| Summary: | Marine gas hydrates are commonly found along continental margins where a unique combination
of stable pressure and temperature conditions and an adequate gas supply exist
to generate the ice-like substance. The vast quantities of methane contained in the hydrate
structure could significantly affect global climate if released. The abundant amount of gas
contained within these structures could also represent a potential future energy resource,
although the cumbersome recovery of gas trapped in the solid hydrate structure currently
designate this process as economically unfeasible. The remote locations of gas hydrates
has hampered studies of the natural setting and consequently many aspects of the physical
processes that control hydrate formation in the marine environment remain unresolved.
A quantitative model for the formation and accumulation of gas hydrates in compacting
marine sediments is presented. Conservation principles are used to develop a mathematical
model which is described by a set of governing equations that represent the physical
processes. The numerical method of lines is employed to obtain time varying solutions
for hydrate volume fraction, temperature, gas and salt. Jump boundary conditions are
imposed at the base of the hydrate stability zone where there can be a discontinuity in
hydrate volume fraction. The widely varying geographical and geological distribution of
hydrates indicates that the amount of organic material and its deposition are the dominant
factors controlling hydrate growth within hydrate stability zones. A variety of these
settings can be achieved through the model presented here by considering different sedimentation
rates and organic carbon contents as inputs to the calculations. The necessary
levels of these quantities required for hydrate formation is investigated by examining a
range of sedimentation rates and total organic carbon contents. Typical values of continental margin organic carbon contents and sedimentation rates
are sufficient for hydrate evolution in both active and passive margins. In both cases,
the gas is supplied by in situ biogenic production. Specific settings are also modelled by
inputing representative values for sedimentation rates and organic carbon contents into
the computations. The formation rates for hydrate regions are believed to correspond
to the length of time required to develop current hydrate volume fractions from biogenic
gas production. The possibility of attaining steady state conditions, observed in results
for typical continental margins, indicates that hydrates may have been in existence for
longer time periods. Hydrates are absent in deep sea regions because of insufficient organic
material and slow sedimentation rates. Increasing the total organic content over typical
continental margin values magnifies the hydrate volume fraction since biogenic production
augments the amount of gas available for hydrate formation by the extra available organic
carbon. When the sedimentation rate is increased, the amount of hydrate produced is
not increased significantly, but the time scales on which the hydrate develops are altered
because of changes in the burial rate of organic material, and correspondingly, the rate at
which gas is produced through the decay of buried organic material. Global distributions
of total organic carbon are analyzed and results obtained from model calculations are used
to predict where the conditions for hydrate formation are favourable.
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