Abstract: The conclusion of "near-source accumulation" of tight oil is inferred based on the reservoir characte-ristics that crude oil is difficult to migrate laterally for a long distance in low-permeability reservoirs, yet there has been no direct geochemical evidence for a long time. Based on the analysis of the characteristics of source rocks and the source differences of oil in the near-source assemblage (Chang 6 member to Chang 8₁ sub-member) of the Tirassic Yanchang Formation in the Ordos Basin, this paper explores the geochemical basis of tight oil near-source accumulation and explains its cause. The results show that the crude oil in the near-source assemblage comes from the in-situ Chang 7 source rocks, and even the oil sources in the adjacent blocks are different. It is difficult for crude oil to undergo large-scale lateral migration in a tight reservoir. The biomarker compounds of Chang 7 source rocks in the Ordos Basin have significant differences in the plane and are distributed in a circularbelt along the sedimentary center. The oil source difference caused by the combination difference ofbiomarker compounds in different facies zones is the fundamental reason for the predecessors' discovery of "near-source accumulation" of tight oil. The "near-source accumulation" of tight oil has changed the previous view that oilsource correlation is a generalization of the whole basin and the source rocks of the same layer, and has important significance for the zoning and fine evaluation of different types of oil and gas resources in a single basin in the future.