Characterizing flow pathways in a sandstone aquifer: Tectonic vs sedimentary heterogeneities

Abstract

Sandstone aquifers are commonly assumed to represent porous media characterized by a permeable matrix. However, such aquifers may be heavy fractured when rock properties and timing of deformation favour brittle failure and crack opening. In many aquifer types, fractures associated with faults, bedding planes and stratabound joints represent preferential pathways for fluids and contaminants. In this paper, well test and outcrop-scale studies reveal how strongly lithified siliciclastic rocks may be entirely dominated by fracture flow at shallow depths (≤180m), similar to limestone and crystalline aquifers. However, sedimentary heterogeneities can primarily control fluid flow where fracture apertures are reduced by overburden pressures or mineral infills at greater depths. The Triassic St Bees Sandstone Formation (UK) of the East Irish Sea Basin represents an optimum example for study of the influence of both sedimentary and tectonic aquifer heterogeneities in a strongly lithified sandstone aquifer-type. This fluvial sedimentary succession accumulated in rapidly subsiding basins, which typically favours preservation of complete depositional cycles including fine grained layers (mudstone and silty sandstone) interbedded in sandstone fluvial channels. Additionally, vertical joints in the St Bees Sandstone Formation form a pervasive stratabound system whereby joints terminate at bedding discontinuities. Additionally, normal faults are present through the succession showing particular development of open-fractures. Here, the shallow aquifer (depth≤180m) was characterized using hydro-geophysics. Fluid temperature, conductivity and flow-velocity logs record inflows and outflows from normal faults, as well as from pervasive bed-parallel fractures. Quantitative flow logging analyses in boreholes that cut fault planes indicate that zones of fault-related open fractures characterize ~50% of water flow. The remaining flow component is dominated by bed-parallel fractures. However, such sub-horizontal fissures become the principal flow conduits in wells that penetrate the exterior parts of fault damage zones, as well as in non-faulted areas. The findings of this study have been compared with those of an earlier investigation of the deeper St Bees Sandstone aquifer (180 to 400m subsurface depth) undertaken as part of an investigation for a proposed nuclear waste repository. The deeper aquifer is characterized by significantly lower transmissivities. High overburden pressure and the presence of mineral infillings, have reduced the relative impact of tectonic heterogeneities on transmissivity here, thereby allowing matrix flow in the deeper part of the aquifer. The St Bees Sandstone aquifer contrasts the hydraulic behaviour of low-mechanically resistant sandstone rock-types. In fact, the UK Triassic Sandstone of the Cheshire Basin is low-mechanically resistant and flow is supported both by matrix and fracture. Additionally, faults in such weak-rocks are dominated by granulation seams representing flow-barriers which strongly compar...

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

“…Also, numerical models suggest the transition between stratabound and nonstratabound joints typically occurs between 0.6 and 2.0 km (Gillespie et al 2001). Optical logs in the shallow St Bees Sandstone aquifer (< 150 m BGL) also show development of karst pathways in correspondence of intersections between bedding planes and stratabound fractures (Medici et al 2016). This suggests how~2.5 Ma of exposure to the processes of enhancement of fracture permeability, typical of shallow aquifers, might have played a key role in influencing transmissivity distribution in this fractured fluvial aquifer, which is markedly decreased where tested at depths > 150 m below the ground surface (Chadwick et al 1994;Streetly et al 2000;Worthington et al 2016).…”

“…Historically, pumping tests undertaken at shallow (< 150 m BGL) depths yield relatively high transmissivity (T) values throughout the St Bees-Sellafield coastal plain (Fig. 1) in both wells penetrating the fluvial (n = 9; T = 30-700 m 2 /day) and aeolian (n = 3; T = 100-220 m 2 /day) deposits (Allen et al 1997;Medici et al 2016). Pumping tests (n = 33; T = 0.46-8.82 m 2 /day) were also undertaken at greater depth (200-400 m BGL; Streetly et al 2000) in the fluvial St Bees Sandstone aquifer.…”

“…12a. The quantitative analysis of well-bore flow logs from the shallow St Bees Sandstone aquifer shows that fault zones typically account for 50% of the overall transmissivity in boreholes cutting the fault planes (Medici et al 2016). Similarly, well tests (n = 33) undertaken by Streetly et al (2000) with packers in correspondence of extensional faults at intermediate (150-400 m BGL) depths in Sellafield BH2 show higher well-test scale hydraulic conductivity (K of 3.76 × 10 −1 m/day) compared to the unfaulted parts of the aquifer (K of 9.2 × 10 −3 -2.24 × 10 −1 m/ day).…”

“…Previous hydro-geophysical characterization studies of sandstone aquifers have mostly focused on the first 150 m below the ground surface, providing information on the effects of bedding parallel fractures and faults on water flow (Gellasch et al 2013;Lo et al 2014;Runkel et al 2006;Hitchmough et al 2007;Medici et al 2016). Several petrohydraulic studies conducted on sandstone lithotypes (including fluvial deposits) have been undertaken on successions at depths ≥ 1,000 m using data from hydrocarbon extraction fields (e.g., Baas et al 2007;Corbett et al 2012;Zheng et al 2000).…”

Forensic Excavation of Rock Masses: A Technique to Investigate Discontinuity Persistence

“…O'Reilly 1980;Park 2005;Kim et al 2007;Wasantha et al 2011;Bonilla-Sierra et al 2015;Tucky and Stead 2016). The persistence and aperture of discontinuities also influence their hydraulic properties and flow pathways (Fu et al 2016;Medici et al 2016). Figure 1a illustrates the usual, and rather simplistic, definition of areal persistence K A which is expressed by:…”