Characterising fracture systems within the Lewisian Gneiss Complex, northwest Scotland: An onshore analogue for the Clair Field.

The Clair Field lies in the Faroe-Shetland Basin, with reservoirs in Devonian and Carboniferous sediments overlying and onlapping a basement high that was upfaulted in the Mesozoic. At Clair, the basement is considered an important control on fluid flow and 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.

Analyses of fault attributes have been undertaken across a range of scales on both the mainland and Hebridean Islands. Fault systems in Lewisian basement and overlying Mesozoic cover sequences have been characterised as an analogue for the Clair Field. This study included analysis of regional NEXTMap digital elevation models of the mainland and the Hebrides along with studies of seismic attribute maps of the Clair top basement horizon, with the aim of characterising large scale fault patterns, including orientation, density and spacing attributes. Fieldwork has been undertaken on the mainland and the Hebrides in order to characterise fault systems and the tectonic history within Lewisian Gneiss and sedimentary cover sequences. This involves 1-D line sample analysis, and terrestrial laser scanning (LIDAR) to allow analysis of the fault networks in different dimensions. Thin section analyses augment this study.

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 mainland and Clair Field datasets. Spacing data show variable population distributions, with outcrop data on the mainland and Hebridean Islands showing 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.

Based on fieldwork observations a brittle deformation history has been reconstructed. In the Outer Hebrides, the presence of the synrift Stornoway Formation has allowed identification of significant brittle faulting that occurred throughout the Mesozoic to as recently as the Tertiary, and this was often affected by the presence of pre-existing structures. Mainland fracture histories are much older with a limited younger set unaffected by previous structures.

Further analyses will determine whether the mainland or the Hebrides provide the closest match for the fault networks seen within the basement and overlying cover sequences of the Clair Field.