Deep reaching fluid flow close to convective instability in the NE German basin-results from water chemistry and numerical modelling

In several areas of the North German Basin salinar brines reach the surface locally. An evaluation of the available data of pore fluids indicates an unstable stratification in terms of fluid densities. In order to study the possible physical causes, numerical simulations of coupled fluid, heat and mass flow were carried out at the basin scale. Modelling results suggest a possible presence of thermally induced free convection controlled by spatial heterogeneity in sedimentary-layer thickness and permeability. Specifically, the results point to a situation where the subsurface fluids are generally close to convective instability, with presence of free convection in those regions where a sufficient, vertically continuous permeability exists. Scenarios that allow for a local presence of free convection may be needed to understand the mechanism behind numerous previously unexplained field observations of deep-groundwater occurrences near the basin surface. The potential presence of large-scale convection cells has implications both for the fundamental understanding of basin processes and socio-economic issues.