Gas hydrates in ocean bottom sediments

Journal Article · Wed Dec 01 04:00:00 EST 1982 · Am. Assoc. Pet. Geol., Bull.; (United States)

DOI:https://doi.org/10.1306/03B5AC8C-16D1-11D7-8645000102C1865D· OSTI ID:5630703

MacLeod, M K

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 Artic 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 gradient, (2) two typical hydrothermal gradients, (3) variable geothermal gradients, and (4) pure methane hydrated with connate seawater. In addition to pressure and geothermal gradient, other variables affecting the stability of gas hydrate are examined. These variables are hydrothermal gradient, sediment thermal conductivity, heat flow, hydrate velocity, gas composition, and connate water salinity. If these variables are constant in a lateral direction and the above assmptions are valid, a local geothermal gradient 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 velocity hydrate layer overlying slower velocity sediments. In addition, free gas may be trapped beneath the hydrate, thereby enhancing the reflection.

• Research Organization: Chevron U.S.A., Concord, CA
• OSTI ID: 5630703
• Journal Information: Am. Assoc. Pet. Geol., Bull.; (United States), Journal Name: Am. Assoc. Pet. Geol., Bull.; (United States) Vol. 66:12; ISSN AAPGB
• Country of Publication: United States
• Language: English

Related Subjects

03 NATURAL GAS
030200* -- Natural Gas-- Reserves
Geology
& Exploration
CHEMISTRY
GAS HYDRATES
GEOCHEMISTRY
GEOPHYSICAL SURVEYS
GEOTHERMAL GRADIENTS
HEAT FLOW
HYDRATES
SEA BED
SEDIMENTS
SEISMIC SURVEYS
SURVEYS
TEMPERATURE GRADIENTS