Seismic methods to detect and quantify gas hydrate in sediments

Abstract

Bottom Simulating Reflectors (BSR) on seismic profiles are interpreted to represent the seismic signature of the base of gas-hydrate formation; a free gas zone may be present just below the BSR. Where no direct measurements are available, detailed knowledge of the seismic properties is essential for quantitative estimations of gas hydrate and free gas in sediments. The discrepancies between the velocity profile and the velocity for water-filled, normally compacted, marine sediments are interpreted as due to the presence of gas hydrate (positive anomalies) and free gas (negative anomalies). These anomalies can be translated in terms of concentration of clathrate and free gas, knowing the velocity trend versus gas hydrate and free gas content. We present two inversion examples. For the Mallik well we obtain hydrate concentrations up to 75%, average values of 37 and 21% from the VSP P- and S-wave velocities, respectively, and 60 and 57% from the sonic-log P- and S-wave velocities, respectively. The above averages are similar to estimations obtained from hydrate dissociation modeling and Archie methods. On the other hand, for the Svalbard case, we obtain average hydrate concentrations of 7% and maximum free-gas saturation of 0.4% and 9% depending on the saturation model. The Q values vary between 200 and 25 with higher values (150–200) above the BSR, and lower values below the BSR (25–40). These results seems to confirm that hydrates cement the grains and attenuation decreases with increasing hydrate concentration.