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Taking organic-rich shale in the first member of Cretaceous Qingshankou Formation in the Gulong Sag, northern Songliao Basin as an example, this study examined the lithofacies classification, petrological characteristics, pore size distribution and their implications on the enrichment of shale oil of lacustrine detrital fine-grained shale. The spatial variation of lithofacies, controlled by Milankovitch cycle and influenced by sediment provenance, has an obvious sequence. The fine-grained sedimentary rocks of studied section could be classified into seven lithofacies according to a three-step classification criterion that consists of total organic carbon (TOC), sedimentary structure and mineral composition. Among them, the laminated siliceous mudstone lithofacies with moderate TOC has high hydrocarbon generation potential and abundant reservoir space, making it the most favorable lithofacies for the enrichment of matrix shale oil. Under the background of abnormally high pressure, the laminated siliceous mudstone lithofacies with moderate TOC deposited between the top of SSC2 and the bottom of SSC3 is stable in lateral distribution in the delta-lacustrine transition zone, with continuous thickness greater than 30 m. The massive siliceous mudstone lithofacies with high and moderate TOC developed in the middle of the these two cycles can act as the roof and floor seal for shale oil, therefore, the study area has good conditions for forming matrix type shale oil reservoirs. Key words: Songliao Basin, Gulong Sag, Cretaceous, Qingshankou Formation, Milankovitch cycle, shale lithofacies, shale oil