3D-temporal structural and petrophysical characterisation of crystalline basement rocks on Smøla Island, Central Norway: Insights into onshore-offshore basement highs and post-Caledonian tectonic evolution
Authors: Hodge Matthew , Venvik Guri , Knies Jochen , van der Lelij Roelant , Schönenberger Jasmin , Viola Giulio 
Published: February 2023

The crystalline basement on Sm&#248;la Island, within the Mid-Norwegian Passive Margin of central Norway, exhibits intricate and polyphase brittle deformation feature arrays ideal for characterising fracture networks, tectonic evolution, and fluid flow and basement storage potential. As Sm&#248;la Island is considered an onshore analogue of offshore basement structural highs, which are currently poorly constrained in terms of unconventional georesource reservoir potential, this work may have important insights for the resource industry, and additionally for advancing basement-hosted greenhouse gas repository opportunities. In this ongoing study, we are integrating various datasets from four Sm&#248;la diamond drill holes and multiscalar surface/subsurface datasets, with K-Ar geochronology, providing a new 3D perspective of brittle deformation evolution through time and in space. We aim to outline a &#8216;toolbox&#8217; methodology for producing robust deterministic 3D geological, and eventually, stochastic petrophysical models for deformed basement rock. Strike trends of pervasive cross-cutting lineaments over Sm&#248;la, identified from airborne magnetic and DTM data prior to their ground-truthing, high-resolution structural data and microscale petrographic analysis from the drill holes, and representative outcrops across Sm&#248;la Island provide geometric, kinematic, genetic, and cross-cutting relationships for a variety of multi-scalar deformation features (including brittle-ductile faults, fracture, and vein arrays). Field evidence and petrographic analysis suggest at least four major brittle deformation episodes (locally exploiting ductile precursors) linked to distinct mineral assemblages: I) epidote (3 types)-chlorite, II) chlorite-hematite-sericite, III) prehnite-calcite, and IV) hematite-calcite-zeolite. K-Ar dating results from seven selected oriented fault gouges indicate multiphase authigenic clay growth on faults oriented E-W, NW-SE, and NE-SW from the Late Carboniferous/Early Permian to the Late Triassic-Early Jurassic, and on N-S, NNE-SSW faults from the Late Carboniferous/Early Permian to the Mid-Cretaceous. Paleostress inversion from heterogeneous fault-slip data sorted according to the identified mineral assemblages indicates a polyphase tectonic evolution that broadly correlates with the known rifting and opening of the North Sea, and hyper-extension of the Mid-Norwegian margin. On-going 3D geological modelling of the oriented fault and fracture arrays coated by different mineral assemblages, through time, will provide a spatial and temporal evolution model for rock deformation on Sm&#248;la. These 3D deterministic geological models will subsequently be utilised to produce meaningful stochastic models, including discrete fracture network models (DFNs), to determine key petrophysical characteristics of the typical basement rocks and of their evolution through time.