Hydrate Bodies Big Enough to Look Like Allochthonous Salt in the Mexican Ridges, Gulf of Mexico

Abstract:

As rising ocean temperatures can destabilize gas hydrates, it is important to understand where large bodies of hydrate may be concentrated. We present evidence of six, previously unreported, gigaton-sized gas hydrate bodies located at shallow depths below the seafloor in the southwestern Gulf of Mexico. These bodies share characteristics with shallow allochthonous salt bodies including their large size, presence of high-impedance boundaries and homogenous interiors. However, when seismic images are constructed using acoustic velocities associated with salt, the resulting images are of poor quality and contain moveout in common offset gathers, indicating velocity error. Using lower-valued acoustic velocities consistent with solid gas hydrate results in higher quality images with little or no moveout. Other indicators supporting the identification of these bodies as hydrate include: 1) a zone of poor reflectivity directly underneath them, which is both typical and expected under solid hydrate, 2) direct observations of gas in a nearby well, 3) other clear indicators of hydrate in the vicinity, including bottom-simulating-reflectors, 4) evidence of migration pathways along faults within the region from a known gas source at depth, and 5) the unlikelihood of salt in this portion of the Gulf of Mexico based on the local geologic history. The total equivalent volume of gas within these bodies is estimated to be 3.3 gigatons or 15 TCF, comparable to the entire proven natural gas reserves of Oman in 2016. Identifying and characterizing large shallow hydrate bodies such as these is increasingly important in order to understand their hazard potential and effect on future climate. Investigation of other seismic data close to continental margins using the velocity analysis techniques used here, or other techniques including full waveform inversion, can help identify the location and quantity of large hydrate bodies to include in biogeochemical modeling and hazard mitigation.