Abstract: Immature oil of Gaosheng-Leijia area has characteristics of high density,high colloid asphalt content,low saturated hydrocarbon content,low full fang ratio,high aromatics and non hydrocarbon contents.Study results indicate the immature oil came from special lithologic oil shale,dolomitic mudstone,grey mudstone,calcareous shale and argillaceous dolomite of Sha 4 member under lake bay subfacies environment.These source rocks not only have strong potential of hydrocarbon generation,but also have superior performance of hydrocarbon expulsion,which provides favorable conditions for the formation of immature oil reservoir.