Abstract: In 2015, it achieved an important breakthrough for the petroleum exploration in Weixi exploration area of SINOPEC. Weizhou Oilfield located in the east slope of Weixinan Sag was discovered.In order to reveal the cause of crude oil, Geochemistry and Geology methods have been used to study the genesis and source of crude oil in this paper. The analysis by GC and GC-MS shows that the crude oil is mainly composed of alkanes distributed smoothly with low carbon number. The crude oil is also characterized by high degree of sterane isomerization, rich in C₃₀4-methyl steranes and low molecular steranes. The abundance of C₂₇, C₂₈, C₂₉-regular steranes of the crude oil samples show a "L" or "V" shape distribution. The correlation between oil-oil and oil-source rock shows that the crude oil of Weizhou Oilfield has a genetic relationship with the high-quality deep to semi-deep lacustrine source rocks of Liushagang Formation (El₂). The crude oil maturity parameters, C₂₉ αββ/(αββ+ααα) -sterane and C₃₁22S/(S+R)-homohopane, indicate that the crude oil has reached the matured stage. The maturity of the crude oil is equivalent to the thermal evolution of source rocks of Liushagang Formation in D sub-sag,estimated to be 0.79~0.97 by the parameter MPI1. Based on the above comprehensive analysis, the crude oil of Weizhou Oilfield is characterized by typical lacustrine facies, mainly from the El₂ source rock in D sub-sag of Weixinan.