Adv. Geosci., 49, 121–127, 2019 https://doi.org/10.5194/adgeo-49-121-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Using core and outcrop analogues to predict flow pathways in the subsurface: examples from the Triassic sandstones of north Cheshire, UK Joanna Thompson, Daniel Parkes, Edward Hough, and Oliver Wakefield British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK Correspondence: Joanna Thompson ([email protected]) Received: 30 May 2019 – Accepted: 2 August 2019 – Published: 19 September 2019 Abstract. Borehole core provides detailed vertical data resources such as pore space and heat extraction. Subsurface which is used to interpret subsurface sand body architec- fluid flow is relevant to the development of new, low-carbon tures, but assumptions are made on the relationship between energy technologies (e.g., geothermal systems located in sed- the lateral and vertical thickness, and the interconnectivity imentary aquifers, underground gas storage and Carbon Cap- of units. The sedimentological complexity of the Sherwood ture and Storage; CCS). In the UK, there is the potential for Sandstone Group succession in this area, passing between the development of these geological settings where there are aeolian and fluvial packages creates local- to regional-scale porous and permeable rocks that may serve as production or heterogeneities which will impact flow pathways within the storage horizons located in proximity to sources of energy rockmass. Measured thickness in boreholes might represent such as shale gas reservoirs, coal seams or renewables; op- an architectural element’s true maximum thickness or more erational viability is increased where these geologies are lo- likely, a partial thickness as a result of incision by over- cated near areas of industrial activity or areas of dense popu- lying facies types or as a result of the borehole sitting to- lation. One such area is in north Cheshire in north-west Eng- wards the margins of individual elements (e.g. tapering mar- land, where the Sherwood Sandstone Group (SSG) aquifer gin of channel elements). Length and thickness data were overlies coal measures and the Bowland shale gas prospect, measured from a suite of primary core data and secondary with thick beds of halite used for gas storage located within published outcrop studies in north-west England. The ad- 25 km. The area is also located in the “Protos” area of in- dition of outcrop studies in combination with the borehole dustry (http://thisisprotos.com/, last access: 21 August 2019), data provides a dataset from which the likely lateral extent located near Liverpool, Manchester and associated conurba- of the architectural frameworks within the Triassic sand- tions. This site has been chosen as the Cheshire Energy Re- stones can be extrapolated. The interpreted high resolution search Field Site (CERFS), part of a GBP 31 million initia- sub-seismic architecture contributes to an increased under- tive to develop technologies to decarbonise energy by the UK standing of flow pathways and the effect these may have on Government (Fig. 1). It is planned that the CERFS will com- groundwater as well as sustainable energy technologies such prise a network of deep and shallow boreholes containing as low-temperature geothermal aquifers, carbon storage and state-of-the-art monitoring equipment to measure groundwa- energy storage. ter (levels, flow, temperature and chemistry), ground mo- tion, and seismicity. The proposed boreholes will range in drilled depth from 25 m to 1200 m (below surface), produc- ing a dense dataset including down-hole geophysical and im- 1 Introduction age logs and recover more than 3000 m of new drill core in the area of interest, largely from the Triassic SSG. A thorough understanding of fluid flow in bedrock geologi- The SSG is a principal aquifer, a hydrocarbon reservoir cal units is relevant to aquifer and reservoir management, al- offshore and onshore and a potential host for CO2 stor- lowing for optimised use of increasingly stressed subsurface age and aquifer geothermal schemes. The sedimentologi- Published by Copernicus Publications on behalf of the European Geosciences Union. 