Modeling secondary migration of oil currently is difficult because the physics governing the movement is complex and information on heterogeneity of carrier beds is always incomplete. To better understand the basic physical process of secondary migration, we discuss displacement patterns based on relative magnitudes among buoyancy, interfacial, and viscous forces using the results of one-dimensional vertical oil-water displacement experiments. Oil was injected at a constant rate from the lower inlet of a glass tube packed with sorted glass beads. The injection pressure and oil outflow rate were measured while we observed the displacement pattern. Runs were done using different grain sizes and injection rates. Two displacement patterns were recognized during the experiments: type A consisted of stable, pistonlike displacement and type B consisted of capillary fingering. The difference coincided with the relative magnitudes of the driving forces, which can be characterized by the dimensionless modified Bond (Bo＇) and Capillary (Cn) numbers. Type A pattern was produced for high Ca/Bo＇ ratios and type B pattern for low Ca/Bo＇ ratios. A flow regime diagram showing the regions of the two displacement patterns was constructed in Ca/Bo＇ space, including the effects of gravity. Our results also showed that excess pressure for the nonwetting phase fluid to intrude into a porous medium was rate dependent.