MIS09-02

日本地球惑星科学連合2016年大会

熱分解起源メタンを包接する結晶構造I型の天然ガスハイドレートの特徴
Characteristics of structure I natural gas hydrate encaged thermogenic methane*八久保 晶弘1、太田 有香1、竹谷 敏2、Vereshchagina Olga3、Jin Young K.4、Obzhirov Anatoly3、小西 正朗
1、坂上 寛敏1、南 尚嗣1、山下 聡1、髙橋 信夫1、庄子 仁1*Akihiro Hachikubo1, Yuka Oota1, Satoshi Takeya2, Olga Vereshchagina3, Young Keun Jin4, Anatoly
Obzhirov3, Masaaki Konishi1, Hirotoshi Sakagami1, Hirotsugu Minami1, Satoshi Yamashita1, NobuoTakahashi1, Hitoshi Shoji1
1.北見工業大学、2.産業技術総合研究所、3.ロシア太平洋海洋学研究所、4.韓国極地研究所
1.Kitami Institute of Technology, 2.National Institute of Advanced Industrial Science andTechnology (AIST), 3.Pacific Oceanological Institute, FEB RAS, 4.Korea Polar Research Institute
Crystallographic structures of natural gas hydrate are usually either structure I or structure II.
The latter can encage larger hydrocarbons, for example, propane, isobutane, n-butane, andneopentane. Because the origin of these molecules is thermogenic, methane ascending with them fromdeeper sediment layer is also thermogenic. Hydrate-bound thermogenic methane has been reported in
the world (Gulf of Mexico, offshore Vancouver Island, Caspian Sea, etc.). C1/C2+ of guest gas inthese sites are less than 10, indicating that compositions of ethane and propane in hydrate-boundhydrocarbons are in an order of several percent. Therefore, crystallographic structure of gas
hydrate composed of thermogenic gas is primarily the cubic structure II.
On the other hand, the structure I gas hydrates retrieved off Joetsu contained thermogenic methane(delta 13C > -50 permil, e.g. Lu et al., 2011). C1/C2+ of hydrate-bound hydrocarbons was more than
2,000, whereas the maximum value of methane delta 13C was -35permil (Hachikubo et al., 2015). It isstill unknown how higher hydrocarbons reduced in the sediment. Gas hydrates have been discovered at
the southwestern Sakhalin Island in the cruises of LV59 (2012), LV62 (2013), LV67 (2014), and LV70(2015) on board R/V Akademik M. A. Lavrentyev in the framework of Sakhalin Slope Gas Hydrate (SSGH)
project. We reported in the last JpGU meeting that hydrate-bound gas contained 13C-rich methane,suggesting thermogenic origin. In this study, we focus on the gas hydrates of the cubic structure I
containing thermogenic methane retrieved from the Tatar Trough, off Sakhalin Island, and comparewith those retrieved off Joetsu.
We obtained hydrate crystals from sediment cores, and stored them in liquid nitrogen. Raman spectraof the crystal showed two peaks of C-H stretching mode, correspond to methane molecules in large
and small cages of the structure I, and small peaks of hydrogen sulfide were also detected. We also
obtained hydrate-bound gas on board and measured their molecular and stable isotope compositions.
C1/C2+ of hydrate-bound hydrocarbons ranged between 200 and 800, suggesting that contribution ofthermogenic C2+ was low. However, delta 13C and delta D of hydrate-bound methane distributed from-48permil to -42permil and from -200 permil to -170 permil, respectively. According to an empiricalclassification of the methane stable isotopes (delta 13C and delta D; Whiticar, 1999),
hydrate-bound methane obtained at the Tatarsky Trough was mainly thermogenic origin.
Characteristics of hydrate-bound methane is similar to those obtained off Joetsu. delta 13C of CO2in sediment gases was high (+20 permil), suggesting interaction between methane and CO2 throughmicrobial activity.
We appreciate the support of the crew onboard R/V Lavrentyev during the LV59, LV62, LV67, and LV70
cruises off Sakhalin Island. This study was supported by the Grant-in-Aid for Scientific Research(B) 26303021 of the Japan Society for the Promotion of Science (JSPS).
Hachikubo, et al. (2015) Molecular and Isotopic Composition of Volatiles in Gas Hydrates and inSediment from the Joetsu Basin, Eastern Margin of the Japan Sea. Energies 8: 4647-4666.

©2016. Japan Geoscience Union. All Right Reserved.

- MIS09-02 -MIS09-02

日本地球惑星科学連合2016年大会

doi:10.3390/en8064647
Lu H, et al. (2011) The characteristics of gas hydrates recovered from Joetsu Basin, eastern marginof the Sea of Japan. In Proceedings of the 7th International Conference on Gas Hydrates, Edinburgh,UK, 17-21 July 2011.
Whiticar MJ (1999) Carbon and hydrogen isotope systematics of bacterial formation and oxidation ofmethane. Chem Geol 161: 291-314. doi:10.1016/S0009-2541(99)00092-3
キーワード：ハイドレート、メタン、サハリン島
Keywords: hydrate, methane, Sakhalin Island©2016. Japan Geoscience Union. All Right Reserved.

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