Abstract: Through the design of an algorithm, the conversion coefficient of NMR T₂ relaxation time and pore throat radius was optimized by using the pore throat distribution data in tight oil reservoirs obtained from mercury injection experiments. The precision of the NMR characterization of pore distribution was improved. Methods were used in the characterization of Cretaceous tight oil pore radius distribution in the southern Songliao Basin. Pore radius of samples with oil saturation less than 10% were concentrated in the 10-300 nm range, while those with oil saturation between 10% and 40% were mainly 20-1 000 nm. Unlike these samples, the pore radius of samples with oil saturation greater than 40% were concentrated in the 20-3 000 nm range.Experimental results showed that the development of different levels of microporosity in tight oil reservoirs controlled the oiliness properties.