Micro-occurrence of Cretaceous tight oil in southern Songliao Basin, NE China

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.