Takashi Uchida¹, Hailong Lu², Ryo Matsumoto³, Ikuo Mizukoshi⁴, Hisashi Ishida⁵

(1) JAPEX Research Center, Chiba, Japan
(2) JNOC Technology Research Center, Chiba, Japan
(3) University of Tokyo, Tokyo, Japan
(4) JAPEX, Tokyo, Japan
(5) JNOC, Tokyo, Japan

ABSTRACT: Occurrence of subsurface gas hydrate at the Nankai Trough: Implication for the geological and geochemical characteristics

The Nankai Trough runs along the Japanese Island, where extensive BSRs have been recognized. High resolution seismic surveys and site-survey wells conducted by the MITI (Ministry of International Trade and Industry) in 1997 have revealed the subsurface gas hydrate distribution at a depth of about 290 mbsf (1,245 mbsl). The MITI Nankai Trough wells were drilled in late 1999 and early 2000. Two pilot holes and the main hole were drilled to 655 mbsf and 2,355 mbsf, respectively, followed by three post-holes drilled to 355 mbsf. Gas hydrate-bearing sand core samples were successfully retrieved from the main hole and the #2 post-hole by using the PTCS core system, allowing for various laboratory analyses and physical measurements. The anomalies of chloride contents in extracted pore waters, core temperature depression, core observations as well as visible gas hydrates confirm the presence of pore-space hydrates as intergranular pore filling within moderate to thick sand layers dominantly in the depth interval from 205 to 268 mbsf which was within the methane hydrate stability zone. The LWD (logging while drilling) and wire-line log data were collected from the #2 pilot hole, main hole, #1 and #3 post-holes. In 1998 the JAPEX/JNOC/GSC Mallik 2L-38 well was successfully drilled at the Mackenzie Delta, Canadian Arctic that clarified the characteristics of in-situ natural gas hydrates beneath the permafrost zone. There are many similarities between the Mackenzie Delta and the Nankai Trough with observations of well-interconnected and highly saturated pore-space gas-hydrate systems within clastic sediments.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado