The NMR relaxometry measurements have been designed and applied to quantitatively determine residual oil distribution during waterflooding in tight oil formations. A tight core sample is first saturated with water to measure its NMR transverse relaxation time (T-2) spectrum. NMR T-2 spectrum is then measured for the core sample after it has been displaced with the fluorinated oil. Subsequently, the core sample is displaced with water until residual oil saturation is achieved, and the NMR T-2 spectrum is measured again at the end of the displacement. Subsequently, the constant-rate mercury injection method is used to experimentally measure the size of the pore and throat in the core sample. The residual oil saturation is determined as a function of pore size by comparing the difference between the first and last NMR T-2 spectrum. It is found from four core samples with permeability of 0.04-1.70 mD that the average pore size is in a range of 129-145 mu m, and the pore throat has a radius of 0.17-0.89 mu m. The original oil saturation is found to be 76-83%, whereas the oil recovery factor is 36-62%; 4-27% of the original oil is distributed in pores larger than 100 mu m, 50-54% in pores from 10 to 100 mu m, and 21-46% in pores and throats smaller than 10 mu m. Residual oil saturation is 1-2% in pores larger than 100 mu m, 29-64% in pores from 10 to 100 mu m, and 34-69% in pores and throats smaller than 10 mu m.