Abstract Modelling the behaviour of carbon dioxide (CO 2 ) injected into sub-surface reservoirs as part of carbon capture and storage (CCS) strategies is often performed using models that incorporate very limited geological detail, particularly at the subseismic (metre to decametre) scale. Those modelling studies that incorporate varying degrees of geological realism show the inherent risks and uncertainties that can result from neglecting heterogeneity and reservoir–caprock topography along the migration path of an injected CO 2 plume. A key problem is that detailed geological data are often not available for the relatively deep saline aquifers that are an important target for CCS. Deep saline aquifers fall between the relatively data-rich environments of shallow freshwater aquifers and hydrocarbon reservoirs and it is in these settings that outcrop analogues may play an important part in reducing the risks and uncertainties associated with CCS. This study uses an example from the Sherwood Sandstone Group (Otter Sandstone Formation) of the Wessex Basin to show how an outcrop study can impart a much greater understanding of heterogeneity in critical reservoir–caprock zones. Here the transition from the Sherwood Sandstone Group (fluvial sandstone reservoir) to the Mercia Mudstone Group (playa lacustrine mudstone seal) is not simple, but includes a major change in fluvial style that introduces considerable heterogeneity at the top of the reservoir. The study shows how laser-scanned outcrop can be used to rapidly construct static geological models that are taken through to flow simulations. In combination, the use of appropriate outcrop analogues and flow modelling can reduce the risks and uncertainties associated with CCS.

Abstract Carbonate is not generally considered as a potential source of large-scale heterogeneity in Permo-Triassic sandstone aquifers. This study shows that carbonate, in the form of early diagenetic calcrete, forms an abundant component of the Triassic Otter Sandstone Formation in south Devon. Three types of calcrete are described from the outcrop of this fluvial sandstone dominated aquifer: rhizocretions; calcrete sheets; and calcrete conglomerates. Data obtained from core plugs show that calcrete reduces sandstone permeability to less than 10 −15 m 2 and porosity to less than 12%. Calcrete can be sufficiently abundant to produce a significant (up to 30%) reduction in the total effective porosity of the aquifer. Moreover, calcrete conglomerates can form laterally extensive (up to 1 km) low-permeability sheets that will represent major baffles to vertical flow.

Abstract A 150 m observation borehole was drilled in Abbey Arms Wood, Delamere, Cheshire, UK in order to explore the local hydrogeological conditions and to understand better the source of the high concentrations of arsenic in some of the local groundwaters. The borehole was located on an outcrop of the Helsby Sandstone Formation (part of the Sherwood Sandstone Group) and was cored into the underlying Wilmslow Sandstone Formation. The aquifers in the area are unconfined and give rise to low-Fe groundwaters with As concentrations in the 10–50 µg l −1 range. The chemical composition of the sediments is quite uniform down to 150 m. The total arsenic content is in the range from 5 to 15 mg kg −1 and averaged 8 mg kg −1 ( n = 60). There is no trend in sediment As concentration with depth, but pore water centrifuged from the core steadily increased in As concentration with depth. The As concentration ranges from 8 µg l −1 at 10 m (unsaturated zone) to 30 µg l −1 at 150 m. The source of the dissolved As remains unclear but the lack of evidence for discrete high-As minerals or zones of mineralization suggests that it is probably derived by desorption from rock-forming minerals in the sandstones, e.g. iron oxides. This may be in response to slightly higher pH (up to 8.0 at depth). If this trend applies throughout the area, restricting the screened interval for abstraction boreholes to the uppermost parts of the saturated zone may reduce As concentrations, but is likely to reduce yields and may also risk encountering groundwaters with high nitrate concentrations.