This paper present a lens model, based on a physical concept of the individual pay members, that is the first model designed to calculate reserves from infill drilling and pattern changes. The model conceptualizes the reservoir as being made up of a large number of randomly located circular disks or lenses sealed at the edge. Calculations of drainability, floodability and secondary/primary ratio are presented for two calibrated lens-size distributions. The results of these calculations are compared with the performance of some Wasson Clearfork trend waterfloods.