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Researching Beaufort Sea methane hydrate USGS and university scientists seek evidence for impact of climate change on known methane hydrate deposits close to coastline Alan Bailey Petroleum News
It has long been known that deposits of methane hydrate, an ice-like substance that traps methane, the primary component of natural gas, lie below the seafloor under the shallow waters of Alaska’s Beaufort Sea. But concerns about the possibility that the warming climate may be causing these hydrates to disassociate, expelling methane into the sea and atmosphere, has prompted a team of scientists to investigate the current status of the deposits, according to a report published in the latest edition of the Department of Energy’s Fire in the Ice methane hydrate newsletter.
With funding support from the Department of Energy, scientists from the U.S. Geological Survey, Stanford University and Texas A&M University have been participating in the work.
Offshore vulnerable While a thick layer of onshore permafrost in the Beaufort Sea region protects the methane hydrates under the land, sea level rise and seawater inundation in the offshore since the end of the last ice age has resulted in the thawing of offshore permafrost and a relatively high risk of hydrate disassociation, the report says.
One scientist, working in the USGS Woods Hole Science Center, is using 1970s-era seismic data supplemented by legacy borehole logs to develop a map of methane hydrate deposits on the Alaska Beaufort Sea shelf. And members of the research team conducted cruises in August 2010 and 2011 to verify the map data and collect new geophysical and geochemical data. The cruises focused on the central Alaska Beaufort Sea shelf between Cape Halkett and Prudhoe Bay, including Harrison Bay, the submerged part of the Colville River Delta and the barrier islands, the DOE newsletter report says.
The water is very shallow in the area of the cruises, with geologic evidence indicating that the sea only inundated the area within the last few thousand years, the report says.
The team used sonar and shallow seismic technologies to map the seafloor and near-surface sediments. To avoid impacts on local subsistence hunting, the team used an electrical spark system rather than conventional seismic air guns as the sound source for the seismic work.
Ice-bonded strata The results of the survey show ice-bonded strata of the Tertiary-age Sagavanirktok formation dipping seaward under the younger and shallower Gubik formation at depths that are consistent with what has been interpreted from the legacy seismic data as the location of ice-bonded sediments. And there is evidence of widespread ice scouring throughout the study area, the report says.
The sonar data indicate the common presence of gas in near-seafloor strata, with variations in rock types under the seafloor and the degree of ice scouring both affecting the gas distribution. However, the seismic data provided no insights into whether this gas came from deep underground, as would be expected if the gas resulted from the disassociation of hydrates, the report says.
Pockmark features on the seafloor in southern Harrison Bay and seaward of the Colville River Delta suggest the venting of gas, but the researchers found no evidence of active venting at the time of the surveys.
Methane concentrations The 2011 survey also used a spectrographic technique to measure the concentrations of methane in seawater along the tracks where geophysical data were collected. Methane concentrations proved quite variable, with high concentrations observed along part of a track offshore the Colville Delta.
It will be necessary to use new techniques to determine whether methane in the atmosphere or seawater in the Beaufort Sea area originates from disassociating hydrates, as the permafrost melts, the report says. Those techniques potentially include isotopic analysis of the methane and the geophysical techniques for the mapping of subsea permafrost.
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