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 milliseconds (ms) high and 50−370 m wide, within the gas hydrate stability zone (GHSZ), with up to 33 ms velocity pull-up affecting reflections many hundreds of ms vertically below the anomalies; and circular depressions, 139−559 m in diameter, below the GHSZ. We interpret the anomalies as subsurface gas hydrate “plugs,” consisting of millions of cubic meters of massive hydrate accumulations within the GHSZ, and the craters beneath the GHSZ as associated dissociation-collapse structures. This discovery reveals a new type and dynamics of massive methane hydrate formation along continental margins and requires a reevaluation of previous interpretations of columnar seismic image distortions as kilometer-long fluid flow features.