Structural geology of the Gullfaks Field, northern North Sea

Abstract

Abstract The large amount of structural data available from the Gullfaks Field have been used to unravel the structural characteristics of the area. Two structurally distinct subareas have been revealed (a major domino system and an eastern horst complex) that show significant differences with respect to fault geometry, rotation and internal block deformation. The main faults have very low dips in the domino system (25–30°) as compared to the horst complex (65°), whereas most minor faults are steep in all parts of the field. Forward modelling indicates that the horst complex balances with rigid block operations. However, the domino area underwent significant internal deformation, reflected by the low acute angle between bedding and faults, and by non-planar bedding geometries. The internal deformation is modelled as a shear synthetic to, but steeper than, the main domino faults. This deformation explains a large-scale (kilometre sized) drag zone that has a triangular geometry in cross-section. Much of this shear deformation occurred by strain-dependent grain reorganization in the poorly consolidated Jurassic sediments, which led to a decrease in porosity. A strain map is presented for the domino area, indicating where the porosity is likely to have been decreased due to internal shear. Hangingwalls are generally more deformed (sheared) than footwalls. This is seen on both the kilometre scale (large-scale drag) and the metre scale (local drag).