Oil Reservoirs, Petroleum Reservoirs, Crude Oil Reservoirs

By: EconomyWatch   Date: 30 June 2010

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Oil reservoirs are pools of hydrocarbons, located beneath the earth’s surface in porous rock structures. The total estimate of petroleum reservoirs includes the total quantity of oil that be can be recovered and that which cannot be recovered. The fraction of crude oil reservoirs that can be extracted from the oil field is classified as reserves.

Oil Reservoirs: Formation and Location

Crude oil reservoirs are usually located at 1,000 to 30,000 feet beneath the surface and vary in terms of shape, size and age. They usually extend over hundreds of kilometers, and include a series of oil wells spread across a large area. The formation of petroleum reservoirs can be divided into four stages:

  • Burial of plankton, algae and other protein rich life forms under layers of sand and mud at a source rock

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  • With continuous accumulation of sand, the buried material heats up at 50°C to 70°C

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    • High temperatures, combined with pressure, triggers a ‘cooking’ process

    • Eventually, the buried residue transforms into liquid hydrocarbons

    • Later on they migrate from the source rock to porous rocks and get trapped in impermeable layers of rock.

    Topographical characteristics play a crucial role in the formation of petroleum reservoirs. It is anticipated that the Ohio River Valley could have been an important crude oil reservoir but liquid hydrocarbons escaped due to the absence of impermeable traps. On the other hand, the subsurface structure of the North Sea withstood years of sea level changes. This resulted in the development of over 150 oilfields in this region.

    Oil Reservoirs: Major Ones

    According to the United States Department of Energy, the 2009 estimate of recoverable oil reservoirs stands at:

    Country Recoverable Oil Reservoirs In Billion Barrels
    Saudi Arabia 266.75
    Canada 178.59
    Iran 138.4
    Iraq 115
    Kuwait 104
    The United Arab Emirates 97.8
    Venezuela 87.03
    Russia 60
    Libya 41.46
    Nigeria 36.2
    Kazakhstan 30
    United States 20.97
    China 16
    Algeria 12.2
    Brazil 12.18

    Oil Reservoirs: Issues

    Due to geographical limitations and reservoir characteristics, only a limited amount of oil can be extracted from a particular region. For instance, most of Brazil’s oil reservoirs are located in ‘pre-salt’ areas, a region that is found below a thick layer of salt, located 4 km below the sea bed. The thickness and depth of salt formation hinders the extraction process. Furthermore, the extraction technologies are effective only to a certain extent.


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