Three-dimensional seismic analysis of the morphology and spatial distribution of chimneys beneath the Nyegga pockmark field, offshore mid-Norway

Three-dimensional (3D) seismic data analysis has identified 415 seismic chimneys in the Nyegga area, on the mid-Norwegian continental margin. The majority of the seismic chimneys connect with pockmarks at the modern seafloor, suggesting that the seismic chimneys represent the acoustic imprint of highly focused, cross-stratal fluid flow. A semi-quantitative spatial analysis of the seismic chimney complex, measuring parameters describing the dimension, geometry and root zone, provides constraints on the geological controls on chimney formation. Most of the seismic chimneys in the Nyegga region descriptively fall into the categories of 'blowout chimneys', because they diagnostically link to topography-controlled leakage locations within strata showing acoustic indications of free gas. We find that approximately 60% of the fluid-escape chimneys emanate from free gas layers below the gas hydrate stability zone. The timing of highly focused fluid expulsion through chimneys and the formation of the Nyegga pockmark field postdate the last glacial maximum at 25 000 years before present.