Editing Petroleum (section)

=== Fossil petroleum ===
[[File:Treibs&Chlorophyll.png|thumb|Structure of a vanadium [[porphyrin]] compound (left) extracted from petroleum by [[Alfred E. Treibs]], father of [[organic geochemistry]]. Treibs noted the close structural similarity of this molecule and [[chlorophyll a]] (right).<ref>{{Cite journal |last=Treibs, A.E. |year=1936 |title=Chlorophyll- und Häminderivate in organischen Mineralstoffen |journal=Angew. Chem. |volume=49 |issue=38 |pages=682–686 |bibcode=1936AngCh..49..682T |doi=10.1002/ange.19360493803|issn = 0044-8249}}</ref><ref>{{Cite journal |last=Kvenvolden, K.A. |year=2006 |title=Organic geochemistry – A retrospective of its first 70 years |url=https://zenodo.org/record/1000677 |url-status=live |journal=Org. Geochem. |volume=37 |issue=1 |pages=1–11 |bibcode=2006OrGeo..37....1K |doi=10.1016/j.orggeochem.2005.09.001 |s2cid=95305299 |archive-url=https://web.archive.org/web/20190607232519/https://zenodo.org/record/1000677 |archive-date=June 7, 2019 |access-date=July 1, 2019}}</ref>]]
Petroleum is a [[fossil fuel]] derived from  [[fossilized]] [[organic material]]s, such as [[zooplankton]] and [[algae]].<ref>{{Cite journal |last=Kvenvolden |first=Keith A. |year=2006 |title=Organic geochemistry – A retrospective of its first 70 years |url=https://zenodo.org/record/1000677 |url-status=live |journal=Organic Geochemistry |volume=37 |issue=1 |pages=1–11 |bibcode=2006OrGeo..37....1K |doi=10.1016/j.orggeochem.2005.09.001 |s2cid=95305299 |archive-url=https://web.archive.org/web/20190607232519/https://zenodo.org/record/1000677 |archive-date=June 7, 2019 |access-date=July 1, 2019}}</ref><ref name="Schobert-2013">{{Cite book |last=Schobert |first=Harold H. |title=Chemistry of fossil fuels and biofuels |publisher=Cambridge University Press |year=2013 |isbn=978-0-521-11400-4 |location=Cambridge |pages=103–130 |oclc=795763460}}</ref> Vast amounts of these remains settled to sea or lake bottoms where they were covered in [[stagnant water]] (water with no dissolved [[oxygen]]) or [[sediment]]s such as [[mud]] and [[silt]] faster than they could [[Decomposition#Anaerobic vs aerobic|decompose aerobically]]. Approximately 1 [[Metre|m]] below this sediment, water oxygen concentration was low, below 0.1&nbsp;mg/L, and [[Anoxic waters|anoxic conditions]] existed. Temperatures also remained constant.<ref name="Schobert-2013" />

As further layers settled into the sea or lake bed, intense heat and pressure built up in the lower regions. This process caused the organic matter to change, first into a waxy material known as [[kerogen]], found in various [[oil shale]]s around the world, and then with more heat into liquid and gaseous [[hydrocarbon]]s via a process known as [[Catagenesis (geology)|catagenesis]]. Formation of petroleum occurs from hydrocarbon [[pyrolysis]] in a variety of mainly [[endothermic]] reactions at high temperatures or pressures, or both.<ref name="Schobert-2013" /><ref name="Braun-1993" /> These phases are described in detail below.

==== Anaerobic decay ====
In the absence of plentiful oxygen, [[Aerobic organism|''aerobic'' bacteria]] were prevented from decaying the organic matter after it was buried under a layer of sediment or water. However, [[Anaerobic organism|''anaerobic'' bacteria]] were able to reduce [[sulfate]]s and [[nitrate]]s among the matter to [[Hydrogen sulfide|H<sub>2</sub>S]] and [[Nitrogen|N<sub>2</sub>]] respectively by using the matter as a source for other reactants. Due to such anaerobic bacteria, at first, this matter began to break apart mostly via [[hydrolysis]]: [[polysaccharide]]s and [[protein]]s were hydrolyzed to [[simple sugars]] and [[amino acid]]s respectively. These were further anaerobically [[oxidized]] at an accelerated rate by the [[enzyme]]s of the bacteria: e.g., amino acids went through [[oxidative deamination]] to [[amino acid]]s, which in turn reacted further to [[ammonia]] and [[Keto acid|α-keto acids]]. [[Monosaccharide]]s in turn ultimately decayed to [[Carbon dioxide|CO<sub>2</sub>]] and [[methane]]. The anaerobic decay products of amino acids, monosaccharides, [[phenols]] and [[aldehyde]]s combined into [[fulvic acid]]s. [[Fat]]s and [[wax]]es were not extensively hydrolyzed under these mild conditions.<ref name="Schobert-2013" />

