Abstract: In the Dongying Sag, reservoirs were formed in different pressure systems.The geochemical heterogeneity of hydrocarbons in the reservoir could be affected by hydrocarbon migration and accumulation.In this paper, we investigated the geochemical differences of hydrocarbon between source rocks and reservoir fluids for further understanding of the driving mechanism of hydrocarbons.Our results suggest that in a continental fault basin (or sag) like the Dongyiing Sag, source rocks occur in the Paleogene, the geochemical effects caused by vertical migration are greater than those by lateral migration.The light/heavy saturated hydrocarbons ratios (nC₂₁+nC₂₂)/(nC₂₈+nC₂₉) and the petroleum maturity indicator sterane ratios C₂₉ββ/(αα+ββ) decrease with the migration distance.The balance between migration and preservation resulted from the interaction of dynamics and resistance affects the composition of hydrocarbon, and vice versa.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.