The evolution of seafloor venting from hydrate-sealed gas reservoirs

We use 3D seismic data to show that three rows of seafloor gas mounds can be traced downward to leak points that lie at the hydrate-gas contact within three individual dipping coarse-grained sand bodies in the Terrebonne Basin, Walker Ridge block 313, northern Gulf of Mexico. We predict the overpressure within the sand bodies by assuming that the gas pressure at the vent points equal the least principal stress. We interpret that free gas accumulates at the base of the hydrate stability zone, trapped by the overlying sand which has a high methane hydrate saturation. The free gas accumulates until the gas pressure at the base of the hydrate stability zone reaches the least principal stress in the overlying mudrock, whereupon hydraulic fractures form and fluids are vented to the surface. The warm rising fluids and perhaps localized exothermic formation of hydrate raise the local salinity and temperature. This process progressively shifts the base of the hydrate stability zone to shallower depths and dissociates the hydrate seal within the sand, which creates new leak points and results in the observed migration of the seafloor vents. Within the southwest corner of the Terrebonne Basin, this process has repeated multiple times within the Blue, Orange, and Green sands. This study shows how the hydrate stability zone can influence the location of fluid expulsion and in turn be affected by the warm, rising, saline fluids sealed by hydrate. (C)& nbsp;& nbsp;2021 Elsevier B.V. All rights reserved.

Automated summary

This study uses 3D seismic data to track seafloor gas mounds and their leakage points in the Terrebonne Basin, Walker Ridge block 313 of the northern Gulf of Mexico. It predicts overpressure within sand bodies and explains the accumulation and release of free gas at the base of the hydrate stability zone. The study shows how the hydrate stability zone can impact fluid expulsion and be influenced by warm, rising, saline fluids sealed by hydrate.