Editing Petroleum (section)

== Reservoirs ==
{{main|Petroleum reservoir}}
{{More citations needed section|date=October 2016}}
[[File:Structural Trap (Anticlinal).svg|thumb|A [[hydrocarbon]] trap consists of a reservoir rock (yellow) where oil (red) can accumulate, and a caprock (green) that prevents it from egressing.]]
Three conditions must be present for oil reservoirs to form:
* A [[source rock]] rich in [[hydrocarbon]] material buried deeply enough for subterranean heat to cook it into oil,
* A [[porous]] and [[permeability (fluid)|permeable]] reservoir rock where it can accumulate,
* A [[caprock]] (seal) or other mechanism to prevent the oil from escaping to the surface. Within these reservoirs, fluids will typically organize themselves like a three-layer cake with a layer of water below the oil layer and a layer of gas above it, although the different layers vary in size between reservoirs. Because most hydrocarbons are less dense than rock or [[water]], they often migrate upward through adjacent rock layers until either reaching the surface or becoming trapped within porous rocks (known as [[oil reservoir|reservoirs]]) by impermeable rocks above. However, the process is influenced by underground water flows, causing oil to migrate hundreds of kilometres horizontally or even short distances downward before becoming trapped in a reservoir. When hydrocarbons are concentrated in a trap, an [[oil field]] forms, from which the liquid can be extracted by [[drill]]ing and [[pump]]ing.

The reactions that produce oil and natural gas are often modeled as first order breakdown reactions, where hydrocarbons are broken down to oil and natural gas by a set of parallel reactions, and oil eventually breaks down to natural gas by another set of reactions. The latter set is regularly used in [[petrochemical]] plants and [[oil refineries]].

Petroleum has mostly been recovered by [[oil drilling]] (natural petroleum springs are rare). Drilling is carried out after studies of structural geology (at the reservoir scale), sedimentary basin analysis, and reservoir characterisation (mainly in terms of the [[porosity]] and [[Permeability (earth sciences)|permeability]] of geologic reservoir structures).<ref>{{Cite journal |vauthors=Guerriero V, etal |year=2012 |title=A permeability model for naturally fractured carbonate reservoirs |journal=[[Marine and Petroleum Geology]] |volume=40 |pages=115–134 |doi=10.1016/j.marpetgeo.2012.11.002}}</ref><ref>{{Cite journal |vauthors=Guerriero V, etal |year=2011 |title=Improved statistical multi-scale analysis of fractures in carbonate reservoir analogues |journal=[[Tectonophysics (journal)|Tectonophysics]] |volume=504 |issue=1 |pages=14–24 |bibcode=2011Tectp.504...14G |doi=10.1016/j.tecto.2011.01.003}}</ref> Wells are drilled into oil reservoirs to extract the crude oil. "Natural lift" production methods that rely on the natural reservoir pressure to force the oil to the surface are usually sufficient for a while after reservoirs are first tapped. In some reservoirs, such as in the Middle East, the natural pressure is sufficient over a long time. The natural pressure in most reservoirs, however, eventually dissipates. Then the oil must be extracted using "[[artificial lift]]" means. Over time, these "primary" methods become less effective and "secondary" production methods may be used. A common secondary method is [[Water injection (oil production)|"waterflood"]] or injection of water into the reservoir to increase pressure and force the oil to the drilled shaft or "wellbore." Eventually "tertiary" or "enhanced" oil recovery methods may be used to increase the oil's flow characteristics by injecting steam, carbon dioxide and other gases or chemicals into the reservoir. In the United States, primary production methods account for less than 40 percent of the oil produced on a daily basis, secondary methods account for about half, and tertiary recovery the remaining 10 percent. Extracting oil (or "bitumen") from oil/tar sand and oil shale deposits requires mining the sand or shale and heating it in a vessel or retort, or using "in-situ" methods of injecting heated liquids into the deposit and then pumping the liquid back out saturated with oil.

=== Unconventional oil reservoirs ===
{{See also|Unconventional oil|Oil sands|Oil shale reserves|Unconventional (oil and gas) reservoir}}
Oil-eating bacteria [[biodegrade]] oil that has escaped to the surface. [[Oil sands]] are reservoirs of partially biodegraded oil still in the process of escaping and being biodegraded, but they contain so much migrating oil that, although most of it has escaped, vast amounts are still present—more than can be found in conventional oil reservoirs. The lighter fractions of the crude oil are destroyed first, resulting in reservoirs containing an extremely heavy form of crude oil, called crude bitumen in Canada, or extra-heavy crude oil in [[Venezuela]]. These two countries have the world's largest deposits of oil sands.<ref>{{Cite web |date=June 19, 2020 |title=Tar sands |url=https://www.strausscenter.org/energy-and-security-project/tar-sands/ |access-date=June 26, 2022 |publisher=The Strauss Center.}}</ref>

On the other hand, [[oil shales]] are source rocks that have not been exposed to heat or pressure long enough to convert their trapped hydrocarbons into crude oil. Technically speaking, oil shales are not always shales and do not contain oil, but are fined-grain sedimentary rocks containing an insoluble organic solid called [[kerogen]]. The kerogen in the rock can be converted into crude oil using heat and pressure to simulate natural processes. The method has been known for centuries and was patented in 1694 under British Crown Patent No. 330 covering, "A way to extract and make great quantities of pitch, tar, and oil out of a sort of stone." Although oil shales are found in many countries, the United States has the world's largest deposits.<ref name="Lambertson">{{Cite news |last=Lambertson |first=Giles |date=February 16, 2008 |title=Oil Shale: Ready to Unlock the Rock |publisher=Construction Equipment Guide |url=http://www.cegltd.com/story.asp?story=10092 |url-status=live |access-date=May 21, 2008 |archive-url=https://web.archive.org/web/20170711112037/http://www.constructionequipmentguide.com/redirect/10092?story=10092 |archive-date=July 11, 2017}}</ref>