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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 12. (December)

First Page: 2649

Last Page: 2662

Title: Gas Hydrates in Ocean Bottom Sediments

Author(s): M. K. MacLeod (2)

Abstract:

Gas hydrates belong to a special category of chemical substances known as inclusion compounds. An inclusion compound is a physical combination of molecules in which one component becomes trapped inside the other. In gas hydrates, gas molecules are physically trapped inside an expanded lattice of water molecules. The pressures and temperatures beneath Arctic water depths greater than 1,100 ft (335 m) and subtropical water depths greater than 2,000 ft (610 m) are suitable for the formation of methane hydrate. Theoretical depths to the base of a gas hydrate layer in ocean bottom sediments are determined by assuming: (1) a constant hydrostatic pressure Previous HitgradientNext Hit, (2) two typical hydrothermal gradients, (3) variable geothermal gradients, and (4) pure methane hydrated with conna e seawater.

In addition to pressure and geothermal Previous HitgradientNext Hit, other variables affecting the stability of gas hydrate are examined. These variables are hydrothermal Previous HitgradientNext Hit, sediment thermal conductivity, heat flow, hydrate Previous HitvelocityNext Hit, gas composition, and connate water salinity. If these variables are constant in a lateral direction and the above assumptions are valid, a local geothermal Previous HitgradientNext Hit can be determined if the depth to the base of a gas hydrate is known.

The base of the gas hydrate layer is seen on seismic profiles as an anomalous reflection nearly parallel to the ocean bottom, cross-cutting geologic bedding plane reflections, and generally increasing in sub-ocean bottom time with increasing water depth. The acoustic impedance is a result of the relatively fast Previous HitvelocityNext Hit hydrate layer overlying slower Previous HitvelocityTop sediments. In addition, free gas may be trapped beneath the hydrate, thereby enhancing the reflection.

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