We present an equilibrium model of methane venting through the hydrate stability zone at southern Hydrate Ridge, offshore Oregon. Free gas supplied from below forms hydrate, depletes water, and elevates salinity until pore water is too saline for further hydrate formation. This system self-generates local three-phase equilibrium and allows free gas migration to the seafloor. Log and core data from Ocean Drilling Program (ODP) Site 1249 show that from the seafloor to 50 m below seafloor (mbsf), pore water salinity is elevated to the point where liquid water, hydrate and free gas coexist. The elevated pore water salinity provides a mechanism for vertical migration of free gas through the regional hydrate stability zone (RHSZ). This process may drive gas venting through hydrate stability zones around the world. Significant amount of gaseous methane can bypass the RHSZ by shifting local thermodynamic conditions.

ABSTRACT. Gas hydrates dominated by methane naturally occur in deep marine sediment along continental margins. These compounds form in pore space between the seafloor and a sub-bottom depth where appropriate stability conditions prevail. However, the amount and distribution of gas hydrate within this zone, and free gas below, can vary significantly at different locations. To understand this variability, we develop a one-dimensional numerical model that simulates the accumulation of gas hydrates in marine sediments due to upward and downward fluxes of methane over time. The model contains rigorous thermodynamic and component mass balance equations that are solved using expressions for fluid flow in compacting sediments. The effect of salinity on gas hydrate distribution is also included. The simulations delineate basic modes of gas hydrate distribution in marine sediment, including systems with no gas hydrate, gas hydrate without underlying free gas, and gas hydrate with underlying free gas below the gas hydrate stability zone, for various methane sources. The results are scaled using combinations of dimensionless variables, particularly the Peclet number and Damkohler number, such that the

Natural gas hydrates occur on most continental margins in organic-rich sediments at water depths>450 m (in polar regions>150 m). Gas hydrate distribution and abundance, however, varies significantly from margin to margin and with tectonic environment. The National Gas Hydrate Program (NGHP) Expedition 01 cored 10 sites in the Krishna-Godawari (K-G) basin, located on the southeastern passive margin of India. The drilling at the K-G basin was comprehensive, providing an ideal location to address questions regarding processes that lead to variations in gas hydrate concentration and distribution in marine sediments. Pore fluids recovered from both pressurized and non-pressurized cores were analyzed for salinity, Cl-, SO4 2-, alkalinity, Ca 2+, Mg 2+, Sr 2+, Ba 2+, Na +, and Li + concentrations, as well as � 13 C-DIC, � 18 O, and 87/86 Sr isotope ratios. This comprehensive suite of pore fluid concentration and isotopic profiles places important constraints on the fluid/gas sources, transport pathways, and CH4 fluxes, and their impact on gas hydrate concentration and distribution. Based on the Cl- and � 18 � depth profiles, catwalk infrared images, pressure core CH4 concentrations, and direct gas hydrate sampling, we show that the occurrence and concentration of gas hydrate varies considerably between sites. Gas hydrate was detected at all 10 sites, and occurs between 50 mbsf and the base of the gas hydrate stability zone

DOE Award No.: DE-FC26-06NT43067 Coupled gas/water/sediment dynamics with rigid hydrate films (Task 7.1 Technical Report)

Multi-phase fluid flow is critical to the formation and concentration of gas hydrate in marine sediments. A transient, multi-phase (hydrate, gas and liquid) fluid and heat flow model is presented to describe hydrate formation in porous media. Fluid flux and physical properties of sediment largely control the dynamics of gas hydrate formation and free gas migration. In fine-grained sediments, hydrate formation leads to rapid permeability reduction and capillary sealing. Free gas accumulates below the hydrate layer until a critical gas column builds up, thereby forcing gas upward to the seafloor. In coarse-grained sediments, large volumes of gas are transported into the hydrate region to produce a significant change in salinity. An interconnected three-phase zone with high hydrate concentration and elevated salinity develops from the base of hydrate stability to the seafloor. Both processes may drive gas venting through the hydrate stability zone. We also extend these models to demonstrate that the likely impact of climatic warming events on marine hydrate reservoirs. If hydrates are originally formed in the two-phase region, dissociated methane cannot be released to the ocean until the warming at the seafloor exceeds a critical value. However, all of hydrates formed within the threephase