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Abstract

Tsuji, Y., T. Namikawa, T. Fujii, M. Hayashi, R. Kitamura, M. Nakamizu, K. Ohbi, T. Saeki, K. Yamamoto, T. Inamori, N. Oikawa, S. Shimizu, M. Kawasaki, S. Nagakubo, J. Matsushima, K. Ochiai, and T. Okui, 2009, Methane-hydrate occurrence and distribution in the eastern Nankai Trough, Japan: Findings of the Tokai-oki to Kumano-nada methane-hydrate drilling program, in T. Collett, A. Johnson, C. Knapp, and R. Boswell, eds., Natural gas hydrates—Energy resource potential and associated geologic hazards: AAPG Memoir 89, p. 228–246.

DOI:10.1306/13201103M893129

Copyright copy2009 by The American Association of Petroleum Geologists.

Methane-hydrate Occurrence and Distribution in the Eastern Nankai Trough, Japan: Findings of the Tokai-oki to Kumano-nada Methane-hydrate Drilling Program

Y. Tsuji,1 T. Fujii,2 M. Hayashi,3 R. Kitamura,4 M. Nakamizu,5 K. Ohbi,6 T. Saeki,7 K. Yamamoto,8 T. Namikawa,9 T. Inamori,10 N. Oikawa,11 S. Shimizu,12 M. Kawasaki,13 S. Nagakubo,14 J. Matsushima,15 K. Ochiai,16 T. Okui17

1Japan Oil, Gas and Metals National Corporation (JOGMEC), Chiba, Japan
2Japan Oil, Gas and Metals National Corporation (JOGMEC), Chiba, Japan
3Japan Oil, Gas and Metals National Corporation (JOGMEC), Chiba, Japan
4Japan Oil, Gas and Metals National Corporation (JOGMEC), Chiba, Japan
5Japan Oil, Gas and Metals National Corporation (JOGMEC), Chiba, Japan
6Japan Oil, Gas and Metals National Corporation (JOGMEC), Chiba, Japan
7Japan Oil, Gas and Metals National Corporation (JOGMEC), Chiba, Japan
8Japan Oil, Gas and Metals National Corporation (JOGMEC), Chiba, Japan
9Japan Petroleum Exploration Co., Ltd., Tokyo, Japan
10Japan Petroleum Exploration Co., Ltd., Tokyo, Japan
11Japan Petroleum Exploration Co., Ltd., Tokyo, Japan
12Japan Petroleum Exploration Co., Ltd., Tokyo, Japan
13Japan Drilling Co., Ltd., Tokyo, Japan
14Japan Drilling Co., Ltd., Tokyo, Japan
15The University of Tokyo, Tokyo, Japan
16Inpex Corp., Tokyo, Japan
17Tokyo Gas Co., Ltd., Tokyo, Japan

ACKNOWLEDGMENTS

Seismic surveys and multiwell drillings of the METI Tokai-oki to Kumano-nada were planned and financed by Japan's Ministry of Economy Trade and Industry (METI). Those programs were conducted as part of a research project entitled Japan's Methane Hydrate Exploitation Program, which is conducted by the Research Consortium for Methane Hydrate Resources in Japan (also known as the MH21 Research Consortium, which consists of Japan Oil, Gas and Metals National Corporation [JOGMEC]; National Institute of Advanced Industrial Science and Technology [AIST]; and Engineering Advancement Association of Japan [ENAA]), in which resource assessment is a major factor. The Japan Petroleum Exploration Co., Ltd. (JAPEX) and Teikoku Oil Co., Ltd. (TOC) jointly operated the drilling. Many geologists and engineers of JAPEX, TOC, and JOGMEC contributed to the drilling operation, its preparation, and data interpretation. Transocean's crew operated JOIDES Resolution, Oceaneering's crew operated ROV dives, Aumann Associates Inc. completed the PTCS, Schlumberger K.K. in Japan undertook the logging program and high-accuracy borehole-temperature-sensing program, and Japan Drilling Co., Ltd. (JDC) conducted the ex perimental well drillings. The analyses of the water chemistry of the cores were conducted by the students of the University of Tokyo under supervision of R. Matsumoto. The authors thank T. Collett and two anonymous reviewers for their critical comments, correction, and encouragements in preparing this article. METI, MH21, and JOGMEC provided the permission for publication of this report.

FMIast (fullbore formation microimager), GVRast (GeoVision Resistivity), ISONICast (IDEALast [integrated drilling evaluation and logging] sonic-while-drilling); seismicVisionast (seismic-while-drilling), CDNast (compensated density neutron).

DSIast (dipole shear sonic imager), CNLast (compensated neutron log), VSIast (versatile seismic imager), and ProVISIONast are all marks of Schlumberger.

ABSTRACT

Obtaining information on the occurrence, distribution, and in-situ concentration of methane hydrate is required for the evaluation of the resource potential of methane hydrate. The primary source of information on marine methane-hydrate occurrence comes from seismic data that have been calibrated with local downhole well-log and core data.

Widespread distributions of bottom-simulating reflectors (BSRs) are known in the Nankai Trough area offshore central Japan from seismic data collected for oil and gas exploration purposes. Since the first Nankai Trough methane-hydrate exploration wells drilled in 1999, systematic surveys were conducted in areas of interest. Following a series of two-dimensional (2-D) and three-dimensional (3-D) seismic surveys in 2001 and 2002, a multiwell drilling program titled Ministry of Economy, Trade and Industry Toaki-oki to Kumano-nada was successfully conducted in early 2004.

In 2004, 16 sites were drilled at water depths ranging from 720 to 2030 m (2362 to 6660 ft), and the hydrate-bearing sediments were easily identified with resistivity and density downhole logs at most of the sites where BSRs were detected, although the distribution and log-inferred thicknesses of the methane-hydrate occurrences varied. Sites with no or very thin methane-hydrate-bearing sediments were also associated with BSRs. Furthermore, some sites with no distinct BSRs also encountered methane hydrates.

It has been shown that BSRs are not always good indicators of concentrated methane-hydrate occurrences and cannot be used to accurately predict in-situ methane-hydrate volumes, although BSR is an important indicator of the existence of methane hydrates.

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