Acoustic evidence for a gas migration and release system in Arctic glaciated continental margins offshore NW-Svalbard

High-resolution 3D and 2D seismic data offshore NW-Svalbard, west of Prins Karls Forland, provide geophysical evidence for geologically controlled fluid migration pathways, gas hydrate occurrence, and an active seabed gas expulsion system. The investigated seabed area covers ∼1600 km2 and lies between Kongsfjorden cross-shelf trough in the north and Isfjorden cross-shelf trough in the south ranging in water depths from ∼200 m on the shelf to 800 m on the upper continental slope. Acoustic evidence for present day methane release from the seabed to the water column comes from more than 220 gas flares at the outer shelf while past methane release activity at the mid-shelf area is evident from pockmarks without flares. The fluid migration pathways towards the seabed can be drawn from sub-seabed acoustic anomalies. Fluid migration towards the upper slope occurs mostly along strata in upslope direction and largely prevails over vertical focused migration. Fluids accumulate in the uppermost part of the slope just westward of the shelf break, where they are trapped beneath the prograding glacigenic sequence, which is not permeable enough for fluids to migrate through. Fluids are expelled on the shelf where the base of the glacigenic sequence outcrops. Some gas-charged fluids may originate from deep-seated hydrocarbon reservoirs and can be temporally stored in gas hydrates in the shallow subsurface. Though evidence for hydrates on the uppermost slope is missing the seismic data from the lower slope clearly suggest the occurrence of gas hydrate.

► Our data provide evidence for the presence of fluids within the upper slope and shelf sediments.
► Prominent indicators are gas flares, high amplitude anomalies, acoustic pipes, pockmarks and BSR.
► Gas may migrate to the area just beneath the shelf break without entering the GHSZ.
► In the uppermost slope area gas is trapped beneath GDF deposits and possibly also beneath gas hydrate.
► Impermeable GDF deposits of the upper continental slope hamper fluid migration directly to the seabed.