Offshore Japan

Map of Nankai Trough

Japan is a volcanic island arc system formed by the subduction of the Pacific and Philippine Sea plates beneath the Eurasian plate. In northern Japan the western edge of the Pacific plate converges with and is being consumed beneath Japan (Eurasian plate) along the Japan Trench. In southern Japan, the western edge of the smaller Philippine Sea plate is in contact with Japan along the Nankai trough. The Nankai trough is marked by a gentle bathymetric expression and somewhat shallower water depths than other oceanic trenches; however, the Nankai Trough is by no means inactive.

The rate of convergence is approximately 4 centimeters per year and great thrust-type subduction-zone earthquakes occur approximately every 100 years. The eastern edge of the Philippine Sea plate, along the Izu-Bonin trench, is where the most active subduction occurs today. Subduction there may be causing tectonic elevation of the northern part of the Nankai trough and the Philippine Sea plate.

Line drawing interpretation of seismic lines showing development and deformation of the accretionary wedge and location of the BSR. OOST – signifies out of sequence thrust – younger fault system that cut pre-existing fault imbricate fault sequence)

Landward of the Nankai Trough lies the Nankai accretionary wedge. Seaward of the trough is the Shikoku Basin, part of the Philippine Sea plate. Mid-Miocene to Holocene turbidites and hemipelagic sediments of the trough and Shikoku Basin have been scraped off during subduction to form the accretionary wedge. Further landward is an older accretionary wedge, the Shimanto Belt. The Cretaceous to Miocene-age Shimanto Belt consists of turbidites cut by imbricated thrust faults and melanges that contain ocean-floor basalts, pelagic limestone, radiolarian chert and shale, and hemipelagic shale. The Shimanto belt is considered an ancient analog to the Nankai Trough today.

Gas hydrates on the shelf and continental slope near Japan are identified by the presence of well-developed Bottom Simulating Reflectors (BSR) that are recorded by high-resolution reflection seismic surveys. The BSRs record the position of the hydrate/gas interface. The strongest reflectors occur in regions where there is enough sediment porosity for gas hydrates to form and where gas is trapped beneath the impermeable hydrate. The hydrate itself forms a seal that prevents further gas migration. BSRs are generally discontinuous which may reflect porosity variations in the sediments. BSRs show the minimum area of gas hydrate occurrence; hydrates are known to occur beyond BSR limits.

South of Japan, deep-sea drilling as part of the Ocean Drilling Program has confirmed the presence of gas hydrates in two localities in the accretionary prism northwest of the Nankai Trough. Gas hydrates have also been confirmed in the back-arc-basin system northwest of Japan. In the Nankai Trough region, gas is leaking from cold-fluid seepage zones, along faults on the seafloor that lie parallel to the trough. Chemical analysis of the chlorine in associated waters and the isotopic character of carbon in the gas suggest the methane is biogenic produced by the disassociation of hydrates. Other features of gas seepage include pockmarked depressions formed by fault movement and gas dissociation, and concentrations of shellfish and other marine life along the faults.

Two wells drilled onshore provide information on the source and possible reservoirs that may exist offshore. The Upper Oligocene and Lower Miocene rocks in the two onshore wells contain 0.5 to 1.0- percent total organic carbon (TOC), and up to 20 percent porosity. It is believed that these source rocks are widely distributed offshore.

Estimates of the volume of natural gas hydrates and associated free gas below the hydrate in the Nankai Trough region are varied and have been calculated at between 16 to 27 trillion cubic meters.

Along the southwest margin of the Nankai Trough, methane hydrates, identified by BSRs, apparently occur in greater concentrations than those identified to the northeast. Several factors may be responsible: longer history of gas migration and concentration or a difference in the timing, rate of elevation, or fault history related to tectonism in the subduction complexes. In the northern regions, the gas hydrates do not appear to be concentrated near the margin of the accretionary wedge as in the south; in the north they show a more random distribution with no direct relation to major structures.

The Drill Ship M.G. Hulme

Japanese scientists drilled 3 wells in 1999 with the dual objectives of characterizing the methane hydrates and investigating the petroleum potential of a deeper formation. The site is near Tokyo Bay, 50km off the coast of Japan at a water depth of 950m. Seismic data display a well-defined BSR, the site is relatively shallow, and there may be conventional hydrocarbons structurally trapped in the deeper strata. This site, Nankai Trough, is not actually in the bathymetric depression, but is in shallow water east of the trough. Depth to the BSR is approximately 290 meters below the sea floor (1240 meters below sea level). Two wells were drilled and cored in order to develop an appropriate casing program. Although no hydrate was observed in the core samples, the low chloride concentration in the pore water suggests the hydrate had dissociated before reaching the surface. The main well was drilled to 3000 meters below sea level; cores were taken between 1110-1272m. Methane hydrate occurred in three zones, totaling 16 feet, between 1152 and 1210m. As with the previous core samples, no hydrate reached the surface. However, distortion of the sediment (from gas flow and dewatering), the large amounts of gas contained in the sediment, the low temperature of the sample, and the low chloride-ion content of the pore water infer the presence of gas hydrates. Gas hydrate formed 20 percent of the bulk volume and 80% of the pore space. Volume of the hydrate is calculated to be 525 million cubic meters per square kilometer, and it is estimated that up to 50 trillion cubic meters of methane may be present in the Nankai Trough.

The future of methane hydrate extraction is promising. However, in order to produce methane from the methane hydrates, safe and dependable technology needs to be developed because dissociation of the methane in shallow water can produce very high pressures.

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