HR: 0800h
AN: C11A-1056    [Abstracts]
TI: Hydrocarbon
AU: * Collett, T S
EM: tcollett@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, MS-939, Box 25046, Denver, CO 80225 United States
AU: Lorenson, T D
EM: tlorenson@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS-999, Menlo Park, CA 94025 United States
AB: Gas hydrates in permafrost regions are believed to be a significant high-latitude reservoir for hydrocarbon gases, including methane which is a ``greenhouse" gas that may play a significant role in global climate warming. Melting permafrost and associated gas hydrates may contribute hydrocarbon ``greenhouse" gases to the atmosphere, however, little is known about the composition or distribution of the natural gases within permafrost. The primary objective of this presentation is to document and compare the composition and source of the hydrocarbon gases associated with gas hydrates both within and immediately below the zone of permafrost in the Prudhoe Bay-Kuparuk River area of northern Alaska. This study included two major geochemical sampling programs in northern Alaska. In the first program the in-situ composition of the gas within and below the zone of permafrost in the Prudhoe Bay-Kuparuk River area was determined, and in the second, a series of surficial geochemical surveys were made over the area of known surficial gas seepage. Geochemical analyses of drill cuttings collected from 11 petroleum industry wells indicate that methane is the principal hydrocarbon gas in the near-surface (0-1,500 m) strata of the North Slope. Stable methane-carbon isotopic analyses of gaseous drill cuttings from several wells suggest that the methane within the permafrost zone is from both microbial and thermogenic sources. To further examine shallow subsurface gas migration and potential atmospheric methane fluxes from permafrost regions, we analyzed the molecular and methane-carbon isotopic composition of the gas from 105 shallow (<2 m) permafrost cores collected across the Eileen fault zone, an area of known surficial gas seepage and more deeply buried gas hydrate occurrence in the Prudhoe Bay-Kuparuk River area. Analyses of these samples yielded high concentrations of methane and other hydrocarbon gases over the surface trace of the Eileen fault zone. Isotopic analysis of methane in the samples collected near the main fault yielded evidence of thermogenic gas. The data from shallow cores, together with the subsurface geochemical data obtained from northern Alaska, confirm that permafrost does control the distribution, volume, and composition of hydrocarbon gases in Arctic regions. In some areas the hydrocarbon gases in ice-bonded permafrost are exclusively microbial in origin, whereas thermogenic gases only occur beneath the ice-bonded interval. In such cases the base of ice-bonded permafrost may act as a trap for free gas accumulations. In other areas, where unique geologic conditions such as faulting occur, thermogenic gases originating from deep hydrocarbon reservoirs may occur within permafrost even at shallow depths. The abundance of both microbial- and thermogenic-sourced gases and their apparent mobility suggests that the melting of permafrost and associated gas hydrate accumulations could release significant volumes of hydrocarbon ``greenhouse" gases to the atmosphere.
DE: 1823 Frozen ground
DE: 4540 Ice mechanics and air/sea/ice exchange processes (0700, 0750, 0752, 0754)
DE: 4820 Gases
DE: 4840 Microbiology and microbial ecology (0465)
SC: Cryosphere [C]
MN: Fall Meeting 2005