Editing Alkane (section)

===Occurrence of alkanes on Earth===
Traces of methane gas (about 0.0002% or 1745&nbsp;ppb) occur in the Earth's atmosphere, produced primarily by [[methanogenesis|methanogenic]] microorganisms, such as [[Archaea]] in the gut of ruminants.<ref>{{cite journal | last1 = Janssen | first1 = P. H. | last2 = Kirs | first2 = M. | year = 2008 | title = Structure of the Archaeal Community of the Rumen | journal = Appl Environ Microbiol | volume = 74 | issue = 12| pages = 3619–25 | doi = 10.1128/AEM.02812-07 |pmc= 2446570 | pmid=18424540| bibcode = 2008ApEnM..74.3619J }}</ref>

The most important commercial sources for alkanes are natural gas and [[Petroleum|oil]].<ref name=m&b/> Natural gas contains primarily methane and ethane, with some [[propane]] and [[butane]]: oil is a mixture of liquid alkanes and other [[hydrocarbons]]. These hydrocarbons were formed when marine animals and plants (zooplankton and phytoplankton) died and sank to the bottom of ancient seas and were covered with sediments in an [[wikt:anoxic|anoxic]] environment and converted over many millions of years at high temperatures and high pressure to their current form. Natural gas resulted thereby for example from the following reaction:
:C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> → 3&nbsp;CH<sub>4</sub> + 3&nbsp;CO<sub>2</sub>

These hydrocarbon deposits, collected in porous rocks trapped beneath impermeable cap rocks, comprise commercial [[oil fields]]. They have formed over millions of years and once exhausted cannot be readily replaced. The depletion of these hydrocarbons reserves is the basis for what is known as the [[energy crisis]].

Alkanes have a low solubility in water, so the content in the oceans is negligible; however, at high pressures and low temperatures (such as at the bottom of the oceans), methane can co-crystallize with water to form a solid [[methane clathrate]] (methane hydrate). Although this cannot be commercially exploited at the present time, the amount of combustible energy of the known methane clathrate fields exceeds the energy content of all the natural gas and oil deposits put together. Methane extracted from methane clathrate is, therefore, a candidate for future fuels.