Characterising fractured crystalline basement evolution through time, Smøla island, Mid-Norway: an integrated approach combining deterministic datasets for stochastic applications

Geologically old crystalline basement rock volumes, such as Smøla island in the mid-Norwegian passive margin, possess complex natural fracture and fault arrays which have formed through polyphase deformation histories. Deconvoluting these fracture networks into discrete formation events is challenging owing to the typical lack of absolute time constraints, which also limits understanding of past petrophysical properties during network evolution. Addressing these challenges, this thesis utilises a two-part temporally constrained study. The first part comprises a deterministic characterisation of Smøla, with the second part focussing on stochastic modelling and reconstruction. By utilising lineament analysis, field mapping, drill hole logging, 3D modelling, petrographic and microstructural studies, and fault gouge K–Ar geochronology, five deformation episodes (D1 to D5) were interpreted with associated synkinematic mineralisation, geometrical, and kinematic trends. D1 (after ~395 Ma) involved epidote–prehnite veins produced from brittle transtension along the regional Møre Trøndelag Fault Complex. D2 (comprising two stages, ~300 Ma and ~200 Ma) involved sericite–chlorite–calcite mineralised structures related to strike–slip faulting. D3 (~128–100 Ma) involved the formation of chlorite–hematite fractures associated with pervasive dip–slip, possibly transpressional, faulting. The later episodes, D4 (after ~75 Ma) and D5 (assumed Cretaceous–Paleogene), produced hematite–zeolite–calcite and quartz–calcite veins resulting from regional crustal extension. The second part then incorporated the deterministic statistical descriptions of the deformation features on Smøla to reconstruct the D4+5 to D1 networks via a stochastic ‘grown’ DFN modelling approach, using progressive fracture back–stripping, from <70–60 Ma to 395 Ma. 3D permeability tensors and 2D connectivity maps extracted from the models revealed the evolution of permeability anisotropy and connectivity during the prolonged fracture saturation of the rock mass. These results correspond to the timing of hydrocarbon migration activity offshore Norway. This conceptual approach establishes a framework to resolve reservoir charging histories of fractured crystalline basement volumes.