Surface and subsurface expressions of gas seepage to the seabed-examples from the Southern North Sea

Expressions of gas seepage observable within North Sea seismic and acoustic data include seabed pockmarks, seepage plumes in the water column, acoustic blanking, shallow enhanced reflectors, and shallow seismic chimneys. Three areas were selected for a marine survey in which 60 vibrocores were taken. Gas content of the seabed sediment samples (methane and ethane concentrations of 2 to10,395 vppm and 0.5 to 2.2 vppm respectively were observed in the headspace gas) and carbon stable isotope ratios of methane (−88.3 to −30.5 ‰) were determined, in order to examine if geochemical anomalies confirm the interpretation of geophysical anomalies. In a few cases they did, whereas in other cases it was questionable whether only slightly elevated concentrations may be considered as a confirmation of the interpretation of geophysical anomalies. In the case of a seabed pockmark methane concentrations in headspace gas were elevated (122.6 vppm) in the samples taken from the centre of the pockmark compared with 5-10 vppm background values away from the pockmark. Very high concentrations of methane (up to 10,395 vppm) were found near an active vent, which had previously been identified via a seepage plume in the water column. Shallow enhanced reflectors in a 2D seismic profile confirm the presence of gas in the subsurface at this site. The methane concentrations (5 to 172 vppm) and carbon isotope ratios ranging from −88.9 to −30.5 ‰ found in samples taken above a Jurassic condensate field provide indications for leakage of a mixture of thermogenic and biogenic hydrocarbons to the seabed.