The free gas zone beneath gas hydrate bearing sediments and its link to fluid flow: 3-D seismic imaging offshore mid-Norway

Hydrate recycling and vertical migration of fluids from deep sources are processes evoked as controllers of the formation and stability of the free gas zone (FGZ) beneath the base of the gas hydrate stability zone (BGHSZ). These processes have been often investigated through analytical and numerical modeling at some locations in continental margins. However, the seismic response of sediments deformed by such mechanisms has been poorly investigated due to the lack of reliable seismic resolution to clearly image anomalies confined to the FGZ. The present study is dedicated to the seismic characterization of the FGZ beneath the BGHSZ at a hydrate site offshore mid-Norway using high resolution P-Cable 3D seismic data. Dim amplitude anomalies, showing mainly lineations in a polygonal pattern of distribution, 50–80 m vertical extension, 150–300 m lateral extension and up to a few kilometers long, are interpreted as evidence of sediment remobilization possibly aided by hydrate dissociation at focused fluid flow zones below former bases of the GHSZ. The exclusive occurrence of the anomalies within a depth range comprising estimated paleo BGHSZs during the last 200 kyr, suggests that the anomalies are associated to paleo-hydrate bearing sediment seals emplaced at depths controlled by overburden. It also supports recently reported indications of main periods of fluid expulsion towards the seafloor occurring between 200 and 130 ka BP.