2. OCCURRENCE, STRUCTURE, AND COMPOSITION OF NATURAL GAS HYDRATE
RECOVERED FROM THE BLAKE RIDGE, NORTHWEST ATLANTIC¹

Ryo Matsumoto,² Takashi Uchida,³ Amane Waseda,³ Tsutomu Uchida,⁴ Satoshi Takeya,⁵ Takashi Hirano,⁶ Kenji Yamada,⁷ Yuriko Maeda,⁸ and Tomoharu Okui⁸

ABSTRACT

Ocean Drilling Program (ODP) Leg 164 recovered a number of large solid gas hydrate from Sites 994, 996, and 997 on the Blake Ridge. Sites 994 and 997 samples, either nodular or thick massive pieces, were subjected to laboratory analysis and measurements to determine the structure, molecular and isotopic composition, thermal conductivity, and equilibrium dissociation conditions. X-ray computed tomography (CT) imagery, X-ray diffraction, nuclear magnetic resonance (NMR), and Raman spectroscopy have revealed that the gas hydrates recovered from the Blake Ridge are nearly 100% methane gas hydrate of Structure I, cubic with a lattice constant of a = 11.95 ± 0.05Å, and a molar ratio of water to gas (hydration number) of 6.2. The ¹⁸O of water is 2.67 to 3.51 SMOW, which is 3.5-4.0 heavier than the ambient interstitial waters. The ¹³C and D of methane are -66 to -70 and -201 to -206, respectively, suggesting that the methane was generated through bacterial CO₂ reduction. Thermal conductivity values of the Blake Ridge hydrates range from 0.3 to 0.5 W/(m·K). Equilibrium dissociation experiments indicate that the three-phase equilibrium for the specimen is 3.27 MPa at 274.7 K. This is almost identical to that of synthetic pure methane hydrate in freshwater.

¹Paull, C.K., Matsumoto, R., Wallace, P.J., and Dillon, W.P. (Eds.), 2000. Proc. ODP, Sci. Results, 164: College Station, TX (Ocean Drilling Program).
²Geological Institute, University of Tokyo 113-0033, Hongo, Tokyo 113-0033, Japan. ryo@geol.s.u-tokyo.ac.jp
³JAPEX Research Centre, Japan Petroleum Exploration Co. Ltd. Hamada, Chiba 261-0025, Japan.
⁴Hokkaido National Industrial Research Institute, Tsukisamu-Higashi, Sapporo 062-8517, Hokkaido, Japan.
⁵The Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido, Japan.
⁶Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Hokkaido, Japan.
⁷Fundamental Research Laboratories, Osaka Gas Co. Ltd., Konohana-ku, Osaka 554-0051, Japan.
⁸Frontier Technology Research Institute, Tokyo Gas Co. Ltd., Suehiro-cho, Tsurumi-ku, Yokohama 230-0045, Japan.

Date of initial receipt: 2 December 1998
Date of acceptance: 8 April 1999
Ms 164SR-247