Methane hydrate formation process in the forearc basin of Southwest Japan: Methane was generated beneath the trench bottom.

抄録

Methane and iodine are accumulated in pore water of the forearc basins in southwest Japan. ¹²⁹I dating revealed that the pore water was much older than the host rock, explained to have been derived from an older accretionary prism¹⁾. On the other hand, methane is microbial origin, generated in the shallow depth of sedimentary basins^2),3). There is no evidence whether methane was derived from the trench-fill sediments fabricating the accretionary prism or generated in the forearc basin. Kaneko^4),5) explained methane was generated in the former, and pore water migrated from shallow depth of the younger accretionary prism to the forearc basin, that did not require large-scale advection of a huge amount of pore water. Based on this idea, the model is revised with considering the relationship between geothermal gradient of the basin and the optimum temperature of methanogen, the amount of free methane released by the development of the accretionary prism, and an interpretation of ¹²⁹I ages and iodine contents.

Main Process

a) Generation of a large amount of microbial methane beneath the trench bottom with a high geothermal gradient.

b) Decreasing of methane solubility by P/T condition changing at the deformation front, generation and migration of free methane, hydrate formation.

c) Continuous supply of free methane to the hydrate layer due to the development of the accretionary prism.

d) Recycling of hydrates and compaction flow of pore water from the accretionary prism to the slope and forearc basins with their development.

e) Formation of dissolved-in-water type natural gas deposit, such as Minami-Kanto gas field, after hydrate dissociation by uplift.

Subordinate process

Additional hydrate formation by supplying methane microbially generated in the forearc and slope basins.

Possibility

Injection of originally thermogenic gas with changed chemical and isotope compositions as microbial gas during migration⁶⁾.

1) Tomaru and Fehn, 2015, Geochim. Cosmochim. Acta, 149, 64–78.

2) Kaneko et al., 2002, J. Jap. Assoc. Petrol. Technol, 67, 97-110.

3) Waseda and Uchida, 2004, Resource Geol., 54, 69-78.

4) Kaneko, 2008, AIST GREEN Rep., 63-64.

5) Kaneko, 2009, Soc. Iodine Sci. Rep. 12, 109-110．

6) Kaneko and Igari, 2020, J. Jap. Assoc. Petrol. Technol, 85, 62-73.