Abstract: Based on the data of biomarkers, sporopollen and algae assemblages and carbon isotopes of source rocks, combined with the study of sedimentary background, the geochemical characteristics of source rocks in different sedimentary facies zones and their contributions to the formation of hydrocarbon reservoirs in the Baiyun Sag are discussed in this paper. The results show that the abundance of organic matter in the middle and deep lacustrine source rocks of the Wenchang Formation is on a medium level, dominated by the organic matter of type Ⅱ₁ coming from algae and aquatic organisms, and characterized by the lightest δ¹³C value of kerogen less than −28‰. However, the organic matters of the shallow lacustrine source rocks of the Enping and Wenchang Formations are from terrestrial higher plants, of which the δ¹³C value of kerogen is about −27‰, and the abundance of organic matter is on medium to high levels, dominated by type Ⅱ₂-Ⅲ of organic matter. The transgressive lacustrine source rocks of the Enping Formation are rich in marine limbic algae and Triaryl dinoflagellate sterane, and the δ¹³C value of kerogen is about −25‰. The δ¹³C value of the crude oil in the Baiyun Sag is changed within the range of −28‰～−27‰. It suggests that the crude oil of the Baiyun Sag mainly comes from the shallow lake source rocks of Wenchang and Enping Formations and the distribution of crude oil types is controlled by sedimentary facies. The deep lake source rocks of Wenchang Formation are the main hydrocarbon contributor to the wells near the sag, while the transgressive source rocks of Enping Formation make little contribution to the formation of reservoir.