Gas Hydrates

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Hydrocarbon gas hydrates can form in deep ocean water but they are not found as a carpet on the sea floor. This is because such hydrates have a lower density than that of seawater (850 kg m−3 compared with 1025 kg m−3). As soon as they form, they float upwards and turn back into methane and water in the lower pressures and warmer temperatures of the upper layers of the ocean. However, within the sediments just beneath the sea floor, crystals of hydrocarbon gas hydrate form and move upwards buoyantly. Frequently the rising crystals form a ‘log-jam’ within the pore space of the sediment and once this occurs, more gas hydrate crystals become trapped in the pore spaces beneath. Eventually, all available pore spaces in the sediment become completely filled by gas hydrate crystals that readily absorb methane into their lattice. Fully saturated gas hydrates can hold up to 200 times their own volume of methane, creating a zone that is denser than seawater and thus gravitationally stable. However, within the sediments just beneath the sea floor, crystals of hydrocarbon gas hydrate form and move upwards buoyantly. Frequently the rising crystals form a ‘log-jam’ within the pore space of the sediment and once this occurs, more gas hydrate crystals become trapped in the pore spaces beneath. Eventually, all available pore spaces in the sediment become completely filled by gas hydrate crystals that readily absorb methane into their lattice. Fully saturated gas hydrates can hold up to 200 times their own volume of methane, creating a zone that is denser than seawater and thus gravitationally stable. The gas required for formation of gas hydrates comes from two principal sources: biogenic and thermogenic. Biogenic gases are those produced in situ by bacterial breakdown of organic matter contained within the sea-floor sediment. The dominant biogenic gas is CH4 (>99%) with traces of CO2 and H2S. Such gases typically form in oceanic areas that have relatively high rates of...
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