Nankai Methane Hydrate Site (or Japanese Methane Hydrate R&D Program at Nankai, Nankai Trough Methane Hydrate Site) is located in the Nankai Trough, Japan.
Introduction to the area The Nankai Trough is located beneath the Pacific Ocean off the southeast coast of Japan, and extends more than 700 km in a southwest-northward trending direction. The Nankai Trough is not only known as an active subduction and earthquake zone, but also for its large clathrate hydrate deposits occurring on the shelf and continental slope. Due to the combination of Japan’s lack of domestic fossil energy sources, along with a projected continued rise in energy consumption, the methane hydrate occurrences in Nankai Trough sedimentary sequences are increasingly the focus of energy resource studies. These resources have the potential to secure a supply of domestic natural gas to meet Japan’s needs for decades to come. The Nankai Trough drilling site is of major interest, as recent research in the area has offered groundbreaking new insights concerning the occurrence of methane hydrate deposits in a marine environment. Additionally, research at Nankai Trough is expected to promote the technical development necessary for the commercial exploration and production of methane gas from hydrates, and to facilitate its use and contribution to a long-term national energy strategy.
General physical and geological 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.
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