page_title: Petroleum reservoir
section_title: Oil field
hierarchical_section_title: Petroleum reservoir / Oil field
caption_attribution_description: English: A field of dozens of oil wells just offshore, at Summerland (Santa Barbara County), before 1906. Original caption reads: "Oil wells at Summerland, Santa Barbara County, California, showing the ocean bluffs in front of Sumerland [sic], the wharves and derricks over the wells."
context_page_description: A petroleum reservoir or oil and gas reservoir is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. Petroleum reservoirs are broadly classified as conventional and unconventional reservoirs. In conventional reservoirs, the naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability, while in unconventional reservoirs, the rocks have high porosity and low permeability, which keeps the hydrocarbons trapped in place, therefore not requiring a cap rock. Reservoirs are found using hydrocarbon exploration methods.
context_section_description: An oil field is a pool of oil under the surface of the earth, trapped in a sealed hollow of impermeable rock. As actually used in practice, the term implies the possibility of sufficient economic benefit worthy of commercial attention. Secondarily, the area on the surface above where oil lies trapped underground, is also called an oil field.
  Because oil reservoirs typically extend over a large area, possibly several hundred km across, full exploitation entails multiple wells scattered across the area. In addition, there may be exploratory wells probing the edges, pipelines to transport the oil elsewhere, and support facilities.
  Because an oil field may be remote from civilization, establishing a field is often an extremely complicated exercise in logistics. This goes beyond requirements for drilling, to include associated infrastructure. For instance, workers require housing to allow them to work onsite for months or years. In turn, housing and equipment require electricity and water. In cold regions, pipelines may need to be heated. Also, excess natural gas may be burned off if there is no way to make use of it—which requires a furnace, chimney and pipes to carry it from the well to the furnace.
  Thus, the typical oil field resembles a small, self-contained town in the midst of a landscape dotted with drilling rigs or the pump jacks, which are known as "nodding donkeys" because of their bobbing arm. Several companies, such as Hill International, Bechtel, Esso, Weatherford International, Schlumberger Limited, Baker Hughes and Halliburton, have organizations that specialize in the large-scale construction of the infrastructure and providing specialized services required to operate a field profitably.
  More than 40,000 oil fields are scattered around the globe, on land and offshore. The largest are the Ghawar Field in Saudi Arabia and the Burgan Field in Kuwait, with more than 60 billion barrels (9.5×10⁹ m³) estimated in each. Most oil fields are much smaller. According to the US Department of Energy (Energy Information Administration), as of 2003 the US alone had over 30,000 oil fields.
  In the modern age, the location of oil fields with proven oil reserves is a key underlying factor in many geopolitical conflicts.
  The term "oilfield" is also used as a shorthand to refer to the entire petroleum industry. However, it is more accurate to divide the oil industry into three sectors:  upstream (crude production from wells and separation of water from oil), midstream (pipeline and tanker transport of crude) and downstream (refining, marketing of refined products and transportation to Oil stations).
caption: An oil field with dozens of wells.  This is the Summerland Oil Field, near Santa Barbara, California, before 1906
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