Structural geology and reservoir properties of the Middle Buntsandstein in Göttingen and the related risks for a hydrogeothermal project

Within the sedimentary succession of Zechstein to Lower Jurassic in the Leinetalgraben, stacked channel sandstones of the Middle Buntsandstein are the most promising geothermal reservoirs for Göttingen. Located in depths of ca. 800-1.400 m, reservoir temperatures are estimated to vary from 35-55°C.

Two seismic sections were interpreted and a 3D-structural model was built in combination with outcrop and scarce well data. Structural analysis confirmed the two-phased tectonic model for graben formation proposed previously, with an early W-E extension and later NW-SE compression. The pinnate eastern boundary faults continue towards the graben centre, resulting in highs and lows within the graben.

Statistical analysis of petrophysical properties of the Middle Buntsandstein was based on core plug data from wells in Lower Saxony. The average sandstone porosity is ca. 9.3% (p10: 1.8%, p90: 21.1%), the average permeability is ca. 7.8 mD (p10: 0.005 mD, p90: 263 mD). The very high range of both parameters most likely reflects not only sample depth but also the abundance of authigenic minerals within the intergranular volume and their degree of dissolution. Due to the complex fault evolution and mudstone interlayers, an enhancement of reservoir properties within fault damage zones cannot be assumed.

Following the wide range of petrophysical data, for a geothermal doublet a flow rate between 1.3 l/s (p90) and 101 l/s (p10) has been calculated, with a mean of 29.5 l/s. However, as typical for geothermal doublets in northern Germany and neighbouring regions, this wide range poses a significant economic risk for a geothermal project.