The existence of natural barriers such as tar deposits in oil reservoirs can create problems in primary oil recovery as well as in the application of EOR methods. Significant reduction in oil recovery is reported from this type of reservoirs due to isolation of the oil zone from the adjacent water aquifer. In this study, the effects of tar viscosity and thickness of a tar zone on oil recovery as well as the pressure variation and average water saturation in the tar and oil zones were studied in a tarmat reservoir laboratory model. Waterflooding experiments were conducted, whereby the three adjacent oil, tar and water zones were simulated by means of a berea composite core saturated with kerosene, a blend of asphalt and crude oil and KCl brine, respectively. In every experiment, brine was injected at a constant rate in the water zone and was forced to penetrate the tar zone to flood the oil zone. The results show a slight decrease in oil recovery as the product of the viscosity and the thickness of the tar zone increases. An opposite and more pronounced trend was found for the average water saturation in the tar zone. The injection pressure was found to go through a maximum shortly after commencement of injection and the maximum value increased with both tar viscosity and tar zone thickness. On the other hand, the effective permeability to water was found to be smaller in tests where the product of the tar viscosity and tar zone thickness is higher. Finally, the water saturation distribution in the oil zone combined with the pressure behavior points to the development of water fingers in both tar and oil zones.