Gas seeps in Norwegian waters – distribution and mechanisms

Abstract

Gas seeps and fluid-flow related seabed features are found over the entire Norwegian exclusive economic\nzone (EEZ). Multibeam water-column data from c. 136 000 km2\n has revealed more than 5 000 gas seeps.\nMost of the gas seeps seem to have biogenic, thermogenic or mixed origin; some may be of abiotic origin.\nThe spatial distribution of the gas seeps appears to correlate with: 1 – structural highs with associated\nfaulting, exposing hydrocarbon reservoir rocks at or near the seabed; 2 – faults serving as conduits for fluid\nflow; 3 – settings where reservoir rocks overlain by less permeable cap rocks sub-crop at the seabed. Other\nmechanisms involve seepage around abandoned exploration wells, and possible abiotic gas generation\nfrom serpentinisation of ultramafic rocks near mid-oceanic ridges. The gas seeping from the Norwegian\ncold seeps is mostly methane and has, in many places, led to the formation of methane-derived authigenic\ncarbonate crusts, which give evidence for either extensive gas seepage in the past or long-lived seepage.\nChemosynthetic communities are commonly associated with cold seeps and may form special habitats\ntogether with the carbonate crusts. Methane seepage has been proposed to contribute significantly to\nthe global carbon budget and may be associated with gas hydrates giving rise to potential geohazards.\nGas seeps have been identified and spatially mapped as acoustic gas flares, using multibeam\nechosounder systems, which have the ability to record reflections from both the water column and\nthe seabed. Water-column data have been recorded in the MAREANO seabed mapping program since\n2010, covering an area of c. 262 000 km<sup>2</sup>\n, with a data volume in the order of 210 Tb. The observations of\nextensive gas flares in the Norwegian EEZ are available to the scientific community and other users through\na dedicated MAREANO data and web access system.

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