Formation of natural gas hydrates in marine sediments 1. Conceptual model of gas hydrate growth conditioned by host sediment properties

The stability of submarine gas hydrates is largely dictated by pressure and temperature, gas composition, and pore water salinity. However, the physic al properties and surface chemistry of deep marine sediments may also affec t the thermodynamic state, growth kinetics, spatial distributions, and grow th forms of clathrates, Our conceptual model presumes that gas hydrate beha ves in a way analogous to ice in a freezing soil. Hydrate growth is inhibit ed within fine-grained sediments by a combination of reduced pore water act ivity in the vicinity of hydrophilic mineral surfaces, and the excess inter nal energy of small crystals confined in pores. The excess energy can be th ought of as a "capillary pressure" in the hydrate crystal, related to the p ore size distribution and the state of stress in the sediment framework. Th e base of gas hydrate stability in a sequence of fine sediments is predicte d 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 h ydrate and free gas into a divariant field extending over a finite depth ra nge dictated by total methane content and pore-size distribution. Hysteresi s between the temperatures of crystallization and dissociation of the clath rate is also predicted. Growth forms commonly observed in hydrate samples r ecovered from marine sediments (nodules, and lenses in muds; cements in san ds) can largely be explained by capillary effects, but kinetics of nucleati on and growth are also important, The formation of concentrated gas hydrate s 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 "freeze-drying" may be detectable through physical changes t o the sediment (low water content and overconsolidation) and/or chemical an omalies in the pore waters and metastable presence of free gas within the n ormal zone of hydrate stability.