Focused fluid flow through the gas-hydrate stability zone on the Vestnesa Ridge, offshore W-svalbard

Abstract

Abstract: Numerous unburied seafloor pockmarks are related to current methane-leakage at Vestnesa Ridge (~79° N), west of Svalbard. About 50 km long and 1-3 km wide, Vestnesa Ridge is a 1-2 km thick sediment drift to the north of the active Molloy transform – on the eastern spreading segment. With its crest at 1200-1300 m depth, the ridge is located on young (<19.6 Ma) oceanic crust.

The entire length of the crest is dominated by more than 100 pockmarks that are up to 700 m in diameter. The depressions in the seabed are caused by the vertical migration of gas through leakage structures; chimneys and pipes. The study area for this project is at the eastern end of Vestnesa Ridge, where the pockmarks are predominantly of a larger scale than those at the western end of the ridge.

The distribution of pockmarks is associated to a wide-spread Bottom-Simulating-Reflector (BSR), which indicates the possible occurrence of gas hydrates by representing the base of the Gas-Hydrate-Stability-Zone (GHSZ). Gas hydrates act as a permeability barrier regarding fluid flow. Presumably, their occurrence in this area is controlling the distribution of gas-leakage. Beneath the GHSZ, free gas is shown by strongly negative reflections in the seismic – confirming this assumption.

Chimneys are leakage-structures connecting the pockmarks to the BSR. The chimneys and their internal fluid flow are related to several faults spreading through the ridge. Faults are well known conduits for fluid flow. Mechanisms for fluid flow through chimney-structures, is however uncertain. Results from RMS-amplitude mapping in this project, indicate that the chimneys internal fluid flow is also strongly influenced by faults.