Characterising Fault Networks in Three Dimensions in the Lewisian Gneiss Complex, NW Scotland: Implications for Petroleum Potential in the Clair Field Basement, Faroe-Shetland Basin

Fractured crystalline basement rocks are increasingly a target for hydrocarbon exploration in the development of existing fields. In the Clair field that lies 75km west of the Shetland Isles, the primary clastic reservoirs of Devonian and Carboniferous age overlie and onlap fractured Lewisian basement. Fractures in the basement are thought to play a significant role in providing conduits for fluid movement across the field.

To help understand the characteristics of the Clair basement structure, the Late Archaean - Early Proterozoic Lewisian Gneiss Complex (LGC) NW Scotland is being tested as a suitable analogue. Analysis of the Clair basement from well data and core suggests that it has affinities with the LGC in terms of age, lithologies and fracturing style. The LGC comprises tonalite-trondhjemite-granodiorite gneisses, mafic-ultramafic dykes, together with subordinate metavolcanic & metasedimentary sequences that were accreted as a series of terranes during the Precambrian. The LGC contains a complex fracture network with three prominent regional fault sets, each of which is associated with characteristic fault rock and mineral assemblages (Figure 1).

The present project focuses on analysing onshore data in 1-, 2- & 3-dimensions and correlating the findings with results from analyses of well & seismic data from the Clair field basement. Onshore data comprises regional NEXTMAP digital elevation models, outcrop sample lines, photomosaics and LIDAR data. Well and outcrop data comprises 1D line samples and have been used to characterise the spatial and textural attributes of the fault networks both onshore and offshore. LIDAR or terrestrial laser scanning has been used at selected outcrops to fully capture the characteristic spatial attributes and connectivity of fault & fracture networks in the onshore LGC in three dimensions.

Onshore results show that NE-SW fault & fracture trends are dominant in all dimensions, with NW-SE faults preferentially developing in areas of pre-existing basement anisotropy, such as phyllosilicate-rich ductile shear zones. Offshore seismic and well data show a similarly dominant NE-SW trend for faults. LIDAR point clouds have been interpreted and show complex fault & fracture networks across the scanned outcrops. These interpreted datasets will be used to provide a detailed comparison of the onshore and offshore fault & fracture networks and to determine the likely conected pathways through the Clair basement.

These results, along with the similarities in fracture mode, fill and other attributes suggest that the fault & fracture network recorded on the mainland LGC provide a fitting analogue for the fractured basement in the Clair Field.