Abstract: Qingshankou Formation shale oil in Gulong Sag in the northern part of Songliao Basin (referred to as Gulong shale oil) exhibits distinct microscopic migration and differential accumulation characteristics, which affect the distribution and enrichment of retained movable hydrocarbons. The geochemical characteristics of washing oil products and pore structure of sealed and pressurized core sequences from key wells in deep lake area have systematically been tested to clarify the micro distribution characteristics of hydrocarbons and reveal the migration and accumulation mechanism of Gulong shale oil. The study results suggest that Gulong shale in the deep lake area has a grain size of smaller than 63 μm, and four lithofacies are subdivided, i.e., layered clayey shale, laminated mixed shale, laminated felsic shale, and layered calcareous shale. Based on the ranking of shale retention hydrocarbon content, the layered clayey shale with high TOC (>2%) has a high total hydrocarbon content (2.8–13.7 mg/g), and the movable hydrocarbon (2.8–12.5 mg/g) mainly occurs in intercrystal pores of clay minerals smaller than 32 nm. The laminated mixed and felsic shale with medium–high TOC (>1%) has a total hydrocarbon content of 3.8–7.3 mg/g, and the movable hydrocarbon (2.7–6.4 mg/g) mainly occurs in pores with diameters smaller than 8 nm and larger than 64 nm in the mixed laminae. The laminated felsic shale with low TOC (<1%) contains only a small amount of movable hydrocarbon (3.1–4.6 mg/g), mainly occurring in pores with a diameter greater than 64 nm and only a small amount in pores less than 8 nm. The distribution of organic matter laminae and later diagenesis are the key factors for the differential hydrocarbon distribution and accumulation. The organic matters were enriched in clayey laminae, and the generated hydrocarbon was preferentially retained in situ in organic matter pores and clay intercrystal pores. Part of the movable hydrocarbon migrated to felsic laminae or carbonate mineral laminae within source rocks. In areas with strong dissolution and well-developed pores, the movable hydrocarbon migrated on a large scale. While in areas with strong clay and carbonate rock cementation and undeveloped pores, the migration amount of movable hydrocarbon was small. Based on the movable hydrocarbon distribution and accumulation mechanism of Gulong shale oil, it can be further clarified that the high-TOC layered clayey shale has a high oil content but small pore size, which is a resource sweet spot, but the low-TOC laminated felsic shale has a low movable oil content; The medium-hig TOC laminated mixed shale and felsic shale have a high movable oil content, large pore size and good fracability, which is the resource and engineering dual sweet spot.

Key words: shale oil; differential accumulation; Songliao Basin; Gulong Sag; sweet spot