Abstract: The source potential of Qingdong sag has not been fully recognized due to low exploration extent. Thus study on the genetic mechanism of immature oil of this sag is important for future exploration. The immature oil of Qingdong sag is mainly discovered on the edges of the sag and the source rocks are believed to be the upper submember of the 4th member of Shahejie Formation. Oilsource correlation shows that immature oil are mainly from immature source rocks of upper submember of the 4th member of Shahejie Formation. On the basis of molecular geochemical and organic petrology analysis, the immature source rocks possesses geological and geochemical condition to generate immature oil, the mechanism of which are as follows: (1) the mudstone/shale of the 4th member of Shahejie Formation has high organic abundance. The organic matters types are sapropelic type(Ⅰ)and sapropelichumic type(Ⅱ1), abundant in hydrogenrich components which are prone to generate hydrocarbons in early period; (2) the symbiotic saline reducing environment of source rocks and evaporates with the strata fluids of high salinity can promote hydrocarbon generating; (3) the soluble organic matters in the source rocks are easy to generate hydrocarbon influenced by low temperature chemical reactions, bacterial activities and catalysis during the early period of evolution. Therefore, more attention should be focused on the immature oil in the future exploration of Qingdong sag.

Key words: immature oil, genetic mechanism, saline reducing environment, soluble organic matter, Qingdong Sag