Editing Methane clathrate (section)

===Oceanic===
Methane hydrate can occur in various forms like massive, dispersed within pore spaces, nodules, veins/fractures/faults, and layered horizons.<ref>{{cite journal |last1=Mishra |first1=C. K. |last2=Dewangan |first2=P |last3=Mukhopadhyay |first3=R |last4=Banerjee |first4=D |title=Available online 7 May 2021 1875-5100/© 2021 Elsevier B.V. All rights reserved. Velocity modeling and attribute analysis to understand the gas hydrates and free gas system in the Mannar Basin, India |journal=Journal of Natural Gas Science and Engineering |date=August 2021 |volume=92 |page=104007 |doi=10.1016/j.jngse.2021.104007 |s2cid=235544441 |url=https://www.sciencedirect.com/science/article/pii/S1875510021002134|url-access=subscription }}</ref> Generally, it is found unstable at standard pressure and temperature conditions, and 1 m<sup>3</sup> of methane hydrate upon dissociation yields about 164 m<sup>3</sup> of methane and 0.87 m<sup>3</sup> of freshwater.<ref>{{Cite book|last=Sloan|first=E. Dendy|url=https://www.worldcat.org/oclc/85830708|title=Clathrate hydrates of natural gases.|date=2008|publisher=CRC Press|others=Carolyn A. Koh|isbn=978-1-4200-0849-4|edition=3rd|location=Boca Raton, FL|oclc=85830708}}</ref><ref>{{cite journal |last1=Mishra |first1=C K |last2=Dewangan |first2=P |last3=Sriram |first3=G |last4=Kumar |first4=A |last5=Dakara |first5=G |title=Spatial distribution of gas hydrate deposits in Krishna-Godavari offshore basin, Bay of Bengal |journal=Marine and Petroleum Geology |year=2020 |volume=112 |page=104037 |doi=10.1016/j.marpetgeo.2019.104037 |bibcode=2020MarPG.11204037M |doi-access=free }}</ref><ref>{{cite journal |last1=Kvenvolden |first1=K A |title=Gas hydrates-geological perspective and global change |journal=Reviews of Geophysics |year=1993 |volume=31 |issue=2 |pages=173–187 |doi=10.1029/93RG00268 |bibcode=1993RvGeo..31..173K |url=https://doi.org/10.1029/93RG00268|url-access=subscription }}</ref> There are two distinct types of oceanic deposits.  The most common is dominated (> 99%) by [[methane]] contained in a structure I [[clathrate]] and generally found at depth in the sediment.  Here, the methane is isotopically light ([[δ13C|δ<sup>13</sup>C]] < −60‰), which indicates that it is derived from the microbial [[redox|reduction]] of [[carbon dioxide|CO<sub>2</sub>]].  The clathrates in these deep deposits are thought to have formed in situ from the microbially produced methane since the δ<sup>13</sup>C values of clathrate and surrounding dissolved methane are similar.<ref name="Kvenvolden 1995" />  However, it is also thought that freshwater used in the pressurization of oil and gas wells in permafrost and along the continental shelves worldwide combines with natural methane to form clathrate at depth and pressure since methane hydrates are more stable in freshwater than in saltwater.<ref name="Hassan 12305–12314"/>  Local variations may be widespread since the act of forming hydrate, which extracts pure water from saline formation waters, can often lead to local and potentially significant increases in formation water salinity.  Hydrates normally exclude the salt in the pore fluid from which it forms. Thus, they exhibit high electric resistivity like ice, and sediments containing hydrates have higher resistivity than sediments without gas hydrates (Judge [67]).<ref name="MH_future_2011">{{citation |title=Methane Hydrates and the Future of Natural Gas |first=Carolyn |last=Ruppel |series=Gas Hydrates Project |publisher=U.S. Geological Survey |location=Woods Hole, MA |url=https://mitei.mit.edu/system/files/Supplementary_Paper_SP_2_4_Hydrates.pdf |access-date=28 December 2014 |url-status=dead |archive-url=https://web.archive.org/web/20151106085926/http://mitei.mit.edu/system/files/Supplementary_Paper_SP_2_4_Hydrates.pdf |archive-date=6 November 2015}}</ref>{{rp|9}}

