Characterising fractured basement using the Lewisian Gneiss Complex, NW Scotland: Implications for fracture systems in the Clair Field basement

Abstract

1-, 2- and 3-dimensional fracture network characteristics of the mainland Lewisian Gneiss Complex (LGC) have been used assess if the LGC is a suitable onshore analogue for the fracture networks within the offshore Clair basement. Faults that cut the Clair basement rocks are becoming increasingly recognised as important structural conduits that connect (oil & gas bearing) sedimentary packages across the main basement structure. The basement of the Clair field is poorly understood, with limited seismic and well datasets; so using an onshore fracture analogue, of the mainland LGC, will hopefully improve the understanding of the Clair basement fracture systems.
To determine the suitability of the mainland LGC as an analogue for the Clair basement four main research questions are asked in this thesis: What is the offshore Clair basement? What are the geological characteristics and attributes of the fracture networks in the mainland LGC? What are the dominant controls on these fracture network characteristics? How can the mainland Lewisian be used as an analogue for the Clair basement?
Onshore datasets (outcrop, terrestrial laser scans & NEXTMap® DEM) exhibit prominent NE-SW and/or NW-SE fault and fracture trends. The Clair basement seismic dataset exhibits comparable NE-SW & NW-SE trending faults, but the basement core samples exhibit a strongly aligned NNE-SSW fracture trend that is not so clearly represented in the onshore datasets. Fracture spacing distributions from the mainland LGC have strong power-law relationships over at least three orders of magnitude. Power-law relationships are also present from Clair basement datasets but the lack of large datasets means that these relationships are considered fairly weak.
Qualitative and Quantitative onshore and offshore analyses suggest that the mainland LGC is a suitable analogue for the Clair basement to some degree, but that their relationship is not a simple one. The results presented in this thesis do not provide a unique solution for the Clair basement fracture networks. Instead the onshore data provide model types that can be used in sensitivity models to ultimately assess which onshore dataset provides the best geological and statistical analogue for the Clair basement.