Many places where natural gas hydrate occurs have a regionally extensive, bottom-simulating seismic
reflector (BSR) at the base of the gas hydrate stability zone (GHSZ). This reflection marks the top of an
underlying free-gas zone (FGZ). Usually, hydrate recycling (that produces gas as the stability field moves
upward relative to sediments) is invoked to explain the presence and properties of the sub-BSR FGZ.
However, this explanation is not always adequate: FGZs are often thicker in passive-margin environments
where hydrate recycling is relatively slow, than in convergent-margin environments where hydrate
recycling is relatively fast (e.g. Blake Ridge compared with Cascadia). Furthermore, some areas with thick
FGZs and extensive BSRs (e.g. west Svalbard) have similar rates of hydrate recycling to northern Gulf or
Mexico, yet the latter has no regional BSR.
Here we discuss a gas-forming mechanism that operates in addition to hydrate recycling, and which
produces a widespread, regional, BSR when gas is transported upward through the liquid phase; this
mechanism is dominant in tectonically passive margins. If the gas-water solubility decreases downward
beneath the GHSZ (this occurs where the geothermal gradient and the pressure are relatively high), low
rates of upward fluid flow enable pore water to become saturated in a thick layer beneath the GHSZ. The
FGZ that this produces achieves a steady-state thickness that is primarily sensitive to the rate of upward
fluid flow. Consequently, geophysical observations that constrain the thickness of sub-BSR FGZs can be
used to estimate the regional, diffuse, upward fluid flux through natural gas-hydrate systems.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:BVAU.2429/1050 |
| Date | 07 1900 |
| Creators | Haacke, R. Ross, Westbrook, Graham K., Hyndman, Roy D. |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | text |
| Rights | Haacke, Ross |
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