Recently, 15.1 billion tons of shale oil geological resources were discovered in the Qingshankou Formation of Gulong Sag, Songliao Basin, and this discovery has attracted considerable attention. Thus far, the sedimentary environment and diagenetic process of the Qingshankou Formation in Gulong Sag, particularly the reservoir space and accumulation mechanism of the shale oil, are unclear, which has seriously affected the exploration and development of shale oil. Based on detailed core and thin section observation, thin section analysis, mineral analysis, and geochemical analysis, thin layers and concretions of dolostone in Qingshankou Formation shale in Gulong Sag are studied. Three types of layers and nodules can be seen in the core and thin sections: the first is composed of very pure micrite, powdery calcite, or dolomite (dolomite greater than 90%); the second are argillaceous dolomite thin layers or nodules (dolomite content of approximately 75–90%); and the third is a highly impure dolomite thin layer (dolomite content between 50–75%). The chemical composition of three kinds of dolomite thin layers and nodules is different: the contents of CaO and MgO of the pure dolomite thin layer and nodules are more than 40%. The contents of CaO and MgO in the thin layer and nodules of argillaceous dolomite are between 30 and 40%. The content of CaO and MgO in the impure dolomite thin layers and nodules is less than 30%. The reservoir space is developed at the micron or nanometer scale in the thin dolomite layers and nodules, although most of these layers are filled with asphalt. The development of thin layers and nodules of dolomites is related to a dry and hot depositional climate and influenced by hydrothermal fluids. The findings presented here provide important information for exploiting the newly discovered shale oil resources. Many dolostone thin layers and nodules have curved silt veins, and the bending coefficient is 1.48, indicating that the dolostone thin layers and nodules have undergone compaction after formation.