Influence of fluid flow on the regional thermal field: results from 3D numerical modelling for the area of Brandenburg (North German Basin)

We analyse the effect of fluid flow on the recent thermal field for the Brandenburg region (North German Basin) which is strongly affected by salt structures. The basin fill is modified by a thick layer of mobilized salt (Zechstein, Upper Permian) that decouples the overburden from deeper parts of the lithosphere and is responsible for thermal anomalies since salt has a distinctly higher thermal conductivity than the surrounding sediments and is impermeable to fluid flow. Numerical simulations of coupled fluid flow and heat transfer are carried out to investigate the influence of fluid flow on the shallow temperature field above the Zechstein salt, based on the finite element method. A comparison of results from conductive and coupled modelling reveals that the temperature field down to the low-permeable Triassic Muschelkalk is influenced by fluids, where the shallow low-permeable Tertiary Rupelian-clay is absent. Overall cooling is induced by forced convective forces, the depth range of which is controlled by the communication pathways between the different aquifers. Moreover, buoyancy-induced effects are found in response to temperature-dependent differences in the fluid density where forced convective forces are weak. The range of influence is controlled by the thickness and the permeability of the permeable strata above the Triassic Muschelkalk. With increasing depth, thermal conduction mainly controls the short-wavelength pattern of the temperature distribution, whereas the long-wavelength pattern results from interaction between the highly conductive crust and low-conductive sediments. Our results provide generic implications for basins affected by salt tectonics.