東部南海トラフにおけるガスハイドレートの生成集積過程に関する地質学的・地球化学的制約

Abstract

From January to May of 2004, METI/JOGMEC-MH21 prepared the METI Exploratory Test Wells Tokai-oki to Kumanonada, including drillings, loggings, and long-term borehole temperature monitoring by DTS/FBG. A high-resolution geochemical study of interstitial water and sediments was conducted at the Dai-ichi Tenryu Knoll and the Daini-Atsumi Knoll. Cl- baselines (original in situ Cl- concentration) at both sites show contrasting and characteristic patterns. The oxygen isotope compositions of the interstitial water seem to show a symmetrical pattern mirror image similar to the Cl- concentration. The fluctuations of oxygen and the Cl- baseline can generally be explained by dilution/enrichment mechanisms caused by gas-hydrate formation and dissociation. The formation and dissociation history of gas hydrate at the two sites is summarized as follows: (1) Sea level had fallen toward the Last Glacial Maximum, and BGHS had traveled upward. Methane and heavy oxygen-enriched water traveled upward in response to gas-hydrate dissociation: (2) Released methane was again trapped above the new BGHS, and gas hydrates were concentrated within sandy sediments. The upper BSR in the Dai-ichi Tenryu Knoll area was formed at this time: (3) BGHS has migrated downward following the transgression over the last 18,000 yrs. In the Daini-Atsumi area, relic-BSR corresponding to the upper-BSR in the Dai-ichi Tenryu Knoll area would have disappeared due to rapid accumulations of gas hydrate generated by a high methane flux, whereas in the Dai-ichi Tenryu Knoll area, it would have remained long after BGHS migration due to a lower methane supply. Eustatic sea-level change has brought about a hydrostatic pressure change, and gas hydrate stability zone would also have changed. However, the amount of additional gas hydrate accumulation would have obliterated or facilitated the development of the relic BSR.