Abstract: There are two sets of immature to low-mature source rock of the lower members of Nantun and Damoguaihe Formations developed in the Bayanhushu depression.Dark mudstone of the Nantun Formation is the best with effective thickness up to 360 m,while that of the lower member of Damoguaihe Formation is worse with effective thickness up to 172 m and its hydrocarbon-generating capacity is the same as that of the Nantun Formation.Average asphalt "A" content in dark mudstone of the Nantun Formation is 0.279 8%,and its sedimentary facies is saline to semi-saline lacustrine.In biomarker of terpane series,γ-cetane and β-cerotane is higher,and in steroid series rearranged steroid is lower,which marks water being salified.Pr/Ph of saturated hydrocarbon chromatogram is below 1,with pyrolytic IH about 500 mg/g,H/C about 1.3.Type of organic carbon is mainly type Ⅱ₁.All reaches immature oil-generating condition.Thermal evolution of source rock is commonly lower,R_o is mostly less than 0.55%,which is just under the late-stage of immature to the early stage of low-mature.However,expulsion capacity of hydrocarbon is relatively lower.Generating capacity of immature oil in the Nantun Formation is generally (1～4.5)×10⁶t/km²,capable of forming middle-small size immature oil field.