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3. Profile of Gas Hydrate Sample Data

3.6  Thickness of Hydrate Zones and Dimensions of Pure Hydrates

Figure 6. Dimensions of pure hydrate samples in comparison to thicknesses of hydrate zones. Click for larger view.

Gas hydrate may have an inconsistent presence over a considerable vertical distance in a sediment column at any particular site. For example, the section of hydrate- bearing sediment is more than 200 m thick at Site 4, Middle America Trench off Mexico. Within such sections individual zones that have a more uniform hydrate presence are often evident. The database table shows the number and thickness of relatively uninterrupted vertical extent of hydrate-bearing sediment at each site. ^[6] The data show the variation in drilled sediment to be from a few centimeters (Site 4, Middle America Trench off Guatemala) to as much as 30 m (Japan Sea). Within a hydrate-bearing zone pure hydrate layers have been estimated to be as much as 3-4-m-thick (Site 3, Middle America Trench off Guatemala). Some recovered pure hydrates were no more than 1-4 mm diameter beads (site 1, Gulf of Mexico). The thickness and mass of pure hydrate for drill hole gas hydrate data are shown in Figure 6. The drill hole data were used exclusively in this histogram because gravity coring devices may not fully penetrate a substantial hydrate zone at the sea floor. Additionally, regardless of the recovery system, if hydrate-bearing marine sediment samples are not maintained at their in situ temperature and pressure, the hydrate particles will begin to dissociate -- and thus begin to diminish in size -- before they can be measured. Estimates of the size of the pure hydrate samples (Figure 6, inset) shows that of the 16 samples measured, 15 had their minimum dimension in the range of millimeters to a few centimeters. Only the layer in the sediment off Guatemala was measured in meters. In contrast, in 14 of 24 cases hydrate zones measured more than 10 cm thick and in 7 cases (≈ 30%) the zones were more than a meter thick. Four were "10's of meters" thick (Figure 6, inset).

The data on zone thickness and dimensions of pure hydrates (Figure 6) suggest that although hydrates may be widespread in a given zone of sediment, they characteristically comprise only a minor volumetric component of the thicker zones.

^[6] Criteria of individual investigators for separating proximal zones was not stated in the cited literature.

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