Structural core analysis from the Gullfaks area, northern North Sea

Comprehensive analyses of more than 8 km of core data from the Gullfaks are a, northern North Sea, have proven invaluable for a thorough understanding of the detailed deformation characteristics. In addition, the integration o f the results with analyses of dipmeter data, well log correlation data, se ismic data and field analogue data yields important information on the gene ral structural characteristics and reservoir properties. Microanalyses show that all faults in the reservoirs are associated with ab undant shear bands (deformation bands) clustered in a narrow damage zone th at seldom exceeds a few tens of metres in width. Where mica is abundant, th e bands are dominated by phyllosilicate framework structures, whereas disag gregation structures dominate cleaner sandstones. Permeability reduction ac ross the shear bands depends on the amount of phyllosilicates present and i s generally negligible in clean sandstones. Only minor amounts of cataclasi s are observed. The width of the shear bands increases with increasing grai n 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 degrees to wards the west, most shear bands must have formed after the rotation of bed ding. This is likely because minor faults formed as (late) accommodation st ructures 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 she ar bands or discrete faulting. The integrated analyses of core data with se ismic data demonstrate that most (curvi-)linear and fault-like features obs erved on seismic attribute maps are in fact noise-related. (C) 2001 Elsevie r Science Ltd. All rights reserved.