Nankai Trough

You et al. (1995) studied bulk samples of fluids incorporated in sediments from ODP Site 808, in the Nankai Trough, southwest of Japan. Pore fluids have somewhat variable isotopic compositions (8 Li = +10 to +21), with a spike of light compositions near the basal decollement. These authors interpreted the decollement zone geochemical anomaly to represent influx of waters with Li derived from leaching of sediments at high temperatures. 

You CF, Chan LH, Spivack AJ, Gieskes JM (1995) Lithium, boron, and their isotopes in ODP Site 808, Nankai Trough sediments and pore waters Implications for fluid expulsion in accretionary prisms. Geology 23 ... 

SpivackAJ, Edmond JM (1986) Determination of boron isotope ratios by thermal ionization mass spectrometry of the dicesium metaborate cation. Anal Chem 58 31-35 Spivack AJ, Kastner M, Ransom B (2002) Elemental and isotopic chloride geochemistry and fluid flow in the Nankai Trough. Geophys Res Lett 29 1-4... 

JNOC press release, Proof of the methane-hydrate-containing layer in the national wildcat, Nankai Trough, based on the National Domestic Petroleum Exploration Program, Japan National Oil Corporation, January 20 (2000). 

Livi KJT, Veblen DR, Ferry JM, Frey M (1997) Evolution of 2 1 layered silicates in low-grade metamorphosed Liassic shales of Central Switzerland. J Metamor Geol 15 323-344 Masuda H, O Neil JR, Jiang W-T, Peacor DR (1996) Relation between interlayer composition of authigenic smectite, mineral assemblages, 1/S reaction rate and fluid composition in silicic ash of the Nankai Trough. Clays Clay Minerals 44 460-469... 

Takahashi, H., Japan Petroleum Exploration Co., L., Yonezawa, T., Corporation, J.N.O., Takedomi, Y., and Ministry of Economy, T.A.I., 2001. Exploration for Natural Gas Hydrate in Nankai-Trough Wells Offshore Japan, Offshore Technology Conference, Volume 13040 Houston, Texas, OTC. 

Indeed, there are reports of He He ratios > crustal production rates (-0.05 Ra Andrews 1985) in fore-arc regions such as the Nankai Trough (1.9 Ra Kastner et al. 1993), the Cook Inlet of Alaska (0.8 Ra Poreda et al. 1988) and Kavachi volcano in the Solomon Islands (6.9 Ra Trull et al. 1990)— this would indicate a direct output of mantle-derived volatiles. However, there are no estimates of the scale of this phenomenon. 

Origin and migration of methane in gas hydrate-bearing sediments in the Nankai Trough... 

In 1995, Japan s Ministry of Economy, Trade and Industry (METI), formerly Ministry of International Trade and Industry (MITI), launched a project to explore for marine gas hydrate accumulations around Japan. From late 1999 to early 2000, an exploratory hole MITI Nankai Trough was drilled on the landward side of the eastern Nankai Trough, offshore Japan (Fig. 1) by Japan National Oil Corporation (JNOC) along with Japan Petroleum Exploration Co., Ltd (JAPEX) as the well operator. The water depth at the drill site was 945 m and the sub-bottom depth of the hole was 2355 m. The seismic bottom simulating reflector (BSR) is present at around 295 mbsf. There were two exploration objectives one was a gas hydrate survey in shallow Quaternary sediments and the other was conventional oil and gas exploration in deeper Tertiary sediments. In addition to the main hole, seven short holes (two site survey, two pilot and three post-survey holes) were also drilled for the gas hydrate survey around the main hole. In this paper, we will clarify the origins of methane in gas hydrates found in the MITI Nankai Trough Well and discuss gas migration and hydrate formation in the sediments. 

Fig. 2. Gas isotope compositions, gas hydrate saturation and TOC in the Nankai Trough, (a) Depth-trends of of CH4 and CO2. Open symbols, PTCS samples solid symbols, headspace gas samples.

Fig. 3. Interpretive plot of molecular ratios of hydrocarbons vs. of methane in gas hydrate from the Nankai Trough and the Blake Ridge (Matsumoto et al, 2000). Genetic classification is modified from Bernard (1978).

Table 1. Gas molecular composition and of methane in pressure-temperature-core sampler (PTCS) from the post-survey hole-2 and headspace gas samples from the main hole of the MITI Nankai Trough Well...

Table 3. Occurrences of gas hydrates, origin of gas, average TOC, geological settings and fluid migration patterns in the Nankai Trough and the Blake Ridge...

Nankai Trough Thin (1-10 m) three High (max. Sand and mud Microbial CO2 0.5% (too low to explain Active margin Active flow to... 

Uchida T., Lu H., Tomaru T., Dallimore S. R. and Nankai Trough Scientific Party (2004) Subsurface occurrence of natural gas hydrate in the Nankai Trough area implication for gas hydrate concentration. Resource Geol. 54,35-44. 

Waseda A. and Uchida T. (2002) Origin of methane in gas hydrates from the Mackenzie Delta and Nankai Trough. Proceedings of the Fourth International Confererwe on Gas Hydrates, Yokohama, pp. 169-174. 

Waseda A., Baba K, Yagi M., Matsumoto R., Lu H., Hiroki Y. and Fuin T. (1998) Geochemical characteristics of gases and sediments in the Nankai Trough, offshore Tokai. Proceedings of the International Symposium on Methane Hydrates Resources in the Near Future , Chiba, pp. 55-59. 

---