Assessment of the present-day thermal field (NE German Basin)—Inferences from 3D modelling

Abstract

We use a refined 3D structural model based on an updated set of observations to assess the thermal field of Brandenburg. The crustal-scale model covers an area of about 250 km (E–W) times 210 km (N–S) located in the Northeast German Basin (NEGB). It integrates an improved representation of the salt structures and is used for detailed calculations of the 3D conductive thermal field with the finite element method (FEM). A thick layer of mobilised salt (Zechstein, Upper Permian) controls the structural setting of the area. As salt has a considerably higher thermal conductivity than other sediments, it strongly influences heat transport and accordingly temperature distribution in the subsurface. The modelled temperature distribution with depth shows strong lateral variations. The lowest temperatures at each modelled depth level occur in the area of the southern basin margin, where a highly conductive crystalline crust comes close to the surface. In general, the highest temperatures are predicted in the north-western part of the model close to the basin centre, where rim syncline deposits around the salt domes cause insulating effects. The pattern of temperature distribution changes with depth. Closely beneath the salt, the temperature distribution shows a complementary pattern to the salt cover as cold spots reflect the cooling effect of highly conductive salt structures. The predicted temperatures at depths beneath 8 km suggest that the influence of the salt is not evident any more. Similar to the temperature distribution, the calculated surface heat flow shows strong lateral variations. Also with depth the variations in thermal properties due to lithology-dependent lateral heterogeneities provoke changing pattern of the heat flow. A comparison with published heat flow and temperature data shows that the model predictions are largely consistent with observations and indicates that conductive heat transport is the dominant mechanism of heat transfer. Local deviations between modelled and observed temperatures are in the range of ±10 K and may be due to the convective heat transport. To assess the potential influence of convective heat transport we zoom in on a specific location of Brandenburg corresponding to the in-situ geothermal laboratory Groß Schönebeck. This local model is used to carry out 3D numerical simulations of coupled fluid flow and heat transfer processes. Our coupled models indicate that conduction is the dominant heat transfer mechanism below Middle Triassic strata. Above the Triassic Muschelkalk, the more than 3000 m of sediments with higher hydraulic conductivity promote the formation of convection cells. Here, especially high degrees of coupling result in remarkable convective heat transport.