Abstract: There are three sets of source rocks developed in lower cretaceous Naimen sag, namely Shahai formation, upper part of Jiufotang formation and lower part of Jiufotang formation, among which the organic abundance of source rocks from Shasha formation is higher, the kerogen type is dominated by type Ⅰ and type Ⅱ1, which is in immature–under mature stage and believes to be the middle grading from comprehensive evaluation. The oil generation capacity of the upper Jiufotang formation is poor, while the organic abundance of source rocks in the lower part of Jiufotang formation is high, the kerogen type is dominated by type Ⅰ-Ⅱ1, which is in mature stage and believes to be good source rocks and the main source rock formation in Naiman sag. The source rock is developed in a strong oxygen-free environment, and the parent of organic matter is mixed with algae and supplemented by higher plants.