Definition - What does Reservoir mean?

Reservoir is a porous, permeable and sedimentary rock in which water or hydrocarbon fluids are accumulated over a long period of time. The most common Reservoir rocks are sandstone and limestone. Fractured granite, though not a sedimentary rock, also contains hydrocarbons. In a typical Oil Reservoir - that contains gas, oil and water - gas (being the lightest) occupies the upper part, oil occupies the middle space while water occupies the lower part of the Reservoir.

Petropedia explains Reservoir

In a Reservoir, hydrocarbons and water are denoted by various colors which are globally accepted and all E&P organizations use same color coding to represent them.

  • Oil is represented by green
  • Gas is represented by red
  • Water is represented by blue

A Reservoir can be broadly classified as:

  • Oil Reservoir – In this type of Reservoir, majority of the hydrocarbons are crude oil and water with little or no amount of natural gas.
  • Gas Reservoir – This type of Reservoir contains only natural gas and water.
  • Condensate Reservoir – This type of Reservoir contains natural gas liquids or gas condensates along with natural gas. As the pressure drops below the dew point, the condensate separates from natural gas.
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