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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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2. Introduction

Toward understanding the processes that lead to highly concentrated or massive deposits of marine gas hydrates, a database of offshore gas hydrate samples was constructed from published observations and measurements (Appendix). More than 90 samples from 15 distinct regions are represented in 13 data categories. This database has permitted identification of clathrate-hydrate tendencies and associations with respect to their geological environment. Herein we provide a profile of the database and focus on some of the more conspicuous aspects of gas hydrate occurrence in marine sediments.

Figure 1. Stability zone for methane hydrate.
Figure 1. Stability zone for methane hydrate. ( Click for larger view.)

Any gas hydrate is stable only within a specific combination of temperatures (T) and pressures (P). The natural bounds in terms of T and P that would control the distribution of gas hydrates is shown in Figure 1, where pressure in this context represents hydrostatic pressure (water depth plus subseabed depth, assuming no excess pore pressures) and temperature is largely controlled by the geothermal gradient. Accordingly, most of the world ocean below 500 m water depth (100 m seawater ≈ 1 MPa) has low enough temperatures and high enough pressures to form and sustain methane hydrate. [2] In high latitudes (colder water temperatures) less pressure is required and the depth of hydrate stability is even shallower.

Because water and methane must be present in the proper ratio range to form a hydrate (formula: CH4 nH20, where 5.75 < n <≈ 19 (Sloan, 1990), [3] a condition of extreme concentration of methane must exist in the interstitial water of the sediments. However, this is only a local, not a global, requirement: this criterion can be met in a microenvironment such as an individual pore. At any scale seawater generally must be supersaturated with respect to methane by a factor of about 20 or more in order for hydrate nucleation to take place (i.e., for a hydrate with a minimal number of gas molecules occupying its cavities to form). An abundant source of methane at some depth greater than that at which the hydrate exists is typically implied (even if such a source itself was created by downward transport of biogenic methane or its antecedents). Moreover, gas hydrates may be ephemeral because pressure and temperature conditions are always changing. If such changes result in displacement of the hydrate outside its stability zone, as through temperature increases induced by subsidence, pressure reduction associated with eustasy, or many other geologic processes, it will decompose. Finally, a hydrate may be moved horizontally or vertically relatively great distances from its original position and/or its original methane source in response to tectonic forces or other geologic process.

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