reservoir - Geology: sub surface formation of porous, permeable rock (usually sandstone, limestone, or dolomite) containing oil or gas within the rock pores. A typical oil reservoir contains gas, oil, and water, which occupy the upper, middle, and lower regions of the reservoir, respectively. The flow of reservoir fluids from the rock to the borehole is driven by gas pressure or water pressure. See enhanced recovery, rotary drilling. Lubrication: primary lubricant repository in a lubrication system, e.g., crankcase, gear box.