Exploration for gas hydrates in the deepwater, northern Gulf of Mexico: Part I. A seismic approach based on geologic model, inversion, and rock physics principles

This article presents the results of applying a five-step process for using high-quality seismic data to locate marine gas hydrates. The process involved (1) reprocessing of seismic data for higher resolution, (2) detailed stratigraphic evaluation and interpretation to locate possible hydrate-bearing zones, (3) seismic attribute analysis to further delineate these zones, (4) seismic inversion to obtain appropriate elastic parameters of these zones in 3D, and (5) quantitative estimation of gas hydrate saturation from seismic data using inversion and rock physics principles. We used seismic data from Keathley Canyon 151 and Atwater Valley 14 in the northern Gulf of Mexico. Although careful analysis did indicate the presence of a bottom simulating reflector (BSR), our study mainly relied on a host of other seismic attributes (e.g., gas chimneys, seismically transparent zones, other features associated with the petroleum system) to characterize the occurrence of gas hydrates in these areas. We tested and verified a viable rock model for hydrate-bearing sediments using data from the Mallik (McKenzie Delta, Canada) and Blake Ridge (ODP Leg 164, southeast U.S. Atlantic margin) wells and modified it for application in the current area. We then used this model to estimate gas hydrate saturation in the host sediments of the northern Gulf of Mexico focus areas using estimates of P-wave and S-wave velocities from inversion of reflection seismic data. In this paper, we present the gas hydrate saturations predicted from the seismic processing methodology prior to 2005 drilling in the focus areas. In frontier areas where no well data are available and lithologic heterogeneities are poorly understood, implementing a seismic-based technique like the one described here to identify potential gas hydrates can provide valuable pre-drill information for site selection and for planning future characterization studies of gas hydrate-bearing sediments.