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Combined experiment method of Environmental Scanning Electron Microscope (ESEM) and Energy Dispersive Spectroscopy (EDS) was proposed to detect tight oil occurrence in micro- and nano-pores of tight sands in Member 4 of Cretaceous Quantou Formation. Observation and analysis of 168 measurement points and seventeen samples from seven typical wells shows that the tight oil occurrences in micro- and nano-pores have two main forms: oil film and oil droplet, oil film is dominant. Intra-granular pores, inter-granular pores and micro-fractures are three kinds of micro storage space, mainly inter-granular pores. The oil films in the intra-granular pores and micro-fractures are irregular and adhesive, and the size is about (1−5 μm) × (1−5 μm), the carbon mass percentage of oil films are mainly 40%−90%. The size of oil droplets is (0.2−1.0 μm) × (0.2−1.0 μm), with relatively small occurrence space. The carbon mass percentage of oil droplets are mainly 15%−30%. Reservoir types and distribution of pores control tight oil occurrence. The carbon contents and thicknesses of oil film decrease from type I to type III reservoirs. The carbon mass percentage and thicknesses are controlled by the median pore throat radius and reservoir quality coefficient. Key words: southern Songliao Basin, Cretaceous, tight oil, oil micro occurrence form, oil occurrence space