Subsurface gas hydrate plugs

Marine gas hydrates represent a huge hydrocarbon reservoir in deep-water continental margins and a potential major source of positive feedback to a warming climate. Using three-dimensional seismic reflection data from the West African hydrate province offshore Namibia, we document seismic amplitude anomalies, 10-32 ms high and 50-370 m wide, above the base GHSZ, with up to 33 ms velocity pull-up affecting reflections many hundreds of ms vertically below the anomalies; and circular depressions, 150-450 m in diameter, below the GHSZ. We interpret these as subsurface gas hydrate 'plugs', consisting of millions of cubic meters of massive hydrate accumulations within the gas hydrate stability zone, and associated dissociation-collapse structures beneath the GHSZ. This discovery reveals a new type- and dynamics of massive methane hydrate formation along continental margins and requires a re-evaluation of previous interpretations of columnar seismic image distortions as km-long fluid flow features.