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Theoretical model for the formation and accumulation of marine gas hydrates in compacting sediments Kingdon, Kevin Andrew


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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