Fracture formation due to differential compaction under glacial load: a poro-elastoplastic simulation of the Hugin Fracture

Karin Landschulze,Marcus Landschulze

doi:10.1007/s11001-020-09422-w

Abstract

The Hugin Fracture, discovered in 2011, is an approximately 3.5 km long seafloor fracture in the North Sea. This fracture was unexpected and, due to the geology in the North Sea no obvious explanation could be found. In our study, we adopt the hypothesis that the Hugin fracture was formed by differential compaction controlled by glacial load. We construct a simplified 2D geomechanical model partly covered by top load (ice sheet) and test this hypothesis. We employ transient poro-elastoplastic simulation with a finite element method. For the simulations, we had to make assumptions regarding the material properties, because the fracture is located in-between well locations. We used descriptions from drilling site survey reports and literature values and performed seismic matching form well paths to the Hugin Fracture. Nearby well data were only partly useful due to incomplete logging in the first 400 m below seafloor. To overcome this problem, we introduced a mixing k-value which allows us to easily change the material properties from pure clay to sand. Changing the mixing k-value for each simulation provided information about the limits and robustness of the simulation results. Simulation results show isotropic stress and strain distribution in the horizontally layered, isotropic part of the model that is totally covered by the ice. In the central, channelized part of the model a composite stress and strain pattern develops with sub-vertical focus areas tangential to channel edges. Low stress, strain and deformation values under total load increase drastically soon after the load starts to decrease, resulting in the development of fractures along the focussed zones. Surface deformation such as formation of compaction ridges above stiff clay-filled channels and depression associated with plastic deformation is observed. A fracture and associated surface deformation develop above the shallowest sand-filled channel, very much resembling the observed geometry at the Hugin Fracture. The simulation supports the formation hypothesis for the Hugin Fracture as a compaction fracture and suggests that thin ice sheets may induce differential compaction to a depth of several hundred meters.

Highlights

Recent studies on the Quaternary history of the North Sea (Ottesen et al 2012; Reinardy et al 2017) and the sediments in the North Sea Basin, reflect a sustained interest from both academia and industry in better understanding theThe geographic area of the Viking Graben and Utsira High is still under investigation as to the ice conditions that prevailed during the Quaternary glaciations (e.g. Graham et al 2011)
Marine Geophysical Research (2021) 42:1 marks in the Southern North Sea demonstrate the possibility of high-porosity interglacial sediment deposition (Haavik and Landrø 2014)
The model results support the hypothesis of the Hugin Fracture being a compaction fracture, but they do not prove this hypothesis to be true

Summary

Introduction

The geographic area of the Viking Graben and Utsira High is still under investigation as to the ice conditions that prevailed during the Quaternary glaciations (e.g. Graham et al 2011). Thickness, movement and baseconditions are not yet fully understood. Sediment deposition from glaciers leave poorly sorted low-porosity tills. Hydrocarbon charged Quaternary sand depositions with ice-scouring. Marine Geophysical Research (2021) 42:1 marks in the Southern North Sea demonstrate the possibility of high-porosity interglacial sediment deposition (Haavik and Landrø 2014). Glacial reworking and loading–unloading cycles due to glacial advance and retreat change sediment properties of the overburden of proposed large-scale CO2 storage formations in the North Sea, Europe (e.g. GCCSI 2015, for locations). Different sediment packages and heterogeneities will undergo different degrees of reworking and compaction that will increase the brittle behaviour of the sediment and may lead to fracturing (Aplin et al 1999; Bjørlykke and Hoeg 1997; Bjørlykke 2006)

Methods

Results

Discussion

Conclusion