Estimation of Hydrate and Gas Saturation

From seismic alone, I can detect the presence of methane hydrates by means of bottom simulating reflectors (BSR). A migrated section of the data from the Blake Outer Ridge (Figure ) clearly shows a BSR, which is connected to the bottom of the hydrate stability field. Furthermore, I can use surface seismic to determine the elastic contrasts across the BSR and infer the cause of the BSR (see Chapter 3). However, in order to estimate the actual amount of hydrate and gas present in the sediments above and below this BSR, respectively, seismic information alone is not sufficient. I need to link the seismic with the developed rock-physics models.

 

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Figure 2 Stacked section after prestack migration.

 

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Figure 3 Interval velocity section.

The information directly available from seismic is the interval velocity. Figure shows the interval velocity which was derived in Chapter 2. The velocity was converted into depth by using a simple vertical stretch from time to depth. This depth conversion is required since the subsequent calculation will require depth as an input. The interval velocity is overlain by a wiggle plot of the migrated section. Above the BSR, the velocity increases to about 1.9 km/s while it decreases to approximately 1.6-1.7 km/s beneath the BSR. As shown in Chapter 3, the BSR is mostly the consequence of hydrate-bearing sediments overlying gas-saturated sediments. The transition of gas-saturated sediment to brine-saturated sediment is visible as a flat reflector beneath the BSR. In the region between 0 and 25 km lateral distance, where no BSR is visible, the velocity is uniformly increasing with increasing depth.