1. n. 
A reservoir-drive mechanism whereby the oil is driven through the reservoir by an active aquifer. As the reservoir depletes, the water moving in from the aquifer below displaces the oil until the aquifer energy is expended or the well eventually produces too much water to be viable.
2. n. 
A primary recovery mechanism in which the pressure from free water is sufficient to move hydrocarbons out of the reservoir, into the wellbore and up to surface. Waterdrive reservoirs can have bottomwater drive or edgewater drive. In a bottomwater-drive reservoir, water is located beneath the oil accumulation, while in an edgewater-drive reservoir, water is located only on the edges of the reservoir.