Digital seismic reflection (boomer) profiles of an active pockmark, in UK block 15/25, North Sea, reveal that the feature was formed prior to the deposition of the most recent sediments, probably by vigorous (or even catastrophic) gas escape. This release may have been triggered by the melting of ground ice when North Atlantic waters first entered the North Sea after the last glaciation, about 13,000 years ago. Possible sources of the gas are investigated by examining the composite log from a nearby petroleum exploration well; it is concluded that, although the gas may .originate from the Kimmeridge Clay, it probably comes from lignites of Tertiary age. Its migration towards the seabed is interrupted by local accumulations at several horizons, the shallowest of which (<80 m below seabed) is trapped beneath clayey sediments of the Coal Pit Formation. The topography of the base of this layer apparently controls the location of gas migration pathways to the seabed. As these lead to pockmarks which formed long ago, and as these pockmarks are still active today, it is probable that the migration pathways have remained throughout the intervening period. Gas accumulating beneath the Coal Pit Formation may migrate laterally to reach the pathways.
Seismic chimney characterisation in the North Sea – Implications for pockmark formation and shallow gas migration