Extension, displacement, and block rotation in the larger Gullfaks area, northern North Sea; determined from map view restoration

Numerical map view restoration of the larger Gullfaks area on the western side of the Viking Graben reveals a Late Jurassic apparent displacement field that is slightly divergent, with displacement vectors trending east-northeast west-southwest to east-southeast west-northwest. The predominant extension direction is east-west in the Gullfaks field, changing to east-southeast-west-southwest in the Gullfaks Sor area. We relate the divergent pattern to extensional collapse over low-angle extensional detachments in the eastern part of the area. The detachments also explain the anomalously high extensions in this same area. Total extension in the horizontal plane is estimated to be 19% on average in the larger Gullfaks area, whereas it is 42 and 33% in Gullfaks and Gullfaks Sor, respectively. Block rotations (about vertical axes) are minor (mostly <5 degrees). Calculated fault-slip directions indicate that major north-south striking faults are mostly dip-slip, whereas accommodation faults or transfer faults, oriented at high angles to the major faults, tend to be oblique slip. In general, numerical or manual map view restoration is useful before choosing and balancing vertical sections. In most of the Gullfaks area, strain is close enough to plane strain and block rotations are small enough for section balancing to be reliable. East-west sections should be chosen across the main Gullfaks field, and east-southeast west-northwest sections in the Gullfaks Sor area.