Cool deltas: Sedimentological, geomorphological and geophysical characterization of ice-contact deltas and implications for their reservoir properties (Salpausselkä, Finland)

Abstract: Sediments deposited by glacial meltwaters (for example, ice‐contact delta deposits) form permeable packages in the subsurface that can act as reservoirs for both water and hydrocarbons. They are also an important source of aggregate for the construction industry. As reservoirs they are challenging to characterize in terms of their structure, flow and storage properties due to their complex depositional history. In this study, ice‐contact deltas of Salpausselkä I and II end moraines in Southern Finland are studied using a combination of geomorphological mapping, sedimentological studies and near surface geophysical methods. Sedimentary logs from isolated outcrops were correlated to ground penetrating radar (GPR) profiles to unravel the internal structure and depositional history of these ice‐contact deltas. Subsequently, electrical resistivity tomography (ERT) and gravity data were analysed to estimate the depth to bedrock and to model porosity distribution within the sediments. Results of the study suggest that the delta deposits have a broad range of porosities (10 to 42%) with lowest values found in the bottomset beds. The most variable porosities are in the subaqueous ice‐contact–fan zone, and consistently high porosities occur in delta foreset/topset facies. Detailed sedimentary logging linked to the GPR data shows heterogeneities such as mud drapes on foresets and kettle holes which are below the resolution of ERT and gravity methods but significantly affect reservoir properties of the deltas. Moreover, oscillation of the ice‐margin may have introduced larger heterogeneities (for example, buried ice marginal ridges, or eskers) into the sedimentary sequence which are atypical for other Gilbert‐type deltas. Finally, subglacially sculpted, highly variable bedrock topography exerts a major control on sediment distribution within the delta making reservoir volume and quality less predictable. This work has implications for present‐day freshwater aquifers and low enthalpy geothermal energy in southern Finland and other deglaciated regions, as well as hydrocarbon exploration of analogous deposits in the subsurface from Pleistocene and pre‐Pleistocene glaciogenic sequences