Seismic characterization of a gas hydrate system in the Gulf of Mexico using wide‐aperture data

SUMMARY Gas hydrates were discovered in a mud mound in the lease block Mississippi Canyon 798, Gulf of Mexico, through piston coring. Subsequently, a seismic experiment was carried out to investigate the dynamics behind the hydrate formation. During the experiment, high‐resolution multichannel seismic reflection data using a 24‐channel, 240 m long streamer and wide‐aperture data using six ocean bottom seismometers were collected along five lines. High‐reflectivity zones (HRZs) are present in the reflection data along all lines. To better constrain the interpretation of the reflection data, the traveltimes from the multichannel and wide‐aperture data sets were jointly inverted to estimate a 2‐D P ‐wave layered velocity model for each line. A minimum‐parameter/minimum‐structure modelling approach yielded simple models and a comparison of the models at their intersection points shows they are consistent to within ±10 m s −1 in velocity and ±20 m in depth. In the final P ‐wave velocity models, the HRZs are associated with a lowering of velocity. In the reflection data, the top of the HRZs show a polarity reversal with respect to the seafloor. Presence of free gas in the HRZs best explains the velocity lowering and polarity reversal. It is speculated that the gas has deeper sources and migrates upwards through conduits formed by salt movement in the vicinity. The upward migrating gas accumulates in the axis of a channel complex and manifests itself as HRZs in the reflection data. The fluids circulating along the conduits push the base of the hydrate stability zone close to the seafloor. From the channel axis, the free gas migrates further upwards and close to the seafloor, and as it comes within the gas hydrate stability zone, it forms hydrates.