; 25; 27]. Both kerogen and solid bitumen often develop a large number of organic pores [20; 21; 22; 23; 25; 27]. These organic pores are interconnected in three-dimensional space, forming a pore network that is considered an important pathway for primary migration [20; 21; 22; 23; 25; 27]. Within the oil-generating window, the pore types of lacustrine shales are predominantly inorganic pores, which are also considered important pathways for primary migration [25; 36; 37; 38; 39]. A large number of fractures can be observed in organic-rich shales [25; 40; 41; 42; 43; 44; 45; 46], which are often filled with liquid hydrocarbons and solid bitumen [25; 42; 43; 47]. Hydrocarbon inclusions can also be observed within the fillings of some fractures [40; 42; 48; 49]. The formation, growth, and eventual interconnection of micro-fractures to create a micro-fracture network have been observed through oil generation and expulsion modeling [17; 46; 50; 51]. By comparing the changes in matrix permeability and fracture permeability of shale samples before and after oil generation and expulsion experiments, it has been concluded that the fracture network is the most important pathway for primary migration [17; 46; 50; 51]. However, the number, length, and width of fractures in shales are relatively limited, and their total volume is small [13; 41; 52]. [PERSON] et al. [41] noted that micro-fractures in shales can form local fracture networks, but on a larger scale, these networks may be isolated from one another. Moreover, some fractures cemented by calcite and quartz show no signs of oil migration [17,43]. There is a significant difference between the total volume of micro-fractures and the amount of free hydrocarbons retained in source rocks [13,41]. This suggests that, in addition to micro-fractures, there are other pathways for primary migration within source rocks. Numerous organic-lean siltstone, sandstone, carbonate laminae, and interpeds can be observed within organic-rich source rocks [4, 5, 6, 7, 24, 25, 26, 53, 54, 55, 56, 57]. Compared with organic-rich mudstones and shales, these organic-lean intervals typically exhibit larger pore sizes as well as higher porosity and permeability [25, 29, 53, 54, 55, 56, 57]. Additionally, the interfaces between these organic-lean intervals and the organic-rich mudstones often feature a higher density of micro-fractures and enhanced pore connectivity [25, 29, 53, 54, 55, 56, 57]. These characteristics make organic-lean intervals important pathways for the primary migration of hydrocarbons [7, 25, 29, 53, 54, 55, 56, 57]. Though considerable progress has been made, there is still no consensus on the migration pathways and corresponding migration models in organic-rich shale systems. Therefore, the Triassic lacustrine Yanchang shale in the Ordos Basin was selected to further investigate the primary migration pathways and develop corresponding migration models. This shale has reached maturity within the oil window and is the most important source rock in the Mesozoic strata of the Ordos Basin in China [4, 58, 59].