Origins of methane

The volatile-trapping mechanism has a further problem associated with the temperature. Very volatile molecules such as N2, CO and CH4 are not easily trapped in laboratory ice simulation experiments unless the ice temperature is 75 K, which is somewhat lower than the estimated Saturnian subnebula temperature. This has led to the suggestion that the primary source of nitrogen within the Titan surface ices was NH3, which became rapidly photolysed to produce H2 and N2 upon release from the ice. The surface gravity is insufficient to trap the H2 formed and this would be lost to space. However, the origin of methane on Titan is an interesting question. Methane is a minor component of comets, with a CH4/CO ratio of clCT1 compared with the present atmospheric ratio of > 102. The D/H ratio is also intermediate between that of comets and the solar nebula, so there must be an alternative source of methane that maintains the carbon isotope ratio and the D/H isotope ratio and explains the abundance on Titan. 

Schoell M (1984) Recent advances in petroleum isotope geochemistry. Org Geochem 6 645-663 Schoell M (1988) Multiple origins of methane in the Earth. Chem Geol 71 1-10 Schoell M, McCaffrey MA, Fago FJ, Moldovan JM (1992) Carbon isotope compositions of 28,30-bisnorhopanes and other biological markers in a Monterey crude oil, Geochim Cosmochim Acta 56 1391-1399... 

Methane. Considerable attention must be given to the origin of methane in terms of organic chemical reactions. The abundance of this compound relative to liquid hydrocarbons is substantial, commonly amounting in mass to more than the liquid hydrocarbons in a given basin. 

Shoji and Langway (1982) described air hydrates found with ice cores off Greenland, while Tailleur and Bowsher (1981) indicated the presence of hydrates associated with coals in permafrost regions. Hondoh (1996) suggested that deep ice hydrates of air in Antarctica can be used to predict the Earth s ancient climate. Such hydrates are formed from air imbedded in snowfall and have been buried at pressure for hundreds of thousands of years. Rose and Pfannkuch (1982) have considered the applicability of the Deep Gas Hypothesis to the origin of methane in hydrates. 

SchoeU M. (1988) Multiple origins of methane in the Earth. Chem. Geology 71, 1 — 10. 

Hulston J. R. (1986) Further isotope evidence of the origin of methane in geothermal area. Proc. 5th hit. Symp. Water-Rock Interaction, Iceland, 270-273. 

In 1995, Japan s Ministry of Economy, Trade and Industry (METI), formerly Ministry of International Trade and Industry (MITI), launched a project to explore for marine gas hydrate accumulations around Japan. From late 1999 to early 2000, an exploratory hole MITI Nankai Trough was drilled on the landward side of the eastern Nankai Trough, offshore Japan (Fig. 1) by Japan National Oil Corporation (JNOC) along with Japan Petroleum Exploration Co., Ltd (JAPEX) as the well operator. The water depth at the drill site was 945 m and the sub-bottom depth of the hole was 2355 m. The seismic bottom simulating reflector (BSR) is present at around 295 mbsf. There were two exploration objectives one was a gas hydrate survey in shallow Quaternary sediments and the other was conventional oil and gas exploration in deeper Tertiary sediments. In addition to the main hole, seven short holes (two site survey, two pilot and three post-survey holes) were also drilled for the gas hydrate survey around the main hole. In this paper, we will clarify the origins of methane in gas hydrates found in the MITI Nankai Trough Well and discuss gas migration and hydrate formation in the sediments. 

Waseda A. and Uchida T. (2002) Origin of methane in gas hydrates from the Mackenzie Delta and Nankai Trough. Proceedings of the Fourth International Confererwe on Gas Hydrates, Yokohama, pp. 169-174. 

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