Estimates of hydrocarbon losses during migration are critical to petroleum resource assessments based on mass balance calculations. Using knowledge acquired from physical experiments, we conducted numerical experiments to qualitatively simulate migration processes on a basin scale, and we have estimated the proportions of oil lost along different parts of the migration pathway. Between the point where oil is expelled from a source rock and its arrival in a trap, migration pathways were divided into three sections, namely vertical and lateral pathways within the area of the effective source rock (W-1), and lateral pathways outside this area (W-2). With reference to the concept of an independent migration unit (IMU), a prism-shaped domain with square top and bottom faces, the proportion of vertical pathways (Q(1)) and lateral pathways (Q(2)) in carrier beds in W-1 may be estimated, using parameters obtained in migration experiments. The proportion of oil lost in W-2 was obtained by statistical analysis of modelled results. It is found that the loss of oil within W-2 decreases rapidly with increasing distance from the contact between W-1 and W-2. Relative oil losses in migration pathways were estimated by considering two typical migration models: a sloping rectangular plate model, and a circular synclinal model. Simulation results show that, for sandstone carrier beds at a basin scale, Q(2) is one order of magnitude less than Q(1), and losses (Q(3)) in W-2 are two orders of magnitude less than those (Q(1)+ Q(2)) in W-1.