In the Dongying Sag, reservoirs were formed in different pressure systems. In this paper, the authors investigated the geochemical differences of hydrocarbon between source rocks and reservoir fluids for further understanding of the driving mechanism of hydrocarbons. The study of differentiation mechanisms during hydrocarbon migration in the Dongying Sag reveals that the discrepancies in different fluid units and its effects on hydrocarbon geochemical characteristics were recognizable, that is helpful for further study of migration and reservoir-forming modes in different zones.