Estimating the amount of gas hydrate and free gas from surface seismic

Christine Ecker, Jack Dvorkin, and Amos Nur

ABSTRACT

In this study we provide a theoretical tool for quantifying the amount of gas hydrate and gas near a bottom simulating reflector (BSR) at the Blake Outer Ridge from surface seismic. We develop rock-physics models that link the elastic wave velocity in high-porosity marine sediments to density, porosity, effective pressure, mineralogy, and water/gas and hydrate saturation of the pore space. Three models of hydrate deposition are examined: (1) hydrate is part of the pore fluid; (2) hydrate becomes part of the solid frame, thus reducing porosity and weakly affecting the stiffness of the sediment; and (3) hydrate cements grain contacts and therefore strongly reinforces the sediments. Using interval velocities obtained from velocity analysis together with the rock-physics models, we obtain maps of lateral hydrate and gas saturation. Model (1) predicts maximum hydrate saturations between 19% and 33%, model (2) saturations between 16% and 25% and model (3) saturations less than 1%. Maximum gas saturation is about 2% of the pore space. These results are consistent with those that can be obtained using known well-log velocities and porosities from this region. Subsequently, in order to evaluate the effect of the different models on the actual seismic amplitudes, we generated synthetic seismograms using Kirchhoff modeling. The AVO responses showed that models (1) and (2) cannot be differentiated by surface seismic. Comparison with real AVO data from the Blake Outer Ridge suggests that only model (1) or (2) can reproduce the actual observed amplitude trends. Therefore, we conclude that the hydrated sediments at the Blake Outer Ridge are only weakly, if at all, stiffened by the presence of hydrate, which can occupy up to 33% of the pore space.