Abstract: The composition of soluble organic matter in mudstones is complex and the study of composition diffe-rence is not only important to understanding hydrocarbon generation and expulsion processes, but also necessary for us to determine shale oil formation and accumulation mechanisms. Lacustrine shale of the Hetaoyuan Formation in the Biyang Sag of Nanxiang Basin was systematically studied to determine the composition difference of soluble organic matter from different media (including inorganic minerals and kerogen). The results were compared with shale oil and sandstone oil, and their geological and geochemical significance discussed. Results show that, it is mainly because of the similar polarity among various compounds of saturated hydrocarbon that we can hardly distinguish the difference of saturated hydrocarbon component of soluble organic matter in different media, which implies shale oil generation and expulsion processes in source rock. However, there are obvious variations of aromatic composition. The aromatic composition of soluble organic matter in organic medium is similar to shale oil/sandstone oil, but the aromatic composition of soluble organic matter in inorganic medium is different from shale oil/sandstone oil, which reflects the processes of hydrocarbon generation and expulsion. Accordingly, in the primary migration in mudstone, the generated hydrocarbons firstly filled inorganic mineral pores, after achieving the adsorption capacity of mineral particles, then entering into the micro-cracks in source rock and/or sandstone interlayer, finally forming oil and gas enrichment, which was the so-called shale oil.