Controls on fluid discharge at cold seep-hydrate systems: 4D seismic monitoring of Woolsey Mound, Gulf of Mexico

Significant increases in methane discharge from gas hydrate systems into the global ocean can influence oceanic carbon dynamics and potentially present significant challenges to ocean biogeochemistry, marine ecosystems, and broader climate. Gas hydrate/cold seep systems are highly dynamic and susceptible to environmental perturbations and processes active in the subsurface. This complexity poses significant challenges in identifying their driving forces and evolution. Through quantitative time-lapse seismic monitoring, we characterized the subsurface fluid dynamics at Woolsey Mound, a gas hydrate/cold seep system in the Gulf of Mexico. Using information from five reflection seismic surveys, we quantified the fluid volumes within the subsurface and reconstructed their migration process from key permeable sedimentary units at different depths up to the seafloor over a time span of 23 years (1991–2014). Our results reveal that fluid discharge is governed by overpressure build-up and subsequent release, enabled by a network of faults and fractures providing connectivity between deep sedimentary units and the seafloor, crossing through the gas hydrate stability zone. Despite gas hydrates reducing the permeability of these faults, overpressure within shallow sedimentary units can induce transient fault permeability and effective fluid migration, thus enhancing fluid discharge at the seafloor. Our analysis identifies the critical role of shallow permeable layers acting as buffers between deep fluid reservoirs and surface discharge points. This buffering mechanism significantly modulates the frequency and intensity of fluid discharge episodes over decadal timescales. Similar processes observed at Woolsey Mound have been hypothesized at cold seep/hydrate systems along active continental margins, suggesting a common model for deep-sourced fluid discharge across environments in different geological and geodynamic contexts. This research advances our understanding of the mechanisms controlling fluid discharge in cold seep/hydrate systems, providing insights into the complex interplay of geological and environmental factors that drive these profoundly dynamic systems.