The energy that causes reservoir fluids to flow out of the reservoir rock and into a wellbore. Gravity drive is simply like draining a tank. Gas drive depends on the expansion of the gas in a reservoir to provide the driving energy; the pressure is reduced as the reservoir is produced. Solution gas drive depends on gas coming out of solution in the oil as the pressure is reduced. Water drive depends on water pressure to force hydrocarbons into the wellbore, and depends on the connectivity of the reservoir with a surrounding aquifer. Water drive is much more efficient at driving oil than is gas drive. Natural reservoir drives are supplemented by injecting gas, water, or other substances in secondary recovery and enhanced oil recovery (q.v.).