Selection Of A Hydrate Natural Laboratory The Nankai Trough

Many areas of hydrate have been identified adjacent to the Japanese islands (Fig. 2). One area has been selected as the site at which drilling and testing of sampling and extractive technologies could be carried out. This area was chosen because: 1. It displays well defined BSR; 2. There is a good stratigraphy in dipping clastic sediments in which gas should flow; 3. Conventional hydrocarbons are also anticipated to occur in deeper strata forming potential structural traps; 4. Drilling would be relatively easier in this shallow water site than in deeper, and more remote, sites elsewhere; 5. Operational support is available from ports nearby. The Nankai Trough area is very close (about 200 km) to Tokyo Bay, which is in the center of the Japanese Pacific coast industrial belt; 6. In addition, in the case that the area proves to be a source of commercial methane, it is immediately adjacent to one of the largest hydrocarbon consuming areas in Japan, and the transport distance of the methane from source to market would be minimal. Although this last aspect was not a primary factor in identifying the area in which the first in-situ R&D should be carried out, strongly positive results would allow virtually complete amortization of the R&D by gas production.

The area selected is referred to as the 'Nankai Trough' by Japanese investigators, even though it occurs in relatively shallow water in water depths slightly less than 1 km and not in the nearby geomorphic Nankai Trough, which is a linear bathymetric depression marking the approximate junction between the Japanese island arc and the Pacific oceanic plate (Figs. 1, 2). The term 'Nankai Trough', however, is used here to remain consistent with Japanese hydrate terminology and usage.

The stratigraphy of the nearby land area is characterized by forearc and accretional Cenozoic sediments (Tsuji et al., 1998). Forearc sediments are Mikura, Kurami-Saigo, Sagara, Kakegawa, and Ogasa Groups in ascending order bounded by unconformities, and the sediments consist of clastics which are mostly alternation of sandstone and siltstone with associated conglomerates and tuff. Accretional sediments are Setogawa, Ooigawa, and Takakusayama Groups, made of ocean floor sediments such as basalt, limestone, and chert, and slope sediments including allocthonous limestone blocks. Two MITI wells, "Sagara" and "Omaezaki-oki", were drilled close to the survey area on land immediately to the north. Both reached the Paleogene, the former to the Takakusayama Group and the latter to the Mikura Group.

In the Sagara district on land, the Sagara oil field, which produced small amounts of oil from the alternating sandstones and siltstones of the Sagara Group (late Miocene to Pliocene). The results of two MITI wells suggest that reservoir rocks and matured source rocks can be anticipated nearby offshore. Upper Oligocene strata in the MITI "Omaezaki-oki" (offshore well in about 560 m water depth about 37 km NNE of the test hydrate drill hole) and Upper Oligocene to Lower Miocene in MITI "Sagara" (coastline well about 58 km NE of the test drill hole, Tjsui et al. 1998, Fig. 1) have 0.5 to 1% of TOC (Total Organic Carbon), and have the potential to be hydrocarbon and methane source rocks. Matured source rock (Ro > 0.5) is expected below 3,000 mbsl in MITI "Omaezaki-oki", and most of the Upper Oligocene and Lower Miocene subsided are on the shelf, area are regarded as thermally matured with respect to gas production. Some areas in the 'Nankai Trough' hydrate area contain thick sediments with sufficient organic detritus which would insure significant methane generation and possibly higher gravity hydrocarbons. Sandstones in the Sagara Group have more than 20% of porosity in MITI "Sagara" and demonstrate proven gas production. The Sagara Group and its constituent sandstones is believed to be widely distributed in this region offshore (Tsuji et al., 1998).

Cold fluid seepage containing biogenic hydrocarbons has been sampled by submersible and ROVs. The methane is believed to be produced from dissociation of hydrates (Ashi and Tokuyama, 1997) because of the chlorinity of associated waters and the isotopic character of carbon in gas. These seepages are usually found in the seafloor along faults that lie parallel to the trend of Nankai Trough and are often associated with concentrations of seafloor shellfish and other life. Large-scale pockmark-like depressions are common and thought to be related to movement on faults and associated hydrate dissociation (Kuramoto et al., 1997). Methane-rich gas taken from a JNOC gravity core sample 813 C values of about 75%, suggests a biogenic origin (Chapter 7).

Although BSR has been recognized in the Nankai Trough research area (Fig. 2) on the basis of observed BSR (Arato et al. 1996; Satoh et al., 1996), hydrate is probably developed more broadly. Estimation of hydrate volumes in the 'Nankai Trough' hydrate study area is on-going (Matsumoto et al. 1996; 1998). Satoh et al. (1996) estimated resources of natural gas hydrates and associated free gas with hydrated layer in the Nankai Trough offshore Shikoku region at from 2.7 x 1012m3 to 1.6 x 1012m3 (Tsuji et al., 1998).

Was this article helpful?

0 0

Post a comment