These deposits are located within a mid-depth zone around 300–500 m thick in the sediments (the [[gas hydrate stability zone]], or GHSZ) where they coexist with methane dissolved in the fresh, not salt, pore-waters. Above this zone methane is only present in its dissolved form at concentrations that decrease towards the sediment surface. Below it, methane is gaseous. At [[Blake Ridge]] on the Atlantic [[continental rise]], the GHSZ started at 190&nbsp;m depth and continued to 450&nbsp;m, where it reached [[phase equilibrium|equilibrium]] with the gaseous phase. Measurements indicated that methane occupied 0-9% by volume in the GHSZ, and ~12% in the gaseous zone.<ref name="Dickens 1997">{{Cite journal |last=Dickens |first=GR |author2=Paull CK |author3=Wallace P |year=1997 |title=Direct measurement of in situ methane quantities in a large gas-hydrate reservoir |journal=Nature |volume=385 |issue=6615 |pages=426–428 |doi=10.1038/385426a0 |bibcode=1997Natur.385..426D |url=https://deepblue.lib.umich.edu/bitstream/2027.42/62828/1/385426a0.pdf |hdl=2027.42/62828|s2cid=4237868 |hdl-access=free }}</ref><ref name=sautter2012>
{{cite web |url=http://oceanexplorer.noaa.gov/explorations/04etta/background/profile/profile.html |title=A Profile of the Southeast U.S. Continental Margin |author=Leslie R. Sautter |work=NOAA Ocean Explorer |publisher=National Oceanic and Atmospheric Administration (NOAA) |access-date=3 January 2015}}</ref>

In the less common second type found near the sediment surface, some samples have a higher proportion of longer-chain [[hydrocarbon]]s (< 99% methane) contained in a structure II clathrate. Carbon from this type of clathrate is isotopically heavier ([[Carbon-13|δ<sup>13</sup>C]] is −29 to −57 ‰) and is thought to have migrated upwards from deep sediments, where methane was formed by thermal decomposition of [[organic matter]]. Examples of this type of deposit have been found in the [[Gulf of Mexico]] and the [[Caspian Sea]].<ref name="Kvenvolden 1995"/>

Some deposits have characteristics intermediate between the microbially and thermally sourced types and are considered formed from a mixture of the two.

The methane in gas hydrates is dominantly generated by microbial consortia degrading organic matter in low oxygen environments, with the methane itself produced by [[methanogen]]ic [[archaea]]. Organic matter in the uppermost few centimeters of sediments is first attacked by aerobic bacteria, generating CO<sub>2</sub>, which escapes from the sediments into the [[water column]]. Below this region of aerobic activity, anaerobic processes take over, including, successively with depth, the microbial reduction of nitrite/nitrate, metal oxides, and then [[sulfate]]s are reduced to [[sulfide]]s.  Finally, methanogenesis becomes a dominant pathway for organic carbon [[remineralization]].

If the sedimentation rate is low (about 1&nbsp; cm/yr), the organic carbon content is low (about 1% ), and oxygen is abundant, aerobic bacteria can use up all the organic matter in the sediments faster than oxygen is depleted, so lower-energy [[electron acceptors]] are not used. But where sedimentation rates and the organic carbon content are high, which is typically the case on continental shelves and beneath western boundary current upwelling zones, the [[pore water]] in the sediments becomes [[Hypoxia (environmental)|anoxic]] at depths of only a few centimeters or less.  In such organic-rich marine sediments, sulfate becomes the most important terminal electron acceptor due to its high concentration in [[seawater]]. However, it too is depleted by a depth of centimeters to meters.  Below this, methane is produced. This production of methane is a rather complicated process, requiring a highly reducing environment (Eh −350 to −450 mV) and a pH between 6 and 8, as well as a complex [[syntrophic]], consortia of different varieties of archaea and bacteria. However, it is only archaea that actually emit methane.

