Impact of glacially induced stress changes on fault-seal integrity offshore Norway: Discussion

Abstract

The world stress map (Zoback, 1992) shows almost ubiquitous anisotropic stress regimes attributable to varying and long-lived global tectonic stresses. It is highly improbable that (1) an isotropic stress state existed anywhere in the Earth's upper crust prior to 2 Ma and (2) the conclusion by Grollimund and Zoback (2003) of no pre-ice cap tectonic stresses in the north Viking Graben is based on an oversimplified model and rather selective, limited data. Grollimund and Zoback (2003) stated that the thick Pleistocene glacial ice overburden had a strong impact on the fault-seal integrity in oil and gas reservoirs at depth. They sought to demonstrate this by modeling. They constructed a 500-km2 (193-mi2) (northeast and northwest sides) model of the crust, centered on the north Viking Graben, northern North Sea (Figure 1). The upper crust was modeled as a laterally unchanging 20-km (12-mi)-thick, elastic-plastic upper crust, overlying a 14-km (8.6-mi) viscoelastic lower crust, a 16-km (10-mi) viscoelastic lithospheric mantle, and a viscous asthenosphere. Figure 1 Comparison of modeled and present-day stress in the Viking Graben, North Sea (north of latitude 58°N; modified from Grollimund and Zoback, 2003). Measured stresses in the uppermost 4 km (2.4 mi) of the upper crust were compared to the model predictions at a depth of 3 km (1.8 mi). The good match suggested to Grollimund and Zoback that at least in the northern North Sea, the stress field is adequately represented by the model despite the exclusion of other possible stress sources and complex crustal geometries. They then concluded, “The initial stress state before ice sheet growth is isotropic…” (p. 495); that is, the three mutually perpendicular components of the stress field, the load S V, and the maximum and minimum horizontal components, S H and S h, respectively, were equal prior to ice …