Rock physics-based seismic trace analysis of unconsolidated sediments containing gas hydrate and free gas in Green Canyon 955, Northern Gulf of Mexico

Abstract

The gas hydrate petroleum system at the 2009 Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) Green Canyon 955 (GC955) site shows a complex seismic amplitude and waveform response of highly negative and positive amplitudes with continuous and discontinuous character within inferred gas-hydrate- and gas-bearing sand reservoirs. Logging-while-drilling (LWD) data obtained during JIP Leg II and conventional 3-D seismic data allowed for the identification of thick highly concentrated hydrate layers by integrating rock physics modeling, amplitude and thin layer analysis, and spectral decomposition. Rock physics modeling with constraints from three JIP LWD holes allowed for the analysis of variations in acoustic amplitude characteristics as a product of hydrate saturation, gas saturation, and reservoir thickness. Using the well log-derived acoustic models, thick highly concentrated gas hydrate with and without underlying free gas accumulations have been identified. These results suggest that thick highly concentrated gas-hydrate-bearing sand units (with thicknesses greater than half of the seismic tuning thickness and gas hydrate saturations greater than 50%) underlain by gas can be differentiated from sands containing only gas, but thin gas-hydrate-bearing sand units with low gas hydrate concentrations (with thicknesses less than half of the seismic tuning thickness and gas hydrate saturations less than 50%) are difficult to identify from post-stack seismic amplitude data alone. Within GC955, we have identified six zones with seismic amplitude anomalies interpreted as being caused by gas hydrate deposits with variable lateral extent, thickness and saturation, and in some cases overlying free-gas-bearing intervals. Synthetic seismic images produced from well-log- and model-derived velocity and density distributions mimic similar reflection characteristics in the corresponding field seismic data.