Characterising fracture systems within fractured crystalline reservoirs: the Lewisian Gneiss Complex, Scotland as an onshore analogue for the Clair Field basement
Exotic hydrocarbon reservoirs, such as crystalline basement, are increasingly a target for hydrocarbon exploration in the development of new and existing fields. The Clair field lies in the Faroe-Shetland Basin, with reservoirs in Devonian- Carboniferous clastics overlying and onlapping a basement high that was upfaulted in the Mesozoic. This basement is known to control fluid flow and the structural development of the field due to its highly fractured nature. Consequently, it is important to characterise fault networks to assess the connected volume within the basement rocks. Fault-fracture systems in the regional basement complex of NW Scotland, the Leiwsian Gneiss Complex, and in overlying Mesozoic cover sequences have been characterised as an analogue for the Clair Field. Variations in lithology, metamorphic grade and tectonic setting all influence the fault network characteristics in the Scottish mainland and Hebridean islands. This study included analysis of regional NEXTMap digital elevation models of surface outcrops together with studies of seismic attribute maps of the Clair top basement horizon in the subsurface. The aim here was to characterise and compare the large scale fault patterns, including orientation, density and spacing attributes. Fieldwork has been undertaken onshore in order to characterise fault systems and the tectonic history within Lewisian Gneiss and sedimentary cover sequences at meso- to micro-scales. This involves 1-D line sample analysis, photomosaic analysis and terrestrial laser scanning (LIDAR) to allow analysis of the fault networks in different dimensions. 1-dimensional and 2-dimensional analyses of regional and outcrop scale data show that NE-SW is a predominant fault trend within the Lewisian in both the onshore and offshore datasets. Spacing data onshore show mainly power-law distributions. These distributions allow us to infer that the outcrop data show scale-invariance for spacing and can therefore be used as an estimate for the fault networks seen at different scales within the Lewisian. Significant differences have been found in faulting styles between postulated Lewisian basement 'terranes' throughout Scotland. In the Outer Hebrides, the presence of the syn-rift Permo-Triassic Stornoway Formation and later sets of Tertiary dykes has allowed identification of significant brittle faulting that occurred throughout the Mesozoic to as recently as the Tertiary. The Mesozoic and younger fractures are much less well developed in the Scottish mainland compared to the Hebrides. This suggests that the latter region may provide the closest match for the fault networks seen within the basement and overlying cover sequences of the Clair Field. These studies have important implications for the understanding of the complex brittle deformation history of basement gneiss complexes. It also illustrate how analogue surface outcrops can be used to understand sub-surface systems and their fluid transport-storage capabilities.