Structural core analysis from the Gullfaks area, northern North Sea

Abstract

Comprehensive analyses of more than 8 km of core data from the Gullfaks area, northern North Sea, have proven invaluable for a thorough understanding of the detailed deformation characteristics. In addition, the integration of the results with analyses of dipmeter data, well log correlation data, seismic data and field analogue data yields important information on the general structural characteristics and reservoir properties. Microanalyses show that all faults in the reservoirs are associated with abundant shear bands (deformation bands) clustered in a narrow damage zone that seldom exceeds a few tens of metres in width. Where mica is abundant, the bands are dominated by phyllosilicate framework structures, whereas disaggregation structures dominate cleaner sandstones. Permeability reduction across the shear bands depends on the amount of phyllosilicates present and is generally negligible in clean sandstones. Only minor amounts of cataclasis are observed. The width of the shear bands increases with increasing grain size. Orientation analyses of the dip of shear bands associated with minor faults show that these are symmetrical around a vertical axis. Since the general bedding rotation as observed in well data and seismic data is 10° towards the west, most shear bands must have formed after the rotation of bedding. This is likely because minor faults formed as (late) accommodation structures during slip along the main faults. The reservoir rocks are affected ductilely (by fault drag) in a zone wider than that affected by shear bands. This shows that part of the deformation was by a homogeneous redistribution of individual grains rather than by shear bands or discrete faulting. The integrated analyses of core data with seismic data demonstrate that most (curvi-)linear and fault-like features observed on seismic attribute maps are in fact noise-related.