122 J. Thompson et al.: Using core and outcrop analogues to predict flow pathways in the subsurface Figure 2. Architectural styles present in the Sherwood Sandstone Figure 1. Location map showing the generalised distribution of the Group, after Lawrence et al. (2006). SSG at rockhead. © UKRI 2019, British Geological Survey, con- tains Ordnance Survey data © Crown Copyright and database rights 2019 [licence number 100021290 EUL]. multi-storey sand body (e.g., Henares et al., 2014; Mar- riott and Hillier, 2014). Previous studies characterising the bedrock in Runcorn, Cheshire (Lawrence et al., 2006) and cal and structural complexity of the SSG succession in this Sellafield, Cumbria (e.g., Medici et al., 2016), indicate that area, with interactions between aeolian and fluvial processes this approach is particularly successful when applied to and development of the Cheshire Basin, creates local and the SSG. Lawrence et al. (2006) proposed a classification regional scale heterogeneity which will impact fluid flow scheme whereby the sandstone architecture of the SSG was within the rockmass. In particular, the influence of deposi- split between “Layercake” and “Jigsaw” styles (Fig. 2) based tional facies on the diagenetic evolution of the units is well- on the lateral continuity of mudstones within the succession, documented (Bouch et al., 2006), and this variation in diage- and this has been used in this observational study to better un- nesis has a direct impact on the physical properties including derstand connectivity between sandbodies in north Cheshire. porosity, permeability and fracture development (Plant et al., 1999). This is especially important when reservoir sandbod- ies are the subject of porosity and permeability reducing pro- 2 Geological setting cesses such as secondary mineral precipitation and deforma- tion. As much of this heterogeneity exists on a sub-seismic The Sherwood Sandstone Group is Early to Mid-Triassic (In- scale, field-scale observations are important to understand duan to Anisian) in age and provides a preserved record of the likely scales of variability of the sandstone in the subsur- dryland sedimentation. During this time, rifting and exten- face. Aeolian units are generally thought of as having rela- sion related to the break-up of Pangea allowed thick accu- tively high porosity and permeability; however, interdune de- mulations of arenaceous sediments to develop in a series of posits can create baffles to flow, and natural fractures within actively subsiding linked half-grabens which fringed the lo- the bedrock could represent barriers to flow (e.g., deforma- cal palaeo-high of the Pennines. North Cheshire lies on the tion bands) or pathways (as in the case of some sediment- Llyn-Rossendale structural high, which separates the north- filled fractures) (Wealthall et al., 2001; Hough et al., 2006). ern part of the Cheshire Basin from the south-eastern part Estimating the dimensions and distribution of the deposi- of the East Irish Sea Basin (Fig. 3). Locally, sediments are tional elements within the sandstone will be an important over 1000 m thick in some depocentres, with younger units input parameter to detailed flow models that give better esti- overlapping interbasinal highs. Regional-scale depositional mates of transmissivity and fluid flow through the aquifer. models suggest a southerly source in the Variscan foldbelt Outcrop analogue studies can be an effective method to some few hundreds of kilometres to the south, with north- gain information that describes the scale and relationship wards draining rivers bifurcated by the Pennine High, divert- between different architectural elements within a stacked, ing sediment westwards towards the Cheshire Basin, Lan- Adv. Geosci., 49, 121–127, 2019 www.adv-geosci.net/49/121/2019/ J. Thompson et al.: Using core and outcrop analogues to predict flow pathways in the subsurface 123 Table 1. Facies scheme used to classify Triassic sandstones in borehole and outcrops in north Cheshire, this study considers only the aeolian facies. All massive sandstones have been attributed to dry interdunes but these may include minor components of aeolian dunes. Lithology Depositional environment Sandstone, low-to high-angle cross-bedded, with grainfall and Aeolian dune grainflow textures preserved Sandstone, low-angle to planar bedded; rare ripple lamination Dry interdune Sandstone, low-angle to planar bedded, rare adhesion ripples Damp interdune Mudstone, planar-laminated, with characteristic desiccation Wet interdune features Sandstone, massive Dry interdune Sandstone, with deformed lamination Wet interdune with deformed lamination due to slope failure/ liquefaction/ overpressure Figure 3. Basin structure of the study area. © UKRI 2019, British Geological Survey, contains Ordnance Survey data © Crown Copy- right and database rights 2019 [licence number 100021290 EUL]. Figure 4. Stratigraphic column for the study area. cashire and Cumbria, and eastwards to the Needwood Basin and East-Midlands Shelf (McKie and Williams, 2009). To the east of the Pennine High, subsidence rates are information describing the SSG has been obtained from bore- thought to have been steady, allowing for the accumulation hole material. of a thick fluvial succession (Wakefield et al., 2015). This contrasts to the succession to the west of the Pennine