Methane-hydrate Occurrence and Distribution in the Eastern Nankai Trough, Japan: Findings of the Tokai-oki to Kumano-nada Methane-hydrate Drilling Program
Yoshihiro Tsuji, Tetsuya Fujii, Masao Hayashi, Ryuta Kitamura, Masaru Nakamizu, Katsuhiro Ohbi, Tatsuo Saeki, Koji Yamamoto, Takatoshi Namikawa, Takao Inamori, Nobutaka Oikawa, Shoshiro Shimizu, Masayuki Kawasaki, Sadao Nagakubo, Jun Matsushima, Koji Ochiai, Toshiharu 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", Natural Gas Hydrates—Energy Resource Potential and Associated Geologic Hazards, T. Collett, A. Johnson, C. Knapp, R. Boswell
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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.