HR: 1340h
AN: C13A-0262    [Abstracts]
TI: Concentration of Natural Gas Hydrate Beneath the Permafrost Zone: Implications for Geochemical and Hydrologic Investigations
AU: * Uchida, T
EM: uchida@rc.japex.co.jp
AF: JAPEX Research Center, 1-2-1 Hamada, Mihama, Chiba, 2610025 Japan
AU: Waseda, A
EM: amane@rc.japex.co.jp
AF: JAPEX Research Center, 1-2-1 Hamada, Mihama, Chiba, 2610025 Japan
AU: Namikawa, T
EM: namikawa-takatoshi@jogmec.go.jp
AF: JOGMEC Technology Research Cetner, 1-2-2 Hamada, Mihama, Chiba, 2610025 Japan
AB: Gas hydrates are ice-like solids made of water molecules containing various gas molecules. The geological evaluations have suggested worldwide methane contents of gas hydrate beneath deep sea floors as well as permafrost-related zones to about twice the total reserves of conventional and unconventional hydrocarbon. Scientific and economic interests are increasing in gas hydrate as a new energy resource and a potential greenhouse gas. In 1998 and 2002 Mallik wells were drilled in the Canadian Arctic that clarified the characteristics of gas hydrate-dominant layers at depths from 890 to 1110 m beneath the permafrost zone. Continuous downhole well log data, anomalies of chloride contents in pore waters, core temperature depression as well as visible gas hydrates have confirmed the highly saturated pore-space hydrate as intergranular pore filling within sandy layers, whose saturations are higher than 70% in pore volume. Muddy sediments scarcely contain gas hydrate. The Nankai Trough runs along the Japanese Island, where forearc basins and accretionary prisms developed extensively and BSRs (bottom simulating reflectors) have been recognized widely. The METI Nankai Trough wells in 2000 also revealed the presence of pore-space hydrate filling intergranular pore of sandy layers. It is remarked that there are many similar features in appearance and characteristics between the Mallik and Nankai Trough areas with observations of well-interconnected and highly saturated pore-space hydrate. It is necessary for evaluating subsurface fluid flow behaviors to know both porosity and permeability of gas hydrate-bearing sandy sediments, and measurements of water permeability for them indicate that highly saturated sands may have permeability of a few millidarcies. Subsequent analyses in sedimentology and geochemistry performed on gas hydrate-bearing sands revealed important geologic and sedimentologic controls on the formation and concentration of gas hydrate. It is suggested that the distribution of a porous and coarser-grained sandy sediments is one of the most important factors to control the occurrence of gas hydrates, as well as physicochemical conditions.
DE: 9315 Arctic region
DE: 1800 HYDROLOGY
DE: 1823 Frozen ground
SC: Cryosphere [C]
MN: 2004 AGU Fall Meeting