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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

3.4  Habit or Mode of Occurrence

Figure 4. Relative frequency of occurrence of different gas hydrate forms  with respect to sediment type.
Figure 4. Relative frequency of occurrence of different gas hydrate forms with respect to sediment type. Click for larger view.

Gas hydrate samples have been described most frequently as consisting of individual grains or particles. In this form they often were further described as existing as inclusions or as being disseminated. Hydrates have been frequently observed as cements and also as laminae or layers. The terms "massive" and "nodule" have also been applied to hydrate samples, as have a variety of other terms. From these descriptions two types of occurrences based on geometry can be defined: 2-dimensional (laminae, layers, plates, mats, etc.), which implies gas hydrate crystallization along or parallel to bedding planes, or in joints or fractures; and 3-dimensional (granules, nodules, "massive"), which encompasses individual particles large enough to distort or crosscut bedding planes. A third category, cement, is also utilized. It represents an occurrence in which the hydrate has filled the sediment interstices apparently with little destruction or modification of the host sediment fabric. The drill hole data showed that no particular category (habit) was dominant. As indicated by Figure 4, almost all of the 2-dimensional samples were associated with fine sediment, either intrinsically or in fractures. 3-dimensional samples were numerous in fine sediments as well, but were also identified in sediment of more than one basic grain size. Hydrate cements clearly were associated with coarser material.

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