In some regions (e.g., Gulf of Mexico, Joetsu Basin) methane in clathrates may be at least partially derive from thermal degradation of organic matter (e.g. petroleum generation), with oil even forming an exotic component within the hydrate itself that can be recovered when the hydrate is disassociated.<ref>Kvenvolden, 1998(incomplete ref)</ref><ref>{{Cite journal |last1=Snyder |first1=Glen T. |last2=Matsumoto |first2=Ryo |last3=Suzuki |first3=Yohey |last4=Kouduka |first4=Mariko |last5=Kakizaki |first5=Yoshihiro |last6=Zhang |first6=Naizhong |last7=Tomaru |first7=Hitoshi |last8=Sano |first8=Yuji |last9=Takahata |first9=Naoto |last10=Tanaka |first10=Kentaro |last11=Bowden |first11=Stephen A. |date=2020-02-05 |title=Evidence in the Japan Sea of microdolomite mineralization within gas hydrate microbiomes |journal=Scientific Reports |language=en |volume=10 |issue=1 |pages=1876 |doi=10.1038/s41598-020-58723-y |pmid=32024862 |pmc=7002378 |bibcode=2020NatSR..10.1876S |issn=2045-2322}}</ref>{{Citation needed|date=October 2011}} The methane in clathrates typically has a biogenic isotopic signature and highly variable δ<sup>13</sup>C (−40 to −100‰), with an approximate average of about −65‰ .<ref>Kvenvolden, 1993(incomplete ref)</ref>{{Citation needed|date=October 2011}}<ref>Dickens 1995 (incomplete ref)</ref><ref>{{Cite journal |last1=Snyder |first1=Glen T. |last2=Sano |first2=Yuji |last3=Takahata |first3=Naoto |last4=Matsumoto |first4=Ryo |last5=Kakizaki |first5=Yoshihiro |last6=Tomaru |first6=Hitoshi |date=2020-03-05 |title=Magmatic fluids play a role in the development of active gas chimneys and massive gas hydrates in the Japan Sea |journal=Chemical Geology |language=en |volume=535 |pages=119462 |doi=10.1016/j.chemgeo.2020.119462 |bibcode=2020ChGeo.53519462S |issn=0009-2541 |doi-access=free}}</ref><ref name="Matsumoto 1995">{{Cite journal |last=Matsumoto |first=R. |year=1995 |title=Causes of the δ<sup>13</sup>C anomalies of carbonates and a new paradigm 'Gas Hydrate Hypothesis' |journal=J. Geol. Soc. Japan |volume=101 |pages=902–924 |doi=10.5575/geosoc.101.902 |issue=11 |doi-access=free}}</ref> Below the zone of solid clathrates, large volumes of methane may form bubbles of free gas in the sediments.<ref name="Dickens 1997"/><ref name="Matsumoto 1996">{{Cite journal |last1=Matsumoto |first1=R. |last2=Watanabe |first2=Y. |last3=Satoh |first3=M. |last4=Okada |first4=H. |last5=Hiroki |first5=Y. |last6=Kawasaki |first6=M. |others=ODP Leg 164 Shipboard Scientific Party |year=1996 |title=Distribution and occurrence of marine gas hydrates - preliminary results of ODP Leg 164: Blake Ridge Drilling |journal=J. Geol. Soc. Japan |volume=102 |pages=932–944 |doi=10.5575/geosoc.102.932 |issue=11 |doi-access=free}}</ref><ref>{{cite web |url=http://ethomas.web.wesleyan.edu/ees123/clathrate.htm |title=Clathrates - little known components of the global carbon cycle |publisher=Ethomas.web.wesleyan.edu |date=2000-04-13 |access-date=2013-03-14}}</ref>

The presence of clathrates at a given site can often be determined by observation of a "bottom simulating reflector" (BSR), which is a seismic reflection at the sediment to clathrate stability zone interface caused by the unequal densities of normal sediments and those laced with clathrates.

