Gas migration in the Terrebonne Basin gas hydrate system, Gulf of Mexico

Abstract:

The Terrebonne Basin is a salt bounded mini-basin in the northeast section of the Walker Ridge protraction area in the Gulf of Mexico (water depth ~2 km), where the Gas Hydrate Joint Industry Project Leg 2 identified gas hydrate via logging-while-drilling in 2009. The Terrebonne Basin is infilled by gently dipping mud-rich sedimentary sequences with several sand units. Gas hydrate was detected in two significant reservoir sands 10s of meters in thickness, a number of thin 1 to 3 meter-thick sands, and in thick, 10-100 meter intervals of marine muds with gas hydrate in near-vertical fractures.

In this research, we combine 3D seismic mapping with wavelet and travel time analysis to interpret gas migration mechanisms in each hydrate-bearing sand. Our analyses suggest that the Orange sand, a main reservoir unit, is sourced from below the gas hydrate stability zone and, the 2.5 meter-thick Red sand (also called ‘Unit A’), is sourced locally. Our primary evidence is from seismic amplitudes across the two sands that show distinctly different patterns. The Orange sand has distinct high amplitudes within the gas hydrate stability zone and negative amplitudes suggesting free gas below the gas hydrate stability zone. The Red sand, in contrast, has no free gas source below the stability zone and the hydrate distribution as described by high amplitudes suggests that hydrate distribution is spotty. This may imply that gas generation is occurring sporadically in the surrounding marine mud units; this matches with a model of the Red sand that suggests it is sourced locally. These preliminary observations require further refinement but they indicate that fundamentally different migration mechanisms are occurring within a single hydrate system.