Seismic-attribute Analysis for Gas-hydrate and Free-gas Prospects on the North Slope of Alaska

Identification and quantification of gas-hydrate and free-gas reservoirs in unconsolidated sediments using seismic data are important, because seismic methods are the most promising remote sensing technique for the delineation of such prospects. For many years, the seismic method has been used to detect free-gas reservoirs because of their association with high-amplitude events and a characteristic amplitude relative to offset behavior. However, seismic-data-driven analysis for detecting and quantifying gas-hydrate accumulations is uncommon. Gas hydrate in the pore space increases elastic velocities and reduces the bulk density. Because of these properties, coupled with the fact that gas hydrate tends to be more highly concentrated in high-porosity clean sands and sandstones, the combined effect of varying gas-hydrate saturation and reservoir thickness on seismic amplitude is complex. Depending on saturation, the seismic expression of a gas-hydrate accumulation can be that of a low-amplitude event (amplitude blanking) or a high-amplitude event (bright spot). This complex amplitude relationship with respect to gas-hydrate and gas saturations is described in this article and is used to estimate saturations and thicknesses of reservoirs on the basis of a thin-bed analysis of three-dimensional seismic data. Seismic interpretation of gas-hydrate and free-gas occurrences and saturations of the reservoirs agree well with geologic considerations as described by Inks et al. (2009). Because the quantitative interpretation is based on a reservoir model appropriate for the North Slope of Alaska, it is emphasized that a caution should be exercised to extend this method to other areas.