Reservoir

A reservoir is an accumulation of oil and/or gas in a permeable and porous rock such as sandstone. A petroleum reservoir normally contains three fluids (oil, gas and water), which separate into distinct sections, owing to their varying specific gravities. Gas occupies the upper part of the reservoir as it is the lightest, oil the middle section, while water occupies the lower section.  See also Permeability, Porosity, Gas Cap, Aquifer, Solution Gas, Proven Reserves.