Long-term fluid expulsion revealed by carbonate crusts and pockmarks connected to subsurface gas anomalies and palaeo-channels in the central North Sea

Gas seepage through the seafloor into the water column is inferred based on acoustic mapping, video observations and geochemical analyses at multiple locations in the Viking Graben and Utsira High areas of the central North Sea. Flares in the Viking Graben occur both inside and along the periphery of a submarine melt water channel where pockmarks (up to 500 m in diameter) and methane-derived carbonate crusts are found on the seafloor, indicating focussing of fluid flow in the vicinity of the channel. The flares can be related to gas accumulations close to the seafloor as well as in Quaternary and deeper strata, observed as high-amplitude reflections on seismic data. Many palaeo-channels, which act as accumulation zones, are observed in the subsurface of both the Viking Graben and Utsira High areas. The deeper origin of gas is partially supported by results of isotope analyses of headspace gas collected from sediment samples of the Viking Graben, which show a mixed microbial/thermogenic origin whereas isotope data on free seeping gas in the Viking Graben indicate a predominantly microbial origin. Based on these lines of evidence, a structure-controlled fluid flow model is proposed whereby hydrocarbons migrate in limited amount from deep thermogenic reservoirs along faults, and these deep fluids are strongly diluted by microbial methane. Moreover, the existence of subsurface pockmarks at several stratigraphic levels indicates long-term fluid flow, interpreted to be caused by gas hydrate destabilisation and stress-related high overpressures.