Abstract: The shale oil enriched in ultrafine-grained sendimentary rocks (grain size<10μm) account for more than 50% of the total shale oil resources; however, there is still a lack of clear understanding of its genetic mechanism and geological assessment methods, thus restricting the breakthrough in shale oil exploration and development. Therefore, the formation mechanism of ultrafine-grained sedimentary rocks was explored based on analyzing the data of shale oil-rich continental basin; according to the material source of ultrafine-grained sediments, the ultrafine-grained sedimentary rocks were divided into four types dominated by terrestrial source, intra-source, hydrothermal-volcanic source and hybrid source rocks; according to the sedimentary dynamics and material composition, they were further subdivided into nine genetic mechanisms, including terrestrial-sourced still water deposition, terrestrial-sourced event deposition, intra-source dead creature deposition, intra-source biochemistry deposition, intra-source evaporation-chemistry deposition, volcanic deposition, subaqueous hydrothermal-volcanic eruption deposition, hybrid-source still water deposition and hybrid-source event deposition. The ultrafine-grained sedimentary rocks of different genetic mechanisms vary significantly in reservoir quality parameters, such as organic matter abundance, oil content, brittleness and reservoir physical properties. Specifically, the ultrafine-grained sedimentary rocks dominated by intra-source dead creature deposition, intra-source biochemistry deposition, partial hydrothermal-volcanic deposition and hybrid-source still water deposition possess good shale oil reservoir potential, as being the important sweet spots of shale oil exploration.