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  4. Sample, BSR, and Phase Boundary Relationships

Figure 7. General stability range of methane hydrates on continental margins. Click for larger view.

The phase boundary comprises the combinations of temperature and pressure at which a state of equilibrium exists between a free gas-seawater mixture and gas hydrate, assuming a proper molar ratio exists between methane and water. This is the subbottom depth at which the temperature has risen to a level, as controlled by the geothermal gradient, that precludes hydrate stability. It is ideally characterized by a zone of free gas and seawater overlain by a zone of gas hydrate. A representation of the phase boundary is shown in Figure 7.

Figure 8. Relationship of estimated in-situ temperatures and pressures of methane hydrate samples to phase boundary. Click for larger view.

Every sample in the database came from the methane hydrate stability zone Figure 8. Most show that in situ they were -- using pressure (depth) rather than temperature as a reference variable -- situated well above the phase boundary. This is one of the more striking revelations from this limited analysis of the database. If we assume that the methane supply originates from depths greater than that of the phase boundary, why are most of the samples found well above the phase boundary? Once the phase boundary is encountered by the rising gas-rich fluid, should not the gas hydrate form there?

Clearly, gas hydrates are not restricted to the domain of the phase boundary. These hydrates; that is, those situated well above the present regional phase boundary, herein are termed internal gas hydrates.

Four possible explanations for an internal hydrate are:

1. it formed at a time when its phase boundary was shallower; it is relict

2. it formed as a consequence of locally-controlled or locally-induced pressures and/or temperatures rather than under regional phase boundary conditions

3. it formed by site-specific gas enrichment, either in the absolute or relative sense

4. it formed elsewhere and was transported to its present location; it is allochthonous.

Figure 9. Summary of vertical spatial relationships of sample zones, BSRs, and calculated positions of the regional phase boundary for each site. Click for larger view.

Another perspective of this is provided in Figure 9. It shows that the average position of a gas hydrate sample is approximately 300 m above its calculated regional phase boundary and that 70% of the samples are more than 100 m above the assumed position of that boundary. Figure 9 also shows the position of the BSR (Bottom Simulating Reflector) in the sediment column with respect to the phase boundary. The BSR is an acoustic reflector that approximately marks the interface between the base of the gas hydrate zone and free gas below; i.e., the phase boundary.

Figure 10. Relationship between position of phase boundary and position of BSR. Click for larger view.

On seismic reflection records it represents the velocity difference between the overlying hydrate-bearing sediment, which has a higher acoustic velocity than saturated sediment alone, and the underlying, probably gas-bearing sediment, which has a lower velocity than normal sediment. The BSR tends to be parallel to the sea floor because its position, in general, is controlled by the regional geothermal gradient and the hydrostatic pressure rather than geologic factors such as bedding planes. Figure 9 shows that the calculated positions of the phase boundaries and the BSR's are fundamentally in concert. ^[8] In Figure 10 the relationship is examined more closely. Due to the small number of data points (n=8) a level of significance was not determined for the apparently high correlation. However, it would appear that there is not only a close match on the scatter plot between the depth of a BSR and the depth of the site phase boundary, but that the ideal relationship between them in a regression equation (slope=l, y-intercept=0) is nearly achieved (dashed line). This may be taken as independent corroboration of the regional phase boundary calculations: verification of the geothermal gradients, the hydrostatic pressures, and the equations used in the calculations is implied. Alternatively, the agreement may be viewed as corroboration of the basic concept of the BSR.

^[8] The general absence of BSR's in association with DSDP holes (Column I, Appendix) is due to a JOIDES safety policy that forbade drilling in areas with BSRs until 1988.

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