Fracture characteristics from two reactivated basement fault zones : examples from Norway and Shetland
Detailed analyses of fracture attributes developed in basement rocks associated with two, crustal-scale faults, have enabled the characteristics and evolution of the fracture system geometry to be documented quantitatively. Data sets of fracture attributes have been collected adjacent to faults within the Møre-Trøndelag Fault Complex (MTFC) in Central Norway, and the Walls Boundary Fault System (WBFS) in Shetland. Both structures are of Palaeozoic origins and contain multiply reactivated fault strands that extend offshore to bound several hydrocarbon-rich sedimentary basins of Mesozoic-Cenozoic age along the North Atlantic margin. Fracture characteristics from the MTFC were measured within one dominant lithology (acid gneiss) and therefore each data set of fracture characteristics is directly comparable. A number of different fracture parameters were measured using either 1-D or 2-D techniques and were collected over four data scales. These data indicate different signatures for the two main faults within the MTFC: the Verran Fault (VF), a highly reactivated structure and the Hitra-Snasa Fault (HSF), which has experienced little reactivation, and also for a smaller, kinematically simple fault, the Elvdalen Fault (EF). The parameters measured are the exponent values from exponentially distributed spacing and length data sets, mean fracture spacing, fracture density, mean fracture length, fracture intensity and fracture connectivity (defined by the numbers of fractures and nodes per cluster, fracture cluster length and the number of nodes per unit area). Based on analyses of these parameters, the VF is characterised by a tall peak in values (or trough for measurements such as mean length and mean spacing), with a wide zone (-500m) of above-background values to the NW of the Verran Fault Plane. The HSF on the other hand is characterised by a tall and narrow zone of above-background values (or below for mean spacing and mean length parameters), which decrease to background levels within 100m either side of the Hitra- Snasa Fault Plane. The EF is also characterised by a narrow but shorter peak in above background values, where the height of the peak is less than half that associated with the VF and HSF. These different signatures are most likely to be related to the differing reactivation histories between the three faults. In addition, the VF shows widespread evidence for multiple phases of fluid-related alteration and mineralisation, suggesting that the fracture network characteristics play an important role in controlling fluid flow in these otherwise relatively impermeable basement rocks. The data sets of fracture characteristics collected adjacent to four faults within the WBFS display general trends consistent with the changes in fracture attributes observed adjacent to faults within the MTFC. However, the results are considered to be less reliable. Firstly, the data sets were collected within seven different lithologies, meaning that the fracture attributes must be considered separately, resulting in small data sets compared to those collected from gneisses within the MTFC. In addition, the four faults studied all have different kinematic histories. The findings of this study show that detailed studies of fractures may potentially be used to fingerprint fault reactivation and enable its' recognition in the subsurface.