Migration of methane gas through the hydrate stability zone in a low-flux hydrate province

New high-resolution seismic data show clear evidence for upward injection of methane gas well into the hydrate stability zone at the stable, low-methane-flux Blake Ridge crest. This movement of gaseous methane, through a thermo-dynamic regime where it should be trapped as hydrate, suggests that dyna...

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Bibliographic Details
Published in:Geology (Boulder) 2002-04, Vol.30 (4), p.327-330
Main Authors: Gorman, Andrew R, Holbrook, W. Steven, Hornbach, Matthew J, Hackwith, Kara L, Lizarralde, Dan, Pecher, Ingo
Format: Article
Language:English
Subjects:
aliphatic hydrocarbons
alkanes
amplitude
applied (geophysical surveys & methods)
Atlantic Ocean
bathymetry
Blake-Bahama Outer Ridge
continental margin
elastic waves
gas hydrates
Geology
geophysical methods
geophysical profiles
geophysical surveys
Geophysics
high-resolution methods
hydrocarbon indicators
hydrocarbons
inverse problem
Kinetics
Marine
marine sediments
Methane
North Atlantic
ocean floors
Oceanography
organic compounds
reflection methods
sediments
seismic methods
seismic profiles
Seismology
surveys
transport
velocity structure
waveforms
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Summary:New high-resolution seismic data show clear evidence for upward injection of methane gas well into the hydrate stability zone at the stable, low-methane-flux Blake Ridge crest. This movement of gaseous methane, through a thermo-dynamic regime where it should be trapped as hydrate, suggests that dynamic migrations of gas play an important role in the interaction of subseafloor methane with the ocean. In the study area, none of the seismic amplitude anomalies that provide evidence for gas migration reaches the seafloor; instead they terminate at the base of a highly reflective, unfaulted capping layer. Seismic inversions of anomalous regions show (1) increased velocities beneath the hydrate stability zone, suggesting less gas, and (2) increased velocities within the hydrate stability zone associated with observed low-amplitude chimneys and bright spots, indicating increased hydrate concentrations. These observations and analyses indicate that methane migrates upward as free gas hundreds of meters into the hydrate stability zone before forming hydrate. The observations strongly imply that given appropriate permeable pathways, free gas can escape into the ocean. Even in a low-flux environment, the hydrate stability zone is not an impermeable barrier to free-gas migration.
ISSN:0091-7613
1943-2682
DOI:10.1130/0091-7613(2002)030<0327:MOMGTT>2.0.CO;2