Geothermal potential of Mesozoic carbonates on the example of the Rüdersdorf Formation (North German Basin): opportunity or not?

The Mesozoic succession of the North German Basin (NGB) yields enormous resources of heat in place bound to Mesozoic sandstone reservoirs located at depths of up to 2.500 m. Compared to highly-permeable sandstone reservoirs, which are exploited for heat production since the 1980s, carbonate reservoirs are underexplored so far. This study evaluates the potential of Mesozoic carbonates on the example of the Rüdersdorf Formation (“Schaumkalk”) in the subsurface of Berlin/Brandenburg.

The data are based on an extensive bed-by-bed outcrop study in the Rüdersdorf open-pit mine to the East of Berlin, and examination of well cores from Berlin-Spandau and Potsdam. The vertical succession exposed in outcrop and well cores shows gradual transitions from marly limestone background facies (“Wellenkalk”) to thick cross-bedded oolitic grain- to packstone reservoir facies (“Schaumkalk”). Based on analyses of carbonate microfacies and reservoir properties, oolitic reservoir facies exhibit high secondary porosities but low permeabilities. The late-diagenetic dissolution of oolites contributed to oomoldic porosity volumes of up to 27,4 % resulting in the typical foamy texture. But due to matrix-supported grain fabrics, the pore connectivity remained low as emphasized by the permeability range of 0.004–7.7 mD. The potential of the reservoir facies is further limited as fluid circulation along open fractures and faults did obviously not contribute to an enhanced matrix permeability. Accounting for this and the limited lateral extent of thick oomoldic reservoir facies, proven in regional well-to-well correlations, the opportunities for reservoir developments in the Rüdersdorf Formation seem to be very limited.