In assessing the economic viability of a waterflood project, a key parameter is the remaining oil saturation (ROS) within each pattern unit. This information helps in identifying the areas with the highest ROS and thus potential for further development. While special core analysis, log-inject-log, and thermal-decay time-log-evaluation techniques are available, they provide only single-point values and a snapshot in time near a wellbore. Also, they can quickly add up to an expensive program. The analytical areal distribution method estimates ROS in a waterflood pattern unit from material balance calculations using well injection and production data with no pressure information required. Well production and injection volumes are routinely measured in oilfield operations, making the method very attractive. The areal distribution technique estimates two major uncertainties: vertical loss of injected water into nontarget areas or areal loss into surrounding patterns, and injected water for gas fill-up. However, developers tested it only in low-pressure conditions, which are increasingly rare in oilfield operations. The main purpose of my research, then, was to verify whether or not the areal distribution method is valid in higher pressure conditions. Simulation of various waterflood patterns confirmed that the areal distribution method with its estimated ROS is capable of precise estimation of actual ROS, but at high pressures it requires consideration of pressure data in addition to injection and production data.