The rock mass as the governing factor for successfully developing deep geothermal systems in Southern Germany

In Southern Germany, geothermal wells target Upper Jurassic carbonates, formerly referred to as Malm, below the Northern Alpine Foreland Basin (NAFB). In the northern, shallower carbonates (3000 m depth), e.g., in the Munich area, hydrothermal systems prevail, and wells yield proper flow rates. Deeper carbonates (< 4000 m) towards the south indicate a petrothermal system due to a decline in porosity and permeability. Inexplicably, fractures only contribute poorly to larger flow rates. This sets the goal for the project Dolomitkluft to analyse the rock mass for options countering low flow rates in two geothermal wells at Geretsried, Bavaria. Therefore, 20 m drill cores from 4600 to 4715 m total vertical depth (TVD) depict the initial structure of the rock mass: the rock parameters, the discontinuity network, and the in situ stresses. Alternating homogeneous, micritic, strong, to inhomogeneous, porous, weak limestones and massive, crystalline dolostones define the stratigraphy. Five joint sets from the cores can be distinguished into the bedding, two paleo anti- and synthetic normal faults and two recent strike-slip faults. The latter, in combination with core disking, allows exclusively a strike-slip (SS) stress regime and delimits the ratio to at least SH : Sv : Sh = 1.7 : 1.0 : 0.7. According to the results, including dilatation- and slip-tendencies, drill trajectories of prosperous geothermal wells should head ESE/WNW, perpendicular to SS-faults, and avoid the presence of normal faults.