Gas-generated fractures and methane venting in a deepwater Arctic setting

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Tian, Xu, M.S. in Engineering

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Methane seepages from seafloor sediments in the Arctic (West Svalbard) have occurred from two million years ago until the present. During glacial-interglacial cycles, gas venting behavior in shallow water (< 400 m) close to the West Svalbard margin can be explained by gas hydrate dissociation due to pressure reduction caused by lowering of relative sea level or isostatic rebound. However, the process of gas venting in deeper water (> 1000 m), especially the link with any gas hydrate dissociation, remains unclear. Here I analyze the sensitivity of hydrate dissociation to temperature and pressure in a model based on Vestnesa Ridge, a deep water setting offshore Svalbard with observed venting. Then I explore whether gas generated by hydrate dissociation can generate tensile fractures and enhance transport of gas to the seafloor. The major results of this analysis are as follows. (1) The location of the base of the hydrate stability zone is more sensitive to temperature compared to pressure. My result shows that a 5 K increase in seafloor temperature would cause the base of hydrate stability zone move up more than 50 m, while a pressure reduction of 7 MPa would be necessary to raise the base of hydrate stability by the same amount. (2) An increase in temperature does not cause a decrease in the maximum hydrate saturation within hydrate stability zone, and the influence of pressure reduction on hydrate saturation is minor. (3). The gas saturation is more sensitive to pressure compared to temperature. A 5 MPa decrease in seafloor pressure would cause a raise in gas saturation by 65%, while a 5 K increase in temperature would cause saturation to increase by 27%. (4). Gas formation that caused by hydrate dissociation can generate tensile fractures if the pressure drop is large (> 8 MPa), corresponding to a decrease in sea level of ~ 795 m. Based on the above results, I conclude that a simple reduction in sea level is not sufficient enough for gas to generate fractures in deep water environment, and that other mechanisms such as thermal perturbation may be involved


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