==== Kerogen formation ====
Some [[Phenols|phenolic compounds]] produced from previous reactions worked as [[bactericide]]s and the [[Actinomycetales]] order of bacteria  also produced antibiotic compounds (e.g., [[streptomycin]]). Thus the action of anaerobic bacteria ceased at about 10&nbsp;m below the water or sediment. The mixture at this depth contained fulvic acids, unreacted and partially reacted fats and waxes, slightly modified [[lignin]], resins and other hydrocarbons.<ref name="Schobert-2013" /> As more layers of organic matter settled into the sea or lake bed, intense heat and pressure built up in the lower regions.<ref name="Braun-1993">{{Cite report |url=http://www.osti.gov/servlets/purl/10169154-cT5xip/ |title=Chemical reaction model for oil and gas generation from type 1 and type 2 kerogen |last1=Braun |first1=R.L. |last2=Burnham |first2=A.K. |date=June 1993 |publisher=Lawrence Livermore National Laboratory |doi=10.2172/10169154 |access-date=March 18, 2018 |archive-url=https://web.archive.org/web/20200517104249/https://www.osti.gov/biblio/10169154-cT5xip/ |archive-date=May 17, 2020 |url-status=live}}</ref> As a consequence, compounds of this mixture began to combine in poorly understood ways to [[kerogen]]. Combination happened in a similar fashion as [[phenol]] and [[formaldehyde]] molecules react to [[urea-formaldehyde]] resins, but kerogen formation occurred in a more complex manner due to a bigger variety of reactants. The total process of kerogen formation from the beginning of anaerobic decay is called '''diagenesis''', a word that means a transformation of materials by dissolution and recombination of their constituents.<ref name="Schobert-2013" />

==== Transformation of kerogen into fossil fuels ====
Kerogen formation continued to a depth of about 1 [[km]] from the Earth's surface where temperatures may reach around 50 [[°C]]. Kerogen formation represents a halfway point between organic matter and [[fossil fuels]]: kerogen can be exposed to oxygen, oxidize and thus be lost, or it could be buried deeper inside the [[Crust (geology)|Earth's crust]] and be subjected to conditions which allow it to slowly transform into fossil fuels like petroleum. The latter happened through '''catagenesis''' in which the reactions were mostly [[Radical (chemistry)|radical]] [[Rearrangement reaction|rearrangements]] of kerogen. These reactions took thousands to millions of years and no external reactants were involved. Due to the radical nature of these reactions, kerogen reacted towards two classes of products: those with low H/C ratio ([[anthracene]] or products similar to it) and those with high H/C ratio ([[methane]] or products similar to it); i.e., carbon-rich or hydrogen-rich products. Because catagenesis was closed off from external reactants, the resulting composition of the fuel mixture was dependent on the composition of the kerogen via reaction [[stoichiometry]]. Three types of kerogen exist: type I (algal), II (liptinic) and III (humic), which were formed mainly from [[algae]], [[plankton]] and [[woody plant]]s (this term includes [[tree]]s, [[shrub]]s and [[liana]]s) respectively.<ref name="Schobert-2013" />

Catagenesis was [[pyrolytic]] despite the fact that it happened at relatively low temperatures (when compared to commercial pyrolysis plants) of 60 to several hundred °C. Pyrolysis was possible because of the long reaction times involved. Heat for catagenesis came from the decomposition of [[radioactive]] materials of the crust, especially [[Potassium-40|<sup>40</sup>K]], [[Thorium-232|<sup>232</sup>Th]], [[Uranium-235|<sup>235</sup>U]] and [[Uranium-238|<sup>238</sup>U]]. The heat varied with [[geothermal gradient]] and was typically 10–30&nbsp;°C per km of depth from the Earth's surface. Unusual [[magma]] intrusions, however, could have created greater localized heating.<ref name="Schobert-2013" />

==== Oil window (temperature range) ====
Geologists often refer to the temperature range in which oil forms as an ''"oil window"''.<ref name="Stanford_Edu">{{Cite web |last=Malyshev |first=Dmitry |date=December 13, 2013 |title=Origin of oil |url=http://large.stanford.edu/courses/2013/ph240/malyshev2/ |url-status=live |archive-url=https://web.archive.org/web/20210921125720/http://large.stanford.edu/courses/2013/ph240/malyshev2/ |archive-date=September 21, 2021 |access-date=September 21, 2021 |website=large.stanford.edu |language=English}}</ref><ref>{{Cite book |url=https://books.google.com/books?id=xwLHnC9qMsgC&pg=PA104 |title=Polar Prospects:A minerals treaty for Antarctica |publisher=United States, Office of Technology Assessment |year=1989 |isbn=978-1-4289-2232-7 |page=104 |access-date=May 12, 2020 |archive-url=https://web.archive.org/web/20200729203620/https://books.google.com/books?id=xwLHnC9qMsgC&pg=PA104 |archive-date=July 29, 2020 |url-status=live}}</ref><ref name="Schobert-2013" /> Below the minimum temperature oil remains trapped in the form of kerogen. Above the maximum temperature the oil is converted to natural gas through the process of [[thermal cracking]]. Sometimes, oil formed at extreme depths may migrate and become trapped at a much shallower level. The [[Athabasca oil sands]] are one example of this.<ref name="Schobert-2013" />