Overview: Resource Assessment of Methane Hydrates at the Eastern Nankai Trough and the Eastern Margin of the Sea of Japan

Abstract

 In 1995, Japan launched a national project to assess methane hydrates as future gas resources, which successfully identified and recovered hydrate-cemented sandy sediments from Pleistocene turbidite units at the Nankai Trough. Extensive geological and geophysical exploration over 20 years reveals a wide distribution of hydrate-induced BSR at the eastern Nankai Trough. The amounts of hydrates in sandy sediments are estimated to be 17 to 39 vol.% based on low salinity anomalies of squeezed pore waters. Hydrate amounts equivalent to 6 to 7 vol.% of sediment volume extend from the seafloor to the base of gas hydrate stability (BGHS). A probabilistic approach to assess amounts of hydrates based on 2D/3D seismic profiles, LWD geophysical parameters, and geochemical analysis of cores provides an approximation of the amounts of resources in place at 40 tcf of methane gas in the entire BSR area (4,687 km²) and at 20 tcf in the hydrate concentrated zone (767 km²) of the eastern Nankai Trough. An integrated academic, industrial, and national program carried out since 2004 identifies another type of hydrate in the eastern margin of the Sea of Japan. Japan Sea hydrates occur as massive aggregates in a chimney-like acoustic blanking zone of a few 100 m across and 100-120 m thick (gas chimney) in Pleistocene hemipelagic sediments. MBES and SBP surveys confirm 1,742 gas chimneys along the eastern margin of the Sea of Japan and around Hokkaido Island. Amounts of hydrates estimated from combined elastic wave velocity Vp and low salinity anomalies are 35 to 74 vol.% of the volume of the gas chimney structure. BSRs in hydrate-bearing gas chimneys exhibit a sharp pull-up structure within the gas chimneys, reflecting high Vp. Assuming that the Vp of massive hydrate is 3.7 km/sec and Vp of the host sediments in gas chimneys is same as Vp of the sediments around gas chimneys, the amounts of hydrates in the gas chimneys are calculated to be 15 to 65 vol.% of chimney volume. High Vp anomalies of hydrate-bearing sediments is also applied to hydrates estimate at the Nankai trough.