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Sub-Sea-Floor Methane in the Bering SeaUSGS Emeritus Describes Possible Gas-Hydrate Accumulations to the Geophysical Society of Alaska
U.S. Geological Survey (USGS) emeritus scientist Dave Scholl described evidence for large accumulations of methane hydrate beneath the floor of the Bering Sea in an invited talk at a January 11 meeting of the Geophysical Society of Alaska in Anchorage. Methane hydrate is a naturally occurring crystalline substance in which molecules of methanea primary component of natural gasare trapped in a lattice of water molecules. Common in Arctic permafrost environments as well as in sea-floor sediment, methane hydrate interests scientists for numerous reasons, among them its potential as a future source of fossil fuel, and its potential to cause submarine landslides and to release large volumes of methanea greenhouse gaswhen conditions cause it to dissociate into free gas and water.
During his talk, Scholl showed seismic-reflection data collected by the U.S. Navy in the 1960s during antisubmarine-warfare research that revealed unexpected features beneath the floor of deep basins (3,500- to 4,000-m water depth) in the Bering Sea. The features, which range from 2 to 8 km in width, look like giant mushrooms in the seismic-reflection records. The "cap" of the mushroom is a stack of upward-arched seismic-reflection horizons with its top approximately 200 m below the sea floor. The "stem" of the mushroom is a continuous column of downward-arched horizons, extending from about 360 to at least 2,000 m below the sea floor. Using modern tools to re-analyze data in which the mushroom-like structures appear, USGS scientists have extracted new details about these features, now interpreted as natural-gas chimneys overlain by deposits of methane hydrate.
Seismic-reflection data are produced by bouncing acoustic (sound) energy off layers of sediment beneath the sea floor and recording the returning echoes. Some acoustic energy is reflected at any horizon where certain physical properties changeat the boundary between different sedimentary layers, for example. In the Bering Sea data, flat-lying horizons predominate, indicating that deep Bering Sea basins are filled with 2 to 4 (or more) km of sediment in little-deformed horizontal beds. The enigmatic "mushrooms" amid the flat-lying beds were dubbed "acoustic Velocity-AMPlitude (VAMP) anomalies" in the 1970s by USGS scientists examining both Navy and USGS seismic-reflection data from the Bering Sea.
Continuing research on the origin of VAMP structures brought recognition that the transition from upward-arching horizons to downward-arching horizons occurs at the same depthapproximately 360 m below the Bering Sea flooras the predicted transition from stable methane hydrate (above) to stable methane gas (below). The form in which methane is most stable is determined by pressure and temperature. In the Bering Sea, the pressure at about 360 m below the sea floor is approximately 400 bars, and the temperature approximately 24°C. Below this "transition depth," higher temperatures cause methane to exist as a gas in pores in the sediment. Above the transition depth, lower temperatures and moderate pressures cause methane to exist as methane hydrate.
Sound waves travel faster through methane hydrate than through water-filled sediment, and slower through gas-filled sediment than through water-filled sediment. Because of these acoustic-velocity differences, seismic-reflection horizons appear to bow upward where methane hydrate occurs, and downward where pore-filling methane gas occurs. The mushroom-like VAMP structures are thus believed to be acoustic images of large deposits of methane hydrate (the cap of the mushroom) directly overlying chimneys of ascending fluids carrying methane gas (the stem of the mushroom). The methane that forms VAMP structures is believed to be generated largely by thermal decomposition of organic matter in sedimentary deposits deeper within the sedimentary basin.
Thousands of VAMPs occur in the Bering Sea. New analyses by the USGS show that a single large VAMP structure involves a volume of methane equivalent to that of a large conventional gas field (approx 0.6-0.9 trillion ft3). If the hydrate hypothesis is correct and VAMP structures do indeed represent occurrences of methane hydrate and gas, then the potential total inventory of natural gas in the deep Bering Sea is enormous. The remoteness and depth of these deposits, however, make economic extraction implausible in the foreseeable future.
For more information about USGS study of the Bering Sea VAMPs, see "Possible Deep-Water Gas Hydrate Accumulations in the Bering Sea," by Ginger Barth, Dave Scholl, and Jonathan Childs, in the Fall 2006 issue of Fire in the Ice, the National Energy Technology Laboratory's Methane Hydrate Newsletter. In addition, an informative article about Scholl's talk to the Alaska Geophysical Society was written by Alan Bailey for the January 21, 2007, issue of Petroleum News (URL http://www.petroleumnews.com/pnfriends/286678373.shtml).
in this issue:
Sub-Sea-Floor Methane in the Bering Sea
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