The western wing of Chepaizi Uplift is next to Sikeshu Sag. In the early stage，it was believed that the hydrocarbon in light oil reservoirs in this area were mainly from the Jurassic source rocks. Through comprehensive analysis of the geochemical indicators，such as the biomarker compounds，carbon isotope，maturity of the crude oil discovered in multiple strata and the main source rocks in Sikeshu Sag，it was discovered that the light oil in the study area could be divided into two types，the source of light oil was clarified and the comprehensive criterion for identifying the crude oil was established.The results show that TypeⅠcrude oil mainly distributes in C，J，K，E formation，which has the characteristics of regular sterane C27-C29 in shape of“inverted L”，low levels of γ-cerane，high Pr/Ph，weight carbon isotope and relatively high maturity，and its parent material was formed in the environment of weak oxidization-weak reduction freshwater lake，and the source of organic matter was mainly higher plants，and TypeⅠcrude oil was mainly from the source rock of J1b Formation in Sikeshu Sag. While TypeⅡcrude oil mainly distributes in the N1s with regular sterane C27–C29 in shape of“V”，high levels of γ-cerane，smaller Pr/Ph，lighter carbon isotope and relatively low maturity of crude oil，and its parent material was formed in the environment of strongly reduced brackish water deep lacustrine，and the source of organic matter was mainly planktonic algae，and TypeⅡcrude oil was mainly from the source rock of the Paleogene E2-3a Formation. The research results further confirm that two sets of effective source rocks，Jurassic and paleogene，have been developed in Sikeshu Sag，and the oil and gas generated by them have been transported for a long distance.