Characterization of a fluvial aquifer at a range of depths and scales: the Triassic St Bees Sandstone Formation, Cumbria, UK

Abstract

Fluvial sedimentary successions represent porous media that host groundwater and geothermal resources. Additionally, they overlie crystalline rocks hosting nuclear waste repositories in rift settings. The permeability characteristics of an arenaceous fluvial succession, the Triassic St Bees Sandstone Formation in England (UK), are described, from core-plug to well-test scale up to~1 km depth. Within such lithified successions, dissolution associated with the circulation of meteoric water results in increased permeability (K~10 −1 -10 0 m/day) to depths of at least 150 m below ground level (BGL) in aquifer systems that are subject to rapid groundwater circulation. Thus, contaminant transport is likely to occur at relatively high rates. In a deeper investigation (> 150 m depth), where the aquifer has not been subjected to rapid groundwater circulation, well-test-scale hydraulic conductivity is lower, decreasing from K~10 −2 m/day at 150-400 m BGL to 10 −3 m/day down-dip at~1 km BGL, where the pore fluid is hypersaline. Here, pore-scale permeability becomes progressively dominant with increasing lithostatic load. Notably, this work investigates a sandstone aquifer of fluvial origin at investigation depths consistent with highly enthalpy geothermal reservoirs (~0.7-1.1 km). At such depths, intergranular flow dominates in unfaulted areas with only minor contribution by bedding plane fractures. However, extensional faults represent preferential flow pathways, due to presence of high connective open fractures. Therefore, such faults may (1) drive nuclear waste contaminants towards the highly permeable shallow (< 150 m BGL) zone of the aquifer, and (2) influence fluid recovery in geothermal fields.

Sedimentary flow heterogeneities in the Triassic U.K. Sherwood Sandstone Group: Insights for hydrocarbon exploration

“…Rapid rates of subsidence also favour preservation of mudstone beds of fluvial overbank origin since channel emplacement by avulsion has reduced potential to rework such deposits (Figure , central part; Colombera, Mountney, & McCaffrey, ). Such overbank mudstone elements that occur interbedded with fluvial channel elements are capable of impeding flow to production wells (Medici et al, , ). Preservation of such overbank deposits, which is favoured by high rates of extensional tectonics (≳100 m/Myr), allows sufficient preservation in the vertical section (>15% of thickness) and lateral extent (>200 m; Colombera, Mountney, & McCaffrey, ) of mudstone elements to make such bodies effective in reducing well‐test‐scale permeability where matrix flow dominates.…”

“…Preservation of such overbank deposits, which is favoured by high rates of extensional tectonics (≳100 m/Myr), allows sufficient preservation in the vertical section (>15% of thickness) and lateral extent (>200 m; Colombera, Mountney, & McCaffrey, ) of mudstone elements to make such bodies effective in reducing well‐test‐scale permeability where matrix flow dominates. Thus, the preservation of such bodies serves to reduce reservoir quality (Medici et al, , ).…”

“…Rapid rates of subsidence also favour preservation of mudstone beds of fluvial overbank origin since channel emplacement by avulsion has reduced potential to rework such deposits (Figure 8, central part; Colombera, Mountney, & McCaffrey, 2015). Such overbank mudstone elements that occur interbedded with fluvial channel elements are capable of impeding flow to production wells (Medici et al, 2016(Medici et al, , 2018. Preservation of such overbank deposits, which is favoured by…”

“…The presence of conductive fractures in the U.K. Sherwood Sandstone aquifer at depths <150 m below the ground surface typically precludes linkage between transmissivity and lithofacies. For example, the fluvial St Bees Sandstone ( T = 20–750 m 2 /day) and the aeolian Calder Sandstone ( T = 100–210 m 2 /day) aquifers in the eastern Irish Sea Basin, which are characterized by significantly different porosities and core‐plug‐scale permeability (Figure b) values, show substantial overlap in well‐test‐derived transmissivity (Allen et al, ; Brassington & Walthall, ; McKie & Williams, ; Medici et al, , ; Tellam & Barker, ). Despite this, intergranular flow dominates in the Sherwood Sandstone aquifer under circumstances of either (a) particularly high intergranular porosity (eastern England Shelf) or (b) a reduction of fracture flow with the increasing depth and lithostatic load (eastern Irish Sea Basin).…”

Three‐dimensional forward stratigraphic modelling of the sedimentary architecture of meandering‐river successions in evolving half‐graben rift basins

“…Assessing and predicting lithological complexity in the fill of rift basins is important for two principal reasons: (a) it enables models to be developed whereby the sedimentary record can be used to reconstruct histories of basin evolution and to help assess the impacts of tectonic control on sedimentation, (b) from an applied standpoint, the deposits of fluvial successions in rift basins form major oil and gas reservoirs (Corbett, Hamdi, & Gurav, 2012;Hamdi, Ruelland, Bergey, & Corbett, 2014;Medici, West, & Mountney, 2018b), serve as important groundwater aquifers (Lockwood, 2001;Medici, West, & Mountney, 2016;Tellam & Barker, 2006), and act as potential sites for long-term carbon sequestration (Bachu, 2000) and for the underground storage of radioactive waste (Bath et al, 2006;Medici, West, & Mountney, 2018a). Point-bar deposits of meandering rivers are of particular importance in applied geology due to their economic potential as major hydrocarbon reservoirs (Jolley, Fisher, & Ainsworth, 2010;Larue & Hovadik, 2006).…”