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Coastal and Marine Geology Program
USGS Woods Hole Science Center

Open-File Report 96-272, Offshore Gas Hydrate Sample Database with an Overview and Preliminary Analysis
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3. Profile of Gas Hydrate Sample Data

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         3.1   Geographic Locations
         3.2    Water Depth and Subseabed Depth
         3.3    Physiographic Provinces and Tectonic Settings
         3.4    Habit or Mode of Occurrence
         3.5    Associated Sediment Textures
         3.6    Thickness of Hydrate Zones and Dimensions of Pure Hydrates
         3.7    Origin: Biogenic vs. Thermogenic

3.5  Associated Sediment Textures

Figure 5. The frequency of occurrence of gas hydrates with respect to sediment texture.
Figure 5. The frequency of occurrence of gas hydrates with respect to sediment texture. Click for larger view.

It is possible that gas hydrate habit may be influenced by the texture of the host sediment, but the assertion that hydrate nucleation itself may be somewhat influenced by the texture of the sediment cannot be supported by the database. Hydrates have been found in association with the spectrum of grain sizes from gravel through clay. More are associated with finer-grained sediment than with coarser sediment. Given the preponderance of fines (silt and clay) over sand and gravel in sediments in general, this finding is not considered conclusive.

Gravel was a common grain size at many sites. Because the focus of this work concerns factors that may affect hydrate formation, any textures made as a product of gas hydrate formation or dissociation are not included in the analysis. Specifically, the breccias described in Appendix (Black Sea, Caspian Sea) may be a consequence of expansion of the pore fluid in veins or joints as it converts to gas hydrate; that is, volume increase in association with hydrate formation [5] may have fractured the sediment. Because carbonate concretions may be a product of the dissociation of hydrates, the observed associations with carbonate gravel (rubble) (Column N, Appendix) does not necessarily have connotations regarding preferences with respect to the genesis of hydrates.

Based on visual descriptions from Deep Sea Drilling Program (DSDP) or Ocean Drilling Project (ODP), three of the of 28 hydrate samples (11%) are associated with sands; however, nearly 30% of the samples were recovered from zones having both sands and finer-grained sediment. Results of visual grain size classification of the sediment by the authors cited in the database are shown in Figure 5.

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