Recent experimental observations have demonstrated that losses of oil in a secondary migration pathway depend largely on the characteristics of the pathway itself. Accurate assessments of the residual saturation are required to quantify losses during migration. In the present paper, the authors report on experimental procedures designed to evaluate the oil saturation at various stages of the migration/invasion process. The experiments made use of both apparatuses filled with an artificial medium composed of glass beads, and cores composed of reservoir sandstones from oilfields in NE China. The saturation of residual oil in the pathways was measured by nuclear magnetic resonance imaging. Experiment results show that mobile oil continues to migrate in a porous medium when the supply of oil has been stopped, but that the migration pathway shrinks and may become disconnected into isolated segments. Once active migration ceases, the residual oil saturation varies significantly from location to location within a pathway, and the average residual oil saturation falls to 30 similar to 60%, depending on grain size and composition. A porous medium composed of large grains commonly contains large pores and pore throats and thus may correlate with low residual oil saturations.