Permeability of rock discontinuities and faults in the Triassic Sherwood Sandstone Group (UK). Insights for management of fluvio-aeolian aquifers worldwide

Fluvio-aeolian sedimentary successions host groundwater aquifers at shallow depths (<~0.15 km), which overlie geothermal and shale-gas reservoirs, and nuclear waste repositories at intermediate depths (~0.15–2.0 km). Additionally, such deposits represent petroleum reservoirs at greater depths (~2.0–4.0 km). The need to improve conceptual understanding of the hydraulic behaviour of fluvial-aeolian sandstone successions over a large depth interval (~0–4 km) is important for socio-economic reasons. Thus, the hydraulic properties of the Triassic Sherwood Sandstone aquifer in the UK have been reviewed and compared to similar fluvio- aeolian successions. The ratio between well-scale and core-plug-scale permeability (Kwell-test/Kcore-plug) acts as a proxy for the relative importance of fracture versus intergranular flow. This ratio (which typically varies from ~2 to 100) indicates significant contribution of fractures to flow at relatively shallow depths (<~0.15 km). Here, permeability development is controlled by dissolution of calcite-dolomite in correspondence of fractures. The observed ratio (Kwell-test/Kcore-plug) decreases with depth, approaching unity, indicating that intergranular flow dominates at ~1 km depth. At depths ≥ ~1 km, dissolution of carbonate cement by rock alteration due to groundwater flow is absent and fractures are closed. Aeolian and fluvial deposits behave differently in proximity to normal faults in the Sherwood Sandstone aquifer. Deformation bands in aeolian dune deposits strongly compartmentalize this aquifer. The hydro-structural properties of fluvio-aeolian deposits are also controlled by mineralogy in fault zones. A relative abundance of quartz vs. feldspar and clays in aeolian sandstones favours development of low-permeability deformation bands.