Identification Of Hydrate

Identification of areas of hydrate in the vicinity of Japan (Fig. 2) is presently

Figure 2. Map showing hydrate deposits around Japan as revealed by presence of hydrate/gas interface BSR. NT, Nankai Trough. Hydrate locations compiled by Mr. Mikio Satoh, Geological Survey of Japan. After: Aoki et al. (1983), Arato et al. (1996), Ashi and Taira (1993), Ashi and Tokuyama (1997), Ashi et al. (1996), Ashi et al. (1999), Baba and Uchida (1998), Honza (1078), Hyndman et al. (1992), Nakamura et al. (1997), Okuda (1996), Sakai (1998), Satoh et al. (1996), Taira et al. (1991), Tamaki et al. (1990), Tsuji et al. (1998), Yamano et al. (1982, 1984, 1992), and unpublished data held by the Geological Survey of Japan.

Figure 2. Map showing hydrate deposits around Japan as revealed by presence of hydrate/gas interface BSR. NT, Nankai Trough. Hydrate locations compiled by Mr. Mikio Satoh, Geological Survey of Japan. After: Aoki et al. (1983), Arato et al. (1996), Ashi and Taira (1993), Ashi and Tokuyama (1997), Ashi et al. (1996), Ashi et al. (1999), Baba and Uchida (1998), Honza (1078), Hyndman et al. (1992), Nakamura et al. (1997), Okuda (1996), Sakai (1998), Satoh et al. (1996), Taira et al. (1991), Tamaki et al. (1990), Tsuji et al. (1998), Yamano et al. (1982, 1984, 1992), and unpublished data held by the Geological Survey of Japan.

confined to those areas where well developed Bottom Simulating Reflectors (BSR) have been identified. This interpretation must be carried out with care because apparent BSRs identified on seismics in geological strata in and around northern Japan are known to be due to the presence of Opal A/CT horizons from drilling and stratigraphic analysis.

On high-resolution reflection seismic survey records mapped, at least two of the following gas hydrate BSR characteristics were recorded: reverse polarity (phase reversal) versus sea bottom reflector, relatively high amplitude of the BSR reflection coefficient, cross-cutting of bedding in the sediments, and geoacoustic profile of high velocity hydrate-rich sediment overlying lower velocity gas-rich sediments. BSR regions adjacent to Japan are usually discontinuous over large areas and not clearly defined as a single unbroken seismic feature (detected as "high amplitude zone" not as single continuous reflector), probably because of the bedding causes a strong associated porosity differentiation. Tight sediments do not allow enough gas to pond below hydrate to cause a well defined BSR to form and may not have been porous enough to allow methane to migrate into the bed to form hydrate in the first instance.

Hydrate investigators in the hydrate research program of Japan are aware that using BSR to identify the presence of hydrate yields a minimum distribution indicator. Hydrate is known to extend well beyond BSR (Max and Lowrie, 1996; Max and Dillon, 1998), which marks a hydrate-gas interface. BSR is commonly found below a bathymetric culmination where the base of hydrate forms a closure (= trap) that allows gas that is prevented by hydrate-tightened porosity from migrating upward into the HSZ or in a bedded series where a compound hydrate-stratigraphic trap sequence can form in dipping beds.

The confirmation that very large volumes of methane hydrate occurs in deep marine sediments (Matsumoto et al., 1996) has been one of the major accomplishments of the Ocean Drilling Program (ODP). Hydrate was recovered by deep sea drilling off the SE coast of the U.S. and the western offshore of the U.S. and Central America (Chapter 2). Hydrate has also been sampled in the course of ODP drilling in two localities to the S of Japan along the active front of the Ryukyu accretionary prism and off NW Japan and at one locality to the NW of Japan in the back-arc basin system (Fig. 2) in two ODP wells and one MITI well. Chemical analyses of samples have confirmed that naturally occurring hydrate is composed almost entirely methane hydrate (Chapter 2).

It is yet too soon to say whether the apparently greater concentration of hydrate along the SW margin of the Nankai Trough compared with the identified locations to the NE reflects a longer history of gas migration, and concentration by hydrate formation in seafloor sediments, or whether there was some difference in the timing, rate of elevation, fault history, etc. of the tectonic mechanisms themselves in the two subduction complexes. Perhaps older subduction complexes simply generate more methane later in their history, perhaps the nature of the sediments in the subduction zone or their thermal history was different. On the other hand, identification is at a preliminary stage and it is perhaps be too early to draw unequivocal conclusions. The database is as yet very small as hydrate around Japan remains largely to be quantified; much may occur well away from the present firm identifications (Fig. 2), which are based primarily upon the existence of hydrate/gas BSR. Also, virtually nothing is known about the age of the methane or action of the methane concentration cycle through a process of hydrate formation, dissociation, and formation of subsequent hydrate in any of the deposits adjacent to Japan.

To the NE of the termination of the Nankai Trough margin (Fig. 1), hydrate does not appear to be concentrated near the margin of the accretionary prism but is rather distributed with no apparent pattern that can be directly related to major structures. A number of fairly large hydrate deposits have been identified in the back-arc basins to the NW of Japan as well as between land area of Japan and the continental margin (Fig 2). However, this apparent distribution and relationship may not hold up when more hydrate is identified in the region.

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