Abstract The engineer and the conservationist agree that effective water drive is thedesirable reservoir production mechanism. Water drive may result either fromthe expansion of edge water, the reservoir water bordering an oil pool, or fromthe artesian flow of edge water from the outcrop. Thus, the extent of thereservoir has an important bearing upon both the rate and the ultimate quantityof water influx to an oil pool. Lens-type reservoirs are generally recognizable, but the complete structureof the more important domal types, having possible limitations from crestal andsynclinal faulting, is not well understood. The delineation from cores, electrical logs and exploitation phenomena of the crestal portions ofreservoirs is increasingly exact. This paper suggests that inference from thesefactual pictures, combined with the scattered well and geophysical data of thesynclines and with consideration of the dynamics of structural formation, maybe applied to solving the complete geologic structure. Since faulting on thedomes and in the synclines may effectively seal and thus limit the size of areservoir and bar additional water influx, consideration of geologic structureand its possible effects is essential to complete analysis and prediction ofreservoir performance. Introduction Implicit in reservoir studies is the fact that the oil must be displacedfrom the reservoir rock by some other fluid, either gas or water, and thisunderstanding yields the terms descriptive of the major reservoir productionmechanisms, gas expansion and water drive. Three recent papers make clear theeconomic and conservation preeminence of water drive as a production mechanism;in fact, the title of the latest of these is "Production under EffectiveWater Drive as a Standard for Conservation Practice." There appears to be an assumption at times among analysts that theeffectiveness of a water drive is solely dependent on the rate of oilproduction; that is, if water drive appears to be insufficient or lacking, alower oil-recovery rate would increase or reveal it. This assumption mightresult from the fact that the analysis has been wholly developed during theperiod of production curtailment; that is, since 1929. The writer takes it thatthe recurrent phrase in De Golyer's paper, "some degree of waterdrive," is related to this assumption. In California, an extraordinaryphrase, "inactive water drive," is rather common and the connotation ofuncertainty regarding our knowledge of reservoir performance is evident. T.P. 1527