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Single-Channel Seismic Interpretation from the Joetsu Gas Hydrate Area, Eastern Margin of Japan Sea

Freire, Antonio F.1; Satoh, Mikio 2; Santos, Luiz A.3; Machyiama, Hideaki 4; Sugai, Toshihiko 1; Matsumoto, Ryo 5
1 Natural Environmental Studies, University of Tokyo, Graduate School of Frontier Sciences, Kashiwa, Japan.
2 Advanced Industrial Science and Technology (AIST), Tsukuba, Japan.
3 Petróleo Brasileiro S/A (PETROBRAS), Rio de Janeiro, Brazil.
4 Japan Marine Earth Science Technology (JAMSTEC), Kochi, Japan.
5 Earth and Planetary Sciences, University of Tokyo, Graduate School of Science, Tokyo, Japan.

Umitaka Spur and Joetsu Knoll are gas hydrate sites in the Joetsu Basin, off Joetsu, eastern margin of Japan Sea. Vents, giant plumes, hydrate outcrops, bacterial mats and carbonate crusts are direct evidence of gas hydrate and methane seeps in the study area. Mounds and pockmarks are also observed over these gas hydrate sites in a strict relation with gas hydrate plumes. A 2D Single Chanel Seismic (SCS) survey was conducted in 2006, 2007 and 2008 cruises using R/V Natsushima of JAMSTEC, and shows indirect indications of gas hydrate deposits, confirmed by the surface and near surface studies. Gas chimneys, BSR’s, amplitude anomalies and “pull up” structures are observed. The BSR’s occurs at around 0.15~0.20ms below sea floor on both Umitaka Spur and Joetsu Knoll. Heat flow measurements by piston cores and push cores indicate a gradient around 10 degrees C/100m, giving a very good indication about the base of gas hydrate stability zone (BGHSZ) at around 150mbsf, same depth inferred using SCS data. Double BSR’s appear on the northeastern part of Joetsu Knoll. It may represent a seismic signal of sea level changes, indicating that the BGHSZ was shallower in the past. In this study we identified deep and shallow faults and fractures, which are pathways for gas migration from deep reservoirs. This pattern is likely to control gas migration, gas hydrate distribution, and gas chimney activity. The last one controls the occurrence of plumes, mounds and pockmarks on the spur and the knoll.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009