page_title: Nankai Trough gas hydrate site
section_title: General physical and geological description
hierarchical_section_title: Nankai Trough gas hydrate site / General physical and geological description
caption_attribution_description: English: The Nankai Trough fault line with its seismogenic active zone, southwest Japan.
context_page_description: Nankai Methane Hydrate Site is located in the Nankai Trough, Japan.
context_section_description: The Nankai Trough area was identified by the United States Department of Energy (DOE) and its United States Department of Energy National Laboratories (NETL) as an important hydrate-bearing region in the Asian-Pacific. DOE gas hydrate resource estimates indicate a hydrate potential of 16 to 27 trillion cubic meters. Research has confirmed the presence of gas hydrates in two different locations within the Nankai Trough: the hydrates can be found in the accretionary prism off Shikoku and Tokai, and also in the back-arc basin system northwest of Japan. Nearly all of the natural gas hydrate accumulations identified in the Nankai Trough region occur at a depth of around 290 to 300 meters below the seafloor (1,240 meters below sea-level). 
  The structure of the Nankai Trough is dominated by the development of this accretionary prism since the Miocene, which formed due to the northwestward trending subduction of the Philippine Sea plate beneath the Eurasian plate. The active subduction zone makes the Trough one of the most active earthquake zones on the planet. The subsiding segment of the Philippine plate is the so-called ‘Shikoku basin’, which defines a former forearc basin that was subsequently filled with sediments from submarine fan systems in the area, although most of the accreted sediments consist of coarse terrestrial clastics.
  In Arctic regions, natural gas hydrate deposits mostly occur in sand-rich units in association with permafrost. In contrast, gas hydrates in a marine environment tend to occur in continental margin sediments consisting primarily of fine-grained clay minerals and organic debris, which has settled onto the seafloor. They appear to be a very common constituent, filling spaces between the grains and within cracks and fissures in sediments on deepwater continental shelves where the temperature and pressure conditions are suitable for gas hydrate formation. It is generally believed that the majority of natural gas hydrate deposits around the world exist in fine-grained sediments in a dispersed and low-concentrated form, with hydrate saturations of between 1 and 12%. The low hydrate saturation can be explained due to the very small pore size and low permeability in clay-rich sediments, which hinder the mobility of both water and gas, necessary for hydrate formation (see UNEP Global Outlook on Methane Gas Hydrates (2012, in progress) for more details). In contrast, Japanese scientists were able to discover thick sequences of interbedded sands and muds while drilling on the southeastern coast on the margin of the Nankai Trough. Natural gas hydrates seem to mostly occur in high concentrations in the sand units, representing up to 70% of the pore space and are largely absent from the mud sequences.
caption: The Nankai Trough Fault Line with its seismogenic active zone
exif: {}