(Geology) An accumulation of oil and/or gas in a 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 gravities. Gas occupies the upper part of the reservoir as it is the lightest, oil the middle section, while water and rocks occupy the lower section. (Hydraulics) A storage tank for control fluids used to operate the well control components and other hydraulically-actuated devices. .