Abstract: To find out the cause of relatively low oil content of the shale and high oil content in dense zone in bakken formation in the Williston basin,is of great significance in order to reveal the characteristics of hydrocarbon expulsion of shale in bakken formation.Through the detailed analysis of basic geological data of petroleum exploration,combined the methods of thin section and geochemistry,the characteristics of hydrocarbon expulsion of shale in bakken formation is discussed.The results are showed as below:high hydrocarbon expulsion efficiency of shale is the root cause of relatively high oil content in the middle dense zone and low oil content of shale.The dismatches between production rate index and organic carbon content,the low ratio of saturated hydrocarbon and aromatic hydrocarbon of soluble organic matter,as well as strong fluorescence of silty dolomite in the middle bakken group,suggest that the shale in bakken formation have happened a lot of hydrocarbon expulsion.Microfracture and kerogen network inducted by overpressure caused by hydrocarbon-generating pressurization of shale constitutes the channel of the oil primary migration,and is also conducive to hydrocarbon expulsion of the shale.