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 endmember systems, we infer that all natural gas hydrate systems, even those in relatively low flux environments like passive margins, are probably advection dominated.

[1] This study quantifies the excess pore pressure resulting from gas hydrate dissociation in marine sediments. The excess pore pressure in confined pore spaces can be up to several tens of megapascals due to the tendency for volume expansion associated with gas hydrate dissociation. On the other hand, the magnitude of excess pore pressure in wellconnected sediment pores is generally smaller, depending primarily on the hydrate dissociation rate and the sediment permeability. Volume expansion due to gas hydrate dissociation in well-connected pore spaces is related via Darcy's law to an increase in pore pressure and its gradient in sediment, which drives an additional upward fluid flow through the sediment layer overlying the gas hydrate dissociation area. The magnitude of this excess pore pressure is found to be proportional to the rate of gas hydrate dissociation and the depth below seafloor and inversely proportional to sediment permeability and the depth below sea level. The excess pore pressure is the greatest at low initial pressures and decreases rapidly with increasing initial pressure. Excess pore pressure may be the result of gas hydrate dissociation due to continuous sedimentation, tectonic uplift, sea level fall, heating or inhibitor injection. The excess pore pressure is found to be potentially able (1) to facilitate or trigger submarine landslides in shallow water environments, (2) to result in the formation of vertical columns of focused fluid flow and gas migration, and (3) to cause the failure of a sediment layer confined by low-permeability barriers in relatively deep water environments.Citation: Xu, W., and L. N. Germanovich (2006), Excess pore pressure resulting from methane hydrate dissociation in marine sediments: A theoretical approach,

No abstract

No abstract

Abstract. We investigate the response of a methane hydrate layer in marine sediments to cyclic seafloor perturbations of temperature and pressure in order to determine the change in seafloor methane flux resulting from gas hydrate dissociation or accumulation. By using a one-dimensional model describing mass, energy, and methane transport through porous sediments we show that seafloor pressure changes have negligible effect on methane transport to the seafloor. The effect of seafloor temperature perturbations is more pronounced than that of pressure. With an initial seafloor temperature of 3øC, which corresponds to current conditions on Earth, a + 4øC seafloor temperature perturbation occurring over 104 years does The principal point is that strong coupling between methane transport and seafloor temperature occurs because significant hydrate accumulation and dissociation take place near the seafloor only when the seafloor temperature is relatively high. This was the case during the late Paleocene, but it is not the case at present.