Well pressure and production histories and transient pressure tests, evaluated by conventional well testing techniques and simulation, are shown to indicate compartmented reservoir geometry arising by depositional and diagenetic processes. Examples are cited of both clastic and carbonate reservoirs but the central focus of this study is on fluvial deposits exhibiting stratigraphic compartmentation. Field examples of compartmented behavior in such reservoirs are demonstrated by wells with spacing corresponding to separations as small as 1/4 mile. Furthermore, directly drained compartments with radii as small as 60 feet are demonstrated. The impact of such compartmentation on development strategies is examined.