Gas hydrates, free gas distribution and fault pattern on the west Svalbard continental margin

A 3-D tomographic analysis of seismic velocity and attenuation fields is presented with the purpose to analyse the intimate relation of gas hydrates and free gas distribution with the fault pattern. The 3-D, four-component seismic data have been acquired offshore western Svalbard. The analysis of the subbottom topography of the base of the stability field of gas hydrates (indicated by the bottom simulating reflector) and the thickness of the underlying free gas bearing zone suggest a fault-induced compartmentalization of the gas reservoir. The proposed fluid and gas circulation scheme assumes deep sourced warm fluids moving upwards mainly along high permeability faults and fractures below the gas hydrate stability zone. Faults change from being a pathway of fluid flow to being barriers to fluid flow in the hydrate stability zone. Consequently, below the hydrate stability zone free gas is transferred from the permeable faults to non-faulted (but fault-bounded) sediments, where it accumulates below the base of the hydrate stability zone, producing a significantly thicker free gas layer. The tomographic approach is therefore proposed as an effective procedure to provide detailed information on 3-D P- and S-wave velocity and attenuation distribution in marine sediments and specifically to examine the details of the distribution of the gas hydrates and of the free gas, for which the dependence on the Vp, Vs and Qp is now fairly well known.