Effective permeability of fluvial lithofacies in the Bunter Sandstone Formation, UK

Abstract

Understanding effective permeability is crucial for predicting fluid migration and trapping in subsurface reservoirs. The Bunter Sandstone of northwestern Europe hosts major groundwater and geothermal resources and is targeted for CO₂ storage projects. Here the effective permeability of fluvial facies within the Bunter Sandstone Formation was assessed using facies-scale models. Twelve lithofacies were modeled based on core and outcrop observations of their geometries and dimensions. Permeability values from minipermeameter measurements were assigned to low- and high-permeability lithologies in each facies. The dimensions of a Representative Elementary Volume (REV) in depositional dip, depositional strike and vertical directions were determined by extracting sub-volumes from the models at different scales, calculating values of effective permeability for each sub-volume, and identifying the sub-volume at which the values of effective permeability stabilise as the REV. The REV dimensions vary with facies type and flow direction, but are typically of order tens of centimetres to metres in size, significantly larger than a typical core plug. Having identified the REV, we analyze the effective permeabilities of the different facies types. Normalized values of effective permeabilities in depositional dip, strike and vertical directions (k_d, k_s, k_v), relative to the permeability of low- and high-permeability lithologies in each facies, display a positive linear correlation with the proportion of high-permeability lithology (clay-poor sandstone) for all facies. Therefore, the proportion of clay-poor sandstone, as measured in core data, can be used to predict facies-scale effective permeability in the Bunter Sandstone Formation, as well as in analogous fluvial deposits globally.