Abstract: Based on core, wireline logging, mud logging, organic geochemical and production testing data, the main controlling factors of hydrocarbon accumulaiton in the Chang 81 reservoir of Yanchang Formation in the Honghe Oilfield were analyzed from the aspects of source rock, reservoir, cap rock and source-reservoir assemblage. The results show that lithologic oil reservoirs are dominant in the study area. The principal source rock is acted by the Zhangjiatan shale at the bottom of Chang 7 formation, where Type I organic matter is predominant with TOC of about 15%. This widely distributed source rock provides both material basis and migration dynamics for the formation of oil reservoirs. The Chang 8₁ reservoir in the study area is of low porosity and ultra-low permeability, and its physical properties are controlled jointly by sedimentation, diagenesis and micro-fracture reworking, which affect its oil-bearing potential accordingly. The thick mudstones in the mid-upper part of Chang 7 formation act as the physical and overpressure sealing cap rocks, providing good preservation conditions. The Zhangjiatan shale at the bottom of Chang 7 formation is in extensively direct contact with the Chang 8₁ reservoir, thus hydrocarbons are supplied in the shape of plane, which is the key to the hydrocarbon accumulation in the Chang 81 reservoir.

Key words: main controlling factors of hydrocarbon accumulation, Chang 8₁ reservoir, Honghe Oilfield, Ordos Basin