###### Abstract  
The pathways and mechanisms of primary hydrocarbon migration, which are still not well understood, are of great significance for evaluating both conventional and unconventional oil and gas resources, understanding the mechanisms of shale oil retention, and predicting sweet spots. To investigate the petrography, geochemistry, and pore systems of organic-rich mudstones and organic-lean sand-silt intervals in core samples from the Yanchang shale in the Ordos Basin, China, we conducted thin-section observation, X-ray diffraction, Rock-Eval pyrolysis, field emission scanning electron microscopy (FE-SEM), and porosity analysis. Sand-silt intervals are heterogeneously developed within the Yanchang shale. The petrology, mineral composition, geochemistry, type, and content of solid organic matter as well as the pore type, pore size, and porosity of these intervals differ significantly from those of mudstones. Compared with mudstones, sand-silt intervals typically have coarser detrital grain sizes, higher contents of quartz, feldspar, and migrated solid bitumen (MSB), larger pore sizes, higher porosity, and higher oil saturation index (OSI). In contrast, they have lower contents of clay minerals, total organic carbon (TOC), free liquid hydrocarbons (S\({}_{1}\)), and total residual hydrocarbons (S\({}_{2}\)). The sand-silt intervals in the Yanchang shale serve as both pathways for hydrocarbon primary migration and \"micro reservoirs\" for hydrocarbon storage. The interconnected inorganic and organic pore systems, organic matter networks, fractures, and sand-silt intervals form the hydrocarbons' primary migration pathways within the Yanchang shale. A model for the primary migration of hydrocarbons within the Yanchang shale is proposed. s 202520251573920251  
determines the amount and distribution of retained hydrocarbons within the source rocks [4; 5; 6; 7; 8]. Therefore, studying the mechanisms of primary migration is of great significance for the evaluation of both conventional and unconventional oil and gas resources, understanding shale oil retention and enrichment mechanisms, and predicting sweet spots [4; 5; 6; 7; 8]. The study and discussion of the mechanisms of primary migration have a long history. Before this century, most research was primarily focused on the phase, driving forces, and pathways of oil migration, as well as the fractionation of oil components caused by migration and the efficiency of hydrocarbon expulsion [9; 10; 11; 12; 13; 14; 15; 16]. Since the beginning of this century, driven by technological advancements and the in-depth study of organic-rich shales, our understanding of their petrography, petrology, pore systems and structures, migration pathways, and organic-inorganic interactions has become more profound [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. In addition, the physical and chemical fractionation of hydrocarbons during the migration process, the possible primary migration pathways within source rocks revealed by geochemical parameters, the organic-inorganic interactions and their effects on primary migration, and the mechanisms of migration have become hot research topics [4; 5; 6; 7; 30; 31; 32; 33; 34; 35]. Previous studies have confirmed that petroleum migrates within source rocks and is expelled to reservoirs and carrier beds through bulk flow as a separate organic fluid phase [16; 19; 31]. Overpressure generated by compaction and hydrocarbon generation is considered the most important driving force for primary migration [8; 14; 16; 17]. Significant progress has been made in the study of migration pathways and mechanisms of primary migration. However, there are still divergences in understanding these aspects [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. Organic-rich mudstones and shales typically have high solid organic matter content, and some of them contain organic pores [19; 20; 21; 22; 23; 24; 25].