Migration modeling by means of the reservoir simulation techniques is not always capable of handling migration of oil and gas on a basin scale through geologic time because it may require too many time steps and become too computer demanding. However, simple algorithmic description of the migration process can be an alternative to reservoir simulation techniques. One such algorithm is presented here for migration modeling on a basin scale. The basis for the algorithm is a grouping of the simulation cells in the basin into two capillary pressure end members, which corresponds with a coarse-grained reservoir type rock and a fine-grained shale type rock, respectively. This capillary pressure model makes it easy to obtain the migration paths, drainage areas, drainage boundaries, spill points, and traps. The migration paths can be considered as approximate streamlines for the migrating fluids.