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.
Due to the current energy crisis caused by global warming and geopolitical events, evaluating the energy potential of marine gas hydrates around the Japan Islands has become an urgent issue. Based on open and public reports, a critical overview is provided of the current status of the exploration of gas hydrates in the eastern Nankai Trough and along the eastern margin of the Japan Sea. In 2001, an exploration and resource assessment of gas hydrates in the eastern Nankai Trough was launched as a national project. In the late 20th century, hydrate-induced BSRs had been widely recognized on the fore-arc basins and slopes of the accretionary wedges of the Nankai Trough. Integrated geological and geophysical exploration reveals that the hydrates of the eastern Nankai Trough occur in the interstitial pore space of sand layers (pore filling type) of the Early Pleistocene turbidite units of 100 to 200 m in thickness above the BSR-BGHS. Hydrate exploration in the Japan Sea entered the National project in 2012, following academic efforts over the preceding 10 years, confirming 1,742 columnar-shaped gas chimney structures (generally a few hundred meters across and roughly 100 m deep) in which a few centimeters to nearly 50 m of thick massive hydrate deposits occur in the Middle Pleistocene silty host sediments. Japan Sea hydrates are considered to have grown in the host sediments, displacing clayey particles (particle displacement type). Gas chimney structures are the main conduits for the migration of deep-seated methane, associated with sea floor methane seeps, chemosynthetic communities, and methane-derived authigenic carbonates. A high concentration of massive hydrate is related to enhanced maturation and accumulation of hydrocarbon deposits under the high heat flow of the Japan Sea basin. Additionally, glacial eustasy of the closed Japan Sea basin facilitated efficient recycling of methane to accumulate massive hydrates.
