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The database shows three prominent characteristics of the marine gas hydrate samples:
they tend to exist well above their regional phase boundary (Fig. 8 and Fig. 9)
they tend to be associated with faults, diapirs, deformation, vents or seeps (Column P, Appendix)
they tend to be a minor component of the thicker sediment zones in which they are found; i.e., the size of individual hydrate particles, thickness of individual layers or veins, or thickness of cemented layers is characteristically measured in millimeters or centimeters, and these particles or layers tend to be disseminated in sediment zones that can be meters to ten's of meters thick (Fig. 6).
The tendency of gas hydrates to be located well above the calculated position of their phase boundary infers that geological factors other than regional temperature and pressure may be involved in their placement. It also may imply that the supply of methane with respect to the criteria for hydrate formation was variable, episodic, interrupted, or exhausted. Thus, any of the several plausible hypotheses for hydrate formation must include an explanation of the geologic circumstances that established the conditions for hydrate formation, the relative vertical placement of the hydrate-bearing sediment, and explanations pertaining to methane supply.
The association of gas hydrates with features (especially faults) that indicate active geologic forces also suggests that conditions other than regional temperatures and pressures may exert control over the placement of gas hydrates. For example, processes such as uplift or upward migration of relatively warm, methane-rich pore fluids to stratigraphic levels where regional temperatures (i.e., controlled by geothermal gradient) are innately lower may be fundamental to hydrate formation.
The third of the more notable characteristics pertains to the nature of the typical hydrate-bearing sediment section. The quintessential site is a relatively thick yet discrete zone of sediment in which the pure hydrate is a minor constituent and occurs as small-sized grains or thin layers (pure hydrate or as a cement). This indicates that the quantity of methane delivered to the sediment may locally (e.g, within a pore, between sediment laminae or within a fracture) exceed that required to induce hydrate nucleation and minimal growth, but that large-scale layers are less likely to form even though there may be an ample and continuous supply of methane. The implication of this typical site is that properties of the host sediment, such as those relating to bulk diffusion or permeability, may exert control on the mode of occurrence and concentration of pure gas hydrates, but that the gross properties of the sediment section, geologic features, and geologic processes control methane supply and dictate the thickness of the hydrate-bearing zone.
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