[[Gas hydrate pingo]]s have been discovered in the Arctic oceans Barents sea. Methane is bubbling from these dome-like structures, with some of these gas flares extending close to the sea surface.<ref>{{cite web |url=https://phys.org/news/2017-06-domes-frozen-methane-blow-outs.html |title=Domes of frozen methane may be warning signs for new blow-outs |publisher=Phys.org |year=2017}}</ref>

====Reservoir size====
[[File:Gas_hydrate_under_carbonate_rock.jpg|thumb|Gas hydrate under carbonate rock on the seafloor of the northern Gulf of Mexico]]
The size of the oceanic methane clathrate reservoir is poorly known, and estimates of its size decreased by roughly an [[order of magnitude]] per decade since it was first recognized that clathrates could exist in the oceans during the 1960s and 1970s.<ref name="Milkov 2004">{{Cite journal |last=Milkov |first=AV |year=2004 |title=Global estimates of hydrate-bound gas in marine sediments: how much is really out there? |journal=Earth-Science Reviews |volume=66 |issue=3–4 |pages=183–197 |doi=10.1016/j.earscirev.2003.11.002 |bibcode=2004ESRv...66..183M}}</ref>  The highest estimates (e.g. 3{{e|18}} m<sup>3</sup>)<ref name="Trofimuk 1973">{{Cite journal |last=Trofimuk |first=A. A. |author2=N. V. Cherskiy |author3=V. P. Tsarev |year=1973 |title=[Accumulation of natural gases in zones of hydrate—formation in the hydrosphere] |journal=[[Doklady Akademii Nauk SSSR]] |volume=212 |pages=931–934 |language=ru}}</ref> were based on the assumption that fully dense clathrates could litter the entire floor of the deep ocean. Improvements in our understanding of clathrate chemistry and sedimentology have revealed that hydrates form in only a narrow range of depths ([[continental shelves]]), at only some locations in the range of depths where they could occur (10-30% of the [[Gas hydrate stability zone]]), and typically are found at low concentrations (0.9–1.5% by volume) at sites where they do occur. Recent estimates constrained by direct sampling suggest the global inventory occupies between {{convert|1e15|and|5e15|m3|e6mi3|sigfig=2|abbr=off}}.<ref name="Milkov 2004"/>  This estimate, corresponding to 500–2500 gigatonnes carbon (Gt&nbsp;C), is smaller than the 5000 Gt&nbsp;C estimated for all other geo-organic fuel reserves but substantially larger than the ~230 Gt&nbsp;C estimated for other natural gas sources.<ref name="Milkov 2004"/><ref name="USGS 2000">USGS World Energy Assessment Team, 2000. US Geological Survey world petroleum assessment 2000––description and results. USGS Digital Data Series DDS-60.</ref> The permafrost reservoir has been estimated at about 400&nbsp;Gt&nbsp;C in the Arctic,<ref>{{cite journal |last1=MacDonald |first1=G. J. |year=1990 |title=Role of methane clathrates in past and future climates |journal=Climatic Change |volume=16 |issue=3 |pages=247–281 |doi=10.1007/bf00144504 |bibcode=1990ClCh...16..247M|s2cid=153361540 }}</ref>{{Citation needed|date=October 2011}} but no estimates have been made of possible Antarctic reservoirs. These are large amounts. In comparison, the total carbon in the atmosphere is around 800&nbsp;gigatons (see [[Carbon#Occurrence|Carbon: Occurrence]]).

These modern estimates are notably smaller than the 10,000 to 11,000&nbsp;Gt&nbsp;C (2{{e|16}} m<sup>3</sup>) proposed<ref>{{Cite journal |last=Buffett |first=Bruce |author2=David Archer |date=15 November 2004 |title=Global inventory of methane clathrate: sensitivity to changes in the deep ocean |journal=Earth and Planetary Science Letters |volume=227 |issue=3–4 |pages=185–199 |url=http://geosci.uchicago.edu/~archer/reprints/buffett.2004.clathrates.pdf |quote=Preferred ... global estimate of 3<sup>18</sup> g ... Estimates of the global inventory of methane clathrate may exceed 10<sup>19</sup> g of carbon |doi=10.1016/j.epsl.2004.09.005 |bibcode=2004E&PSL.227..185B}}</ref> by previous researchers as a reason to consider clathrates to be a geo-organic fuel resource (MacDonald 1990, Kvenvolden 1998). Lower abundances of clathrates do not rule out their economic potential, but a lower total volume and apparently low concentration at most sites<ref name="Milkov 2004"/> does suggest that only a limited percentage of clathrates deposits may provide an economically viable resource.