Editing Petroleum (section)

== Composition ==
Petroleum consists of a variety of liquid, gaseous, and solid components. Lighter hydrocarbons are the gases [[methane]], [[ethane]], [[propane]] and [[butane]]. Otherwise, the bulk of the liquid and solids are largely heavier organic compounds, often hydrocarbons (C and H only). The proportion of light hydrocarbons in the petroleum mixture varies among [[oil fields]].<ref name="Norman-2001">{{Cite book |last=Norman |first=J. Hyne |title=Nontechnical guide to petroleum geology, exploration, drilling, and production |date=2001 |publisher=Penn Well Corp |isbn=978-0-87814-823-3 |edition=2nd |location=Tulsa, OK |pages=1–4 |oclc=49853640}}</ref>

An [[oil well]] produces predominantly crude oil. Because the pressure is lower at the surface than underground, some of the gas will come out of [[Solution (chemistry)|solution]] and be recovered (or burned) as ''associated gas'' or ''solution gas''. A [[gas well]] produces predominantly [[natural gas]]. However, because the underground temperature is higher than at the surface, the gas may contain heavier hydrocarbons such as pentane, [[hexane]], and [[heptane]] ("[[natural-gas condensate]]", often shortened to ''condensate.'') Condensate resembles gasoline in appearance and is similar in composition to some [[volatility (chemistry)|volatile]] [[light crude oil]]s.<ref>{{Cite book |last=Speight |first=James G. |url=https://books.google.com/books?id=uG-KDwAAQBAJ&q=Condensate+resembles+gasoline&pg=PA13 |title=Heavy Oil Recovery and Upgrading |publisher=Elsevier |year=2019 |isbn=978-0-12-813025-4 |page=13 |language=en |access-date=November 18, 2020 |archive-url=https://web.archive.org/web/20211122221156/https://books.google.com/books?id=uG-KDwAAQBAJ&q=Condensate+resembles+gasoline&pg=PA13 |archive-date=November 22, 2021 |url-status=live}}</ref><ref>{{Cite book |last=Hilyard |first=Joseph |url=https://books.google.com/books?id=F91w410iRLsC&q=Condensate+resembles+gasoline+in+appearance+and+is+similar+in+composition+to+some+volatile+light+crude+oils.&pg=PA31 |title=The Oil & Gas Industry: A Nontechnical Guide |date=2012 |publisher=PennWell Books |isbn=978-1-59370-254-0 |page=31 |language=en}}</ref>

The hydrocarbons in crude oil are mostly [[alkane]]s, [[cycloalkane]]s and various [[aromatic hydrocarbon]]s, while the other organic compounds contain [[nitrogen]], [[oxygen]], and [[sulfur]], and traces of metals such as iron, nickel, copper and [[vanadium]]. Many oil reservoirs contain live bacteria.<ref>{{Cite book |last1=Ollivier |first1=Bernard |title=Petroleum Microbiology |last2=Magot |first2=Michel |date=2005 |publisher=American Society of Microbiology |isbn=978-1-55581-758-9 |location=Washington, DC |doi=10.1128/9781555817589}}</ref> The exact molecular composition of crude oil varies widely from formation to formation but the proportion of [[chemical element]]s varies over fairly narrow limits as follows:<ref name="Speight-1999">{{Cite book |last=Speight |first=J. G. |title=The chemistry and technology of petroleum |date=1999 |publisher=Marcel Dekker |isbn=978-0-8247-0217-5 |edition=3rd ed., rev. and expanded |location=New York |pages=215–216, 543 |oclc=44958948}}</ref>

{| class="wikitable"
|+ Composition by weight
|-
! Element !! Percent range
|-
|[[Carbon]] || 83 to 85%
|-
|[[Hydrogen]] || 10 to 14%
|-
|[[Nitrogen]] || 0.1 to 2%
|-
|[[Oxygen]] || 0.05 to 1.5%
|-
|[[Sulfur]] || 0.05 to 6.0%
|-
|[[Metal]]s || < 0.1%
|}

Four different types of hydrocarbon appear in crude oil. The relative percentage of each varies from oil to oil, determining the properties of each oil.<ref name="Norman-2001" />

{| class="wikitable"
|+ Composition by weight
|-
! Hydrocarbon !! Average !! Range
|-
|[[Alkane]]s (paraffins)|| 30% || 15 to 60%
|-
|[[Naphthene]]s || 49% || 30 to 60%
|-
|[[Aromatic]]s || 15% || 3 to 30%
|-
|[[Asphaltene|Asphaltics]] || 6% || remainder
|}

[[File:Total World Oil Reserves Conventional Unconventional.png|thumb|Unconventional resources are much larger than conventional ones.<ref>{{Cite magazine |last=Alboudwarej, Hussein |display-authors=etal |date=Summer 2006 |title=Highlighting Heavy Oil |url=http://www.slb.com/~/media/Files/resources/oilfield_review/ors06/sum06/heavy_oil.ashx |format=PDF |archive-url=https://web.archive.org/web/20120411145144/http://www.slb.com/~/media/Files/resources/oilfield_review/ors06/sum06/heavy_oil.ashx |archive-date=April 11, 2012 |access-date=July 4, 2012 |periodical=Oilfield Review}}</ref>]]
[[File:Isooctane-3D-balls.png|thumb|[[2,2,4-Trimethylpentane]], a [[hydrocarbon]] with the [[octane number]] of 100. Black spheres are [[carbon]] and white spheres are [[hydrogen]] atoms.]]

