Christine Ecker

Seismic Characterization Of Methane Hydrate Structures

Abstract

Bottom simulating reflectors (BSR) seem to be associated with the base of the stability zone of methane hydrates. These methane hydrates represent a potential future energy resource and might have a strong ``greenhouse'' effect on global climate. In this dissertation I integrate seismic and rock-physics to analyse seismic data from the Blake Outer Ridge, offshore Florida. I infer the cause of the BSR visible in the data and estimate the amount of hydrate present in the sediment structure.

Using stacking velocity analysis, 2-D impedance inversion and AVO analysis, I show that the Blake Outer Ridge BSR is caused by hydrate-bearing sediments overlying gas-saturated sediments. Furthermore, the analysis suggests that a flat reflector beneath the BSR represents the transition of gas- to brine-saturated sediments. Using the interval velocity obtained from the RMS velocities in conjunction with physical rock models, I provide a theoretical tool to quantify the amount of hydrate present. I examine three micromechanical models of hydrate formation: (A) hydrate is part of the pore fluid, (B) hydrate becomes part of the solid frame, and (C) hydrate cements grain contacts together. Model A predicts maximum hydrate saturation between 20% and 26%, model B saturations between 15% and 20%, and model C saturations of less than 1%. Subsequently, I analyze the stability of these estimates to errors in interval velocity. Such errors can cause the estimations to vary as much as $\pm$ 14%, indicating that accurate velocity determination is crucial. The saturation estimation technique is furthermore validated by using known sonic velocities and porosities from two wells at the Blake Ridge. Finally, in order to differentiate between the different models, I use forward 1-D modeling and AVO analysis. Both models A and B can qualitatively reproduce the BSR amplitude trend observed in the seismic data, however, they cannot be distinguished by means of their seismic responses.

This dissertation provides a tool for characterizing hydrate properties and estimating the amount of hydrate in the pore space. Although the study has been applied to data at the Blake Ridge, it could easily be transfered to a different geographic region.