In:
Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), Vol. 104, No. B3 ( 1999-03-10), p. 5081-5095
Abstract:
Using a new analytical formulation, we solve the coupled momentum, mass, and energy equations that govern the evolution and accumulation of methane gas hydrate in marine sediments and derive expressions for the locations of the top and bottom of the hydrate stability zone, the top and bottom of the zone of actual hydrate occurrence, the timescale for hydrate accumulation in sediments, and the rate of accumulation as a function of depth in diffusive and advective end‐member systems. The major results emerging from the analysis are as follows: (1) The base of the zone in which gas hydrate actually occurs in marine sediments will not usually coincide with the base of methane hydrate stability but rather will lie at a more shallow depth than the base of the stability zone. Similarly, there are clear physical explanations for the disparity between the top of the gas hydrate stability zone (usually at the seafloor) and the top of the actual zone of gas hydrate occurrence. (2) If the bottom simulating reflector ( BSR ) marks the top of the free gas zone, then the BSR should occur substantially deeper than the base of the stability zone in some settings. (3) The presence of methane within the pressure‐temperature stability field for methane gas hydrate is not sufficient to ensure the occurrence of gas hydrate, which can only form if the mass fraction of methane dissolved in liquid exceeds methane solubility in seawater and if the methane flux exceeds a critical value corresponding to the rate of diffusive methane transport. These critical flux rates can be combined with geophysical or geochemical observations to constrain the minimum rate of methane production by biogenic or thermogenic processes. (4) For most values of the diffusion‐dispersion coefficient the diffusive end‐member gas hydrate system is characterized by a thin layer of gas hydrate located near the base of the stability zone. Advective end‐member systems have thicker layers of gas hydrate and, for high fluid flux rates, greater concentrations near the base of the layer than shallower in the sediment column. On the basis of these results and the very high methane flux rates required to create even minimal gas hydrate zones in some diffusive end‐member systems, we infer that all natural gas hydrate systems, even those in relatively low flux environments like passive margins, are probably advection dominated.
Type of Medium:
Online Resource
ISSN:
0148-0227
DOI:
10.1029/1998JB900092
Language:
English
Publisher:
American Geophysical Union (AGU)
Publication Date:
1999
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