Abstract. The stability of submarine gas hydrates is largely dictated by pressure and temperature, gas composition, and pore water salinity. However, the physical properties and surface chemistry of deep marine sediments may also affect the thermodynamic state, growth kinetics, spatial distributions, and growth forms of clathrates. Our conceptual model presumes that gas hydrate behaves in a way analogous to ice in a freezing soil. Hydrate growth is inhibited within finegrained sediments by a combination of reduced pore water activity in the vicinity of hydrophilic mineral surfaces, and the excess internal energy of small crystals confined in pores. The excess energy can be thought of as a "capillary pressure" in the hydrate crystal, related to the pore size distribution and the state of stress in the sediment framework. The base of gas hydrate stability in a sequence of fine sediments is predicted by our model to occur at a lower temperature (nearer to the seabed) than would be calculated from bulk thermodynamic equilibrium. Capillary effects or a build up of salt in the system can expand the phase boundary between hydrate and free gas into a divariant field extending over a finite depth range dictated by total methane content and pore-size distribution. Hysteresis between the temperatures of crystallization and dissociation of the clathrate is also predicted. Growth forms commonly observed in hydrate samples recovered from marine sediments (nodules, and lenses in muds; cements in sands) can largely be explained by capillary effects, but kinetics of nucleation and growth are also important. The formation of concentrated gas hydrates in a partially closed system with respect to material transport, or where gas can flush through the system, may lead to water depletion in the host sediment. This "freezedrying" may be detectable through physical changes to the sediment (low water content and overconsolidation) and/or chemical anomalies in the pore waters and metastable presence of free gas within the normal zone of hydrate stability.

A thermodynamic model for hydrate formation is used to compute the solubility of methane in pore water in equilibrium with gaseous methane or methane hydrate or both. Free energy of water in the hydrate phase and of methane in gas bubbles are corrected to account for salt effects and capillary effects. Capillary effects increase the solubility of methane in fluid in equilibrium with either hydrate or gas. Natural sediments have a broad distribution of pore sizes, and the effective pore size for capillary effects is a function of the fraction of the pore space filled by hydrate or gas (phase fraction). The equilibrium conditions for hydrate+water+gas equilibrium thus depend on hydrate and gas phase fraction. Data acquired on Blake Ridge during Ocean Drilling Program Leg 164 show that the base of the hydrate stability there is shifted by -2øC or more with respect to the expected temperature and this shift has been attributed to capillary effects. We show that this explanation would require a very small effective pore radius (20 nm at 30 MPa). Mercury porosimetry indicates that the percolation threshold for Blake Ridge silty claystone is reached at 20-25% phase fraction and corresponds to a 100 nm pore radius. Hydrate and gas phase fraction determined with several independent methods are all lower than this percolation threshold, implying that gas and hydrate fill pores larger than 100 nm. We conclude that additional inhibition factors other than pore size effects must be involved to explain the -2øC bottom-simulating reflector (BSR) shift as an equilibrium phenomenon. Capillary effects may, however, explain other observations such as large variations of the gas hydrate content in the sediment with lithology and porosity and the distribution of hydrate between interstitial hydrate and segregated masses. Capillary effects should also oppose the migration of gas bubbles when gas phase fraction is less than the percolation threshold and make unnecessary the assumption of a hydrate seal impermeable to fluids. Alternatively, we can go some way to explaining the offset position of the BSR by relaxing the assumption that the system is in thermodynamic equilibrium. Nucleation kinetics of hydrate and/or free gas bubbles may be inhibited by confinement of the methane-bearing fluid in small pores. Equilibration may also be limited by possible rates of diffusional transport of gas, water, and salt components or be perturbed by significant flows of fluid or heat through the sediments. 1Now at 1988] that may play a significant role in global climate change, both present [Paull et al., 1991] and past [Dickens et al., 1997a]. The distribution of hydrate with depth in the sediment has been reliably determined at very few sites [Dickens et al., 1997b] and processes by which hydrates accumulate and dissociate in the sediments are not yet well understood. Occurrence of gas hydrate in the sediment is generally recognized from the presence of a bottom-simulating reflector (BSR) and this reflector is generally assumed to represent the thr...

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