We evaluated three different forms of hydrate deposition in the pore space, their effect on the elastic properties of hydrate structures and their resulting AVO responses.
The analysis suggests that already small amounts of hydrate cementation result in a significant increase of both the P- and S-wave velocity compared to the velocities of either brine- or gas-saturated sandstone. The cementation caused an immediate stiffening of the rock. Consequently, the synthetic seismograms displayed an AVO effect at the bottom of the hydrate zone that was decreasing with increasing offset. The opposite behavior was observed for the hydrate deposition in the pore space. Strong P-wave velocity contrasts could only be reached by high hydrate saturation. There was barely a change in S-wave velocity at small hydrate saturations, while the S-wave velocity decreased with increasing hydrate saturation. This velocity behavior resulted in an increase of reflection amplitude with increasing offset at the BSR. Both models showed that in case where the hydrate overlays brine sediment, the overall reflection amplitudes are significantly smaller than that in which the hydrate overlays gas sediment.
Comparison of the synthetic results with real in-situ observations at the Blake Outer Ridge showed that the hydrate cementation model cannot sufficiently recreate the amplitude behavior. However, the deposition of hydrate in the pores appears to create an amplitude behavior that can be observed in reality. This leads to two important conclusions about in-situ hydrate structures at the Blake Outer Ridge: (1) the sediment containing hydrate is uncemented and thus mechanically weak, and (2) the permeability of this sediment is very low because hydrate clogs large pore-space conduits, which explains the presence of free gas underneath the hydrate layer at the Blake Outer . Furthermore, the real data indicate the absence of a strong reflection at the top of the hydrate. We conclude that the high concentration of hydrate in the sediment immediately above the BSR gradually decreases with decreasing depth.