Hydrate and Gas Saturations

After having determined the magnitude of the anomalies caused by the presence of hydrate and gas in the pore space, I can calculate the actual hydrate and gas saturation. This is done by again assuming 100% brine saturation everywhere in the sediment. The assumed brine saturation is decreased and the hydrate or gas saturation increased until I can minimize the porosity anomalies, as calculated by either the averaging or the trace-by-trace technique, within their described uncertainty bands and until I can fit the seismic interval velocities. The resulting estimates in hydrate and gas saturations using the residuals of the averaging method are shown in Figure , while those obtained using the residuals of the trace-by-trace approach are displayed in Figure .

The averaging method shows maximum hydrate saturation above the BSR between 45 km and 52 km lateral distance. Hydrate model A, in which hydrate is part of the pore fluid, results in a maximum hydrate saturation of approximately 26%, while model B (hydrate becomes part of the solid) yields a more conservative estimate of about 20%. Since this model slightly stiffens the sediment frame, less hydrate is required in order to increase the velocity in the hydrate-saturated sediments. In the case of hydrate cementing the sediment grains, less than 1% hydrate saturation is required to satisfy the seismically-obtained velocities. Since cementation has a drastic effect on the stiffness of the sediment, only very little hydrate can considerably increase the velocity. Small hydrate concentrations of approximately 7% are obtained in the region between 32 km and 42 km lateral distance. Some anomalies caused by trace misfits which are of the same magnitude as the hydrate-related anomalies between 32 km and 42 km, appear as hydrate anomalies as well (between 0 km and 5 km, and between 25 km and 27 km). These ``anomalous'' zones, but also the hydrate zone between 32 and 45 km, would have been suppressed had I chosen to allow a higher uncertainty for the hydrate residuals (as described in section 4.3.2). The gas saturation beneath the BSR is approximately 1% to 2% throughout the low velocity zone.

 

avsat1
Figure 11 Hydrate and gas saturation obtained using the average normal porosity trend.

 

tracesat1
Figure 12 Hydrate and gas saturation obtained using the trace-by-trace normal porosity trend.

The trace-by-trace method, on the other hand, results in clear hydrate saturations both between 32 km and 42 km, and 45 km and 52 km lateral distance. Model A gives a maximum hydrate saturation of approximately 20%, model B of about 15 %, and model C again of less than 1%. In the region between 32 km and 42 km, the hydrate saturation is about 10%. Additionally, the gas saturation is estimated to be about 1% to 2% and is thus stable in both the averaging method and the trace-by-trace method.

A comparison of the average and the trace-by-trace method shows that the latter yields hydrate estimates that can be up to 5% more conservative. Both methods result in maximum lateral hydrate saturation in the area between 45 and 52 km. The trace-by-trace method results in clearly distinguishable hydrate anomalies between 32 km and 42 km. Combining both method appears to be a good way to estimate a reasonable range of possible hydrate saturations and give upper and lower uncertainty bounds on the estimates.