Methane migration within the submarine gas-hydrate stability zone under deep-water conditions

Possible pathways of methane migration and associated gas-hydrate accumulation are considered. Gas-hydrates precipitate in water filtration routes when the methane concentration in water exceeds its solubility, which significantly decreases within the hydrate stability zone towards the sea floor. In the vicinity of the ascending free gas flows, gas-hydrates are formed within a diffusion aureole resulting from the difference between chemical potentials of gaseous and water-dissolved methane. The role of diffusion is also essential in the vicinity of gas-saturated water flows as well as within and above the sections where biochemical methane is intensively generated. A chemical potential gradient of dissolved methane (which is a necessary condition for gas-hydrate accumulation) is created due to inhomogeneity of the geological medium manifested as temperature and water salinity gradients or changes of permeability. The thermobaric gas-hydrate stability zone is a geochemical barrier for hydrocarbon gas emigrating from sediments into the water column. However, methane is not completely preserved in a hydrate slate. It is unknown yet which fraction of migrating methane is detained on this barrier.