Abstract: In order to identify hydrocarbon accumulation mechanism of the lower Es3 in Sanhecun oilfield of Jiyang depression, Bohai Bay basin, by using drilling and biomarker data and based on the research on petroleum sources and migration pathways, and combined with analysis of hydrocarbon accumulation period, the characteristics of hydrocarbon migration and accumulation of the reservoir are understood. The results show that the oil and gas of the lower Es3 in Sanhecun oilfield originated from the matured source rocks of the upper Es4 in Bonan sag. Faults and unconformities are very developed in the study area. Thick framework sand bodies occur in the lower Es3 where faults, unconformity surfaces and framework sand bodies have relatively good hydrocarbon transport abilities; the hydrocarbon accumulation period of the lower Es3 is the depositional period of Ed, during which the hydrocarbon migrated to the sandy conglomerate fans overlain by thick mudstones in Sanhecun sag through a 3-D transport system composed of faults, unconformities and high-permeable sandy conglomerate fans. As a result, the structural-stratigraphic reservoir was formed.