HR: 0800h
AN: C11A-1062    [Abstracts]
TI: Origin Of Methane Gas And Migration Through The Gas Hydrate Stability Zone Beneath The Permafrost Zone
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, 1-2-2 Hamada, Mihama, Chiba, 2610025 Japan
AB: In 1998 and 2002 Mallik wells were drilled at Mackenzie Delta in the Canadian Arctic that clarified the characteristics of gas hydrate-dominant sandy layers at depths from 890 to 1110 m beneath the permafrost zone. Continuous downhole well log data as well as visible gas hydrates have confirmed pore-space hydrate as intergranular pore filling within sandy layers whose saturations are up to 80% in pore volume, but muddy sediments scarcely contain. Plenty of gas hydrate-bearing sand core samples have been obtained from the Mallik wells. According to grain size distributions pore-space hydrate is dominant in medium- to very fine-grained sandy strata. Methane gas accumulation and original pore space large enough to occur within host sediments may be required for forming highly saturated gas hydrate in pore system. The distribution of a porous and coarser-grained host rock should be one of the important factors to control the occurrence of gas hydrate, as well as physicochemical conditions. Subsequent analyses in sedimentology and geochemistry performed on gas hydrate-bearing sandy core samples also revealed important geologic and sedimentological controls on the formation and concentration of natural gas hydrate. This appears to be a similar mode for conventional oil and gas accumulations. It is necessary for investigating subsurface fluid flow behaviors to evaluate both porosity and permeability of gas hydrate-bearing sandy sediments, and the measurements of water permeability for them indicate that highly saturated sands may have permeability of a few millidarcies. The isotopic data of methane show that hydrocarbon gas contained in gas hydrate is generated by thermogenic decomposition of kerogen in deep mature sediments. Based on geochemical and geological data, methane is inferred to migrate upward closely associated with pore water hundreds of meters into and through the hydrate stability zone partly up to the permafrost zone and the surface along faults and permeable sandy pathways. It should be remarked that there are many similar features in appearance and characteristics between the terrestrial and deep marine areas such as Nankai Trough with observations of well-interconnected and highly saturated pore-space hydrate.
DE: 0702 Permafrost (0475)
DE: 0714 Clathrate
DE: 1829 Groundwater hydrology
DE: 3004 Gas and hydrate systems
DE: 4820 Gases
SC: Cryosphere [C]
MN: Fall Meeting 2005