The use of dipmeter data to constrain the structural geology of the Gullfaks Field, northern North Sea

Abstract

Analyses of 23 km of dipmeter data from 48 wells on the Gullfaks Field, northern North Sea, have helped to constrain and understand the structural geology of the area. The analyses have verified the general structure obtained by seismic interpretation; i.e. a main, western domino system of rotated fault blocks and non-planar bedding and an eastern horst complex, separated by a collapsed anticline structure. In addition, subseismic structures have been revealed by the integrated use of dipmeter data, well log correlation, seismic data and core data. Within the domino system, dip of bedding decreases towards the main faults to the west (large-scale drag). The effect of such large-scale drag decreases with depth, probably due to a vertical increase in consolidation and mechanical strength at the time of deformation. Within the accommodation zone, a modified fold structure is seen, whereas strata within the horst complex are generally subhorizontal. More than half of all faults on the Gullfaks Field have developed a zone of local drag. Generally, only the northerly-trending faults are associated with drag. The interval affected by drag is typically some tens of meters wide, and is consistently wider in the hanging wall than the footwall. Due to the effect of drag, total offset may be up to one order of magnitude larger than the amount of missing section identified from well log correlation. There is no apparent relationship between interval affected by drag and lithology. However, drag is less abundant in more consolidated rocks at deeper stratigraphic levels. Minor faults are less abundant than expected from a power-law down-scaling of seismically detectable faults, and are not particularly common within the large-scale drag zones. It is suggested that a significant amount of subseismic deformation occurs on a scale below resolution of dipmeter data, probably by ductile flow rather than discrete faulting. North-trending minor faults are most common and will restrict fluid flow in an E–W direction due to cataclasis and phyllosilicate smear associated with abundant deformation bands (micro-faults).