Sediments with gas hydrates: Internal structure from seismic AVO

Christine Ecker, Jack Dvorkin, and Amos Nur

christin@sep.stanford.edu

ABSTRACT

We use rock-physics-based synthetic seismic modeling to interpret AVO data of methane hydrate structures. Two micromechanical models of gas hydrate deposition are examined: (1) the hydrate cements grain contacts and strongly reinforces the sediment; and (2) the hydrate is located away from the grain contacts and only weakly affects the stiffness of the sediment's frame. We calculate the effect of pore fluid on seismic velocities from Gassmann's equation. Synthetic seismograms are generated from 1-D elastic modeling (Thompson-Haskell reflectivity method) and the AVO responses at the BSR are obtained from Zoeppritz equations. Comparison with real seismic AVO data from the Blake Outer Ridge, offshore Florida and Georgia, shows that the second model of hydrate deposition can reproduce the real AVO effect qualitatively. The inferred topology of hydrate deposition leads to two important conclusions: (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. This second conclusion explains the fact that free gas is trapped underneath the BSR at the Blake Outer Ridge. The data furthermore indicate the absence of strong reflections 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.