In this dissertation, I characterized methane hydrate seismic data from the Blake Outer Ridge, offshore Florida and Georgia. I determined the cause of the BSR visible in the seismic data and provided a tool for estimating the amount of hydrate present in the pore space. This study is one of the first attempts to characterize hydrate structures by linking surface seismic and rock-physics.
The seismic data are characterized by a strong BSR with negative reflection polarity. Using stacking velocity analysis, I determined that the hydrate-bearing sediment above the BSR is characterized by an increase in P-wave velocity to approximately 1.9 km/s which decreases to about 1.6-1.7 km/s in the underlying sediment. By means of 2-D impedance inversion and AVO analysis after careful preprocessing of the data, I showed that the BSR is caused by hydrate-bearing sediment overlying gas-saturated sediment, which accounts for the observed drop in velocity. The seismic amplitudes could thus be explained by a decrease in P-wave velocity across the BSR and a simultaneous increase in S-wave velocity. Furthermore, the analysis suggests that a flat reflector underneath the BSR represents the transition of the gas-saturated sediment to brine-saturated sediment.
Using the obtained velocity information together with physical rock models, I presented a method to estimate the amount of hydrate present above the BSR and free gas beneath the BSR. I examined three different models of hydrate formation in the pore space: (A) hydrate is part of the pore fluid, (B) hydrate is part of the solid frame, and (C) hydrate cements grains together. Model A predicts maximum hydrate saturations between 20% and 26%, model B saturations between 15% and 20% and model C saturations of less than 1%. The saturation also vary significantly laterally along the BSR. The free gas saturation underneath the BSR is approximately 1% to 2%. Investigation of the robustness of these saturation estimates in the presence of velocity errors indicated that the saturation estimates can vary as much as 14% (note that % refers to the actual saturation itself and not to the percentage of saturation). Therefore, accurate velocity determination is crucial for the technique to give adequate results. Using additional velocity and porosity information from well-logs 994 and 995 at the Blake Outer Ridge, I evaluated the validity of the proposed technique for estimating saturations. This investigation suggests that the maximum uncertainties in the hydrate estimates are less than 10% which indicates that the technique is quantitatively accurate.
In order to differentiate between the different models of hydrate saturation in the pore space, I used forward 1-D modeling and AVO analysis. Models A and B can qualitatively reproduce the BSR AVO trend as observed in the seismic data, however, both models cannot be distinguished by means of their seismic responses. The inferred structure of the hydrated sediment suggests that the sediment is mechanically weak. The permeability is likely to be low from hydrate clogging large pore-space conduits, explaining why free gas is trapped underneath the BSR.