The alkanes from [[pentane]] (C<sub>5</sub>H<sub>12</sub>) to [[octane]] (C<sub>8</sub>H<sub>18</sub>) are [[oil refinery|refined]] into gasoline, the ones from [[nonane]] (C<sub>9</sub>H<sub>20</sub>) to [[hexadecane]] (C<sub>16</sub>H<sub>34</sub>) into [[diesel fuel]], [[kerosene]] and [[jet fuel]]. Alkanes with more than 16 carbon atoms can be refined into [[fuel oil]] and [[lubricating oil]]. At the heavier end of the range, [[paraffin wax]] is an alkane with approximately 25 carbon atoms, while [[Bitumen|asphalt]] has 35 and up, although these are usually [[Fluid catalytic cracking|cracked]] in modern refineries into more valuable products. The lightest fraction, the so-called petroleum gases are subjected to diverse processing depending on cost. These gases are either [[flare stack|flared off]], sold as [[liquefied petroleum gas]], or used to power the refinery's own burners. During the winter, butane (C<sub>4</sub>H<sub>10</sub>), is blended into the gasoline pool at high rates, because its high vapour pressure assists with cold starts.

The ''aromatic hydrocarbons'' are [[degree of unsaturation|unsaturated hydrocarbons]] that have one or more [[benzene ring]]s. They tend to burn with a sooty flame, and many have a sweet aroma. Some are [[carcinogenic]].

These different components are separated by [[fractional distillation]] at an oil refinery to produce gasoline, jet fuel, kerosene, and other hydrocarbon fractions.

The components in an oil sample can be determined by [[gas chromatography]] and [[mass spectrometry]].<ref>[http://www.norden.org/pub/ebook/2003-516.pdf Use of ozone depleting substances in laboratories] {{webarchive|url=https://web.archive.org/web/20080227052412/http://www.norden.org/pub/ebook/2003-516.pdf |date=February 27, 2008 }}. TemaNord 2003:516.</ref> Due to the large number of co-eluted hydrocarbons within oil, many cannot be resolved by traditional gas chromatography. This [[unresolved complex mixture]] (UCM) of hydrocarbons is particularly apparent when analysing weathered oils and extracts from tissues of organisms exposed to oil.

Crude oil varies greatly in appearance depending on its composition. It is usually black or dark brown (although it may be yellowish, reddish, or even greenish). In the reservoir it is usually found in association with natural gas, which being lighter forms a "gas cap" over the petroleum, and [[saline water]] which, being heavier than most forms of crude oil, generally sinks beneath it. Crude oil may also be found in a semi-solid form mixed with sand and water, as in the [[Athabasca oil sands]] in Canada, where it is usually referred to as crude [[bitumen]]. In Canada, bitumen is considered a sticky, black, tar-like form of crude oil which is so thick and heavy that it must be heated or diluted before it will flow.<ref>{{Cite web |year=2007 |title=Oil Sands – Glossary |url=http://www.energy.gov.ab.ca/OilSands/1106.asp |archive-url=https://web.archive.org/web/20071101112113/http://www.energy.gov.ab.ca/OilSands/1106.asp |archive-date=November 1, 2007 |access-date=October 2, 2008 |website=Mines and Minerals Act |publisher=Government of Alberta}}</ref> Venezuela also has large amounts of oil in the [[Orinoco oil sands]], although the hydrocarbons trapped in them are more fluid than in Canada and are usually called [[extra heavy oil]]. These oil sands resources are called [[unconventional oil]] to distinguish them from oil which can be extracted using traditional oil well methods. Between them, Canada and [[Venezuela]] contain an estimated {{convert|3.6|Toilbbl}} of bitumen and extra-heavy oil, about twice the volume of the world's reserves of conventional oil.<ref>{{Cite web |year=2008 |title=Oil Sands in Canada and Venezuela |url=http://oilsands.infomine.com/countries/ |archive-url=https://web.archive.org/web/20081219113841/http://oilsands.infomine.com/countries/ |archive-date=December 19, 2008 |access-date=October 2, 2008 |publisher=Infomine Inc.}}</ref>