Geological reservoir layering for simulation and development usually models depositional sequences. Another approach looks for flow units, i.e., rock units with uniform flow properties. The two are not always identical and should be distinguished. Numerical core descriptions show that porosity and permeability in Reeves Field are determined by both depositional and diagenetic parameters. Flow units defined in this way cut across depositional cycles and cannot be traced between wells with 40-acre spacing. High oil cuts are obtained from 20-acre infill wells despite a long-term waterflood. Reservoir behavior is controlled by the geometry of these flow units, not by depositional cycles.