Reservoir:A porous and permeable underground formation containing an individual and separate natural accumulation of producible hydrocarbons (oil and/or gas) which is confined by impermeable rock or water barriers and is characterized by a single natural pressure system.
A subsurface, porous, permeable rock body in which oil and/or gas is stored, most reservoir rocks are limestones, dolomites, sandstones, or a combination of these.
The three basic types of hydrocarbon reservoirs are oil, gas, and condensate.
An oil reservoir generally contains three fluids – gas, oil, and water – with oil the dominant product.
In the typical oil reservoir, these fluids occur in different phases because of the variance in their gravities.
Gas, the lightest, occupies the upper part of the reservoir rocks; water, the lower part; and oil, the intermediate section.
In addition to its occurrence as a cap or in solution, gas may accumulate independently of the oil; if so, the reservoir is called a gas reservoir.
Associated with the gas, in most instances, are salt water and some oil.
In a condensate reservoir, the hydrocarbons may exist as a gas, but, when brought to the surface, some of the heavier ones condense to a liquid.