Geophysical investigation of the Hugin Fracture, a soft-sediment seafloor fracture on the Utsira High, North Sea. Implications for subsurface fluid migration

Abstract

The 2011 discovery of the Hugin Fracture, a 3.5 km long seafloor fracture on the Utsira High, shows that large-scale, unexpected features can still be found in ostensibly wellmapped, highly industrialized offshore areas like the Norwegian North Sea. Situated on an up to 1000 m thick glaciogenic overburden, the fracture proves seabed fluid flow and a seemingly brittle behaviour of the unconsolidated Holocene-Quaternary sediments. The present thesis includes geophysical investigation of the fracture and underlying Pleistocene sediments down to the Utsira Formation and a poroelastoplastic deterministic simulation of the fracture formation.

Based on high-resolution synthetic aperture sonar data, sub-bottom profiler and 3D seismic data, the seafloor track of the fracture is connected to subvertical fractures connected to the margin of a sand body identified as an alluvial fan at 40 m below the seafloor. Interpretation of seismic attributes and well log data suggests differential compaction of the stratigraphy. Given the location and age of the sediments, burial compaction is likely to have been enhanced by glacial loading. The Hugin Fracture is proposed to represent a compaction fracture formed after deposition of fan sediments some 20-29 ka ago.

Older Pleistocene sediments below the Hugin Fracture may conceivably host similar fractures. Local absence of otherwise abundant bright spots in the underlying Pliocene succession could indicate leak-off of gas from this stratigraphic level to the seafloor through a network of channels and fractures in the overburden. A minor fault in the top 100 m of the Utsira sands was identified some 700 m below the Hugin Fracture, the 3D seismic data indicate that it has not propagated into the overlying stratigraphy. The presented observations suggest that the seal properties of the Pleistocene overburden of the Utsira Fm. are compromised at the Hugin Fracture location.

A 2D geomechanical model has been constructed from an interpreted 3D seismic section over the Hugin Fracture to test the fracture formation hypothesis of ice-load induced differential compaction. Layer properties were chosen from literature values for sand and stiff clay and a six-layered background model with nine channels/tunnel valleys at different depths was constructed. The poroelastoplastic simulation uses a ramp function representing an up to 80 m thick ice sheet with a growth and decay rate of 0.8 MPa/ka in a single loading/unloading cycle.

Simulation results show isotropic stress and strain distribution in the horizontally layered, isotropic part of the model totally covered by the ice. In the central, channelized part of the model a composite stress and strain pattern develops with subvertical focus areas tangential to channel edges. The low stress, strain and deformation values under total load increase drastically soon after the load starts to decrease, resulting in development of high plastic strain accumulations in the focussed zones. Surface deformation such as formation of compaction ridges above stiff clay filled channels and depressions associated with plastic deformation is observed. A fracture and associated surface deformation develop above the shallowest sand-filled channel, resembling the observed geometry at the Hugin Fracture.

The simulation supports the formation hypothesis for the Hugin Fracture as a compaction fracture. The resulting stress-strain pattern suggests that thin ice sheets may induce differential compaction and plastic strain accumulation tangential to channel edges to a depth of several hundred meters. Repeated glaciations of the study area should have produced distinct stress-strain patterns in the Quaternary sediments and below according to the ice load, its geometry and the stratigraphic heterogeneity. We consider it likely that similar compaction fractures like the Hugin Fracture may be encountered in a wider area, and at larger depths. Other areas with similar glaciation history should therefore hold similar fractures. The results of this study should be of interest for subsea fluid migration and overburden sealing quality for e.g. geological storage of carbon dioxide that has been suggested for a larger part of the Utsira Formation.