Interpreting structural controls on hydrate and free-gas accumulation using well and seismic information from the Gulf of Mexico

The effect of the structural setting on gas hydrate and free-gas accumulation near the East Casey fault zone at Keathley Canyon in the Gulf of Mexico is investigated using well logs and 2D and 3D acoustic-impedance inversions. We interpret two zones from the well logs: a probable hydrate zone and a dissociating hydrate zone. We interpret hydrates as fracture fillings in the clay-dominated formation with maximum saturation of approximately 30% of the available pore space, and maximum volumetric concentration of approximately 12%. The maximum free-gas concentration below the interpreted bottom simulating reflector (BSR) is less than 4%. Resistivity measurements alone are inconclusive; complimentary measurements, such as sonic, reduce hydrate interpretation ambiguities. Seismic data in 3D and the derived acoustic-impedance volume are interpreted in terms of a BSR, a high P-impedance hydrate zone, and free gas. We interpret two bright, low P-impedance features terminating at the BSR as free-gas accumulations. The contrast in average P-impedance across the fault suggests a change in lithology across the fault zone, and free-gas transport along, and to the west of, the fault. Variations in depths of interpreted free-gas zones suggest either a significant lateral variation in the hydrate stability across the fault zone, or coexistence of hydrates and free gas within the hydrate stability zone, or both. The dynamics of the tectonic environment imply nonequilibrium conditions of time-dependent temperature, pressure, or fluid transport.