EXPOSITION TIMES AND KARSTIFICATION SIGNATURES OF THE MALM-AQUIFER IN SOUTHERN GERMANY AND THEIR IMPLICATIONS FOR RESERVOIR CHARACTERISATION

The deep Upper Jurassic Malm carbonates are the most important reservoir rocks for hydrothermal energy in Southern Germany. They consist of up to 600 m thick, locally dolomitised limestones. A mostly thick-bedded to massive platform facies can be distinguished from a marly thinner bedded basin facies. In exploration strategy, karstification is known to significantly increase the productivity of a well; hence its history is a crucial point in reservoir characterisation. It can help to better understand and interpret the Intra-Malm seismic signature and so, to better setup geological and hydrogeological models for fluid flow simulation and well path optimisation.

The Malm crops out in the Suebian and Franconian Alb and dips gently southwards under the Tertiary Molasse Basin (North Alpine Foreland Basin) reaching a depth of more than 5,000 m at the boundary to the alpine orogeny. The available data like seismic surveys, drilling reports and core material, geophysical borehole data and the hydraulic formation tests together with analogue outcrop studies support the reservoir properties assessment.

Reservoir quality varies strongly, depending on sedimentary facies, diagenesis, dolomitisation, karstification, and tectonic impregnation. In the beginning of the hydrothermal exploration, a long-lasting exposition of the carbonates during the Cretaceous was assumed to drive an intense karstification at the top of the reservoir leading to an inherent higher productive zone. However, this has shown to be a simplification, since karstification features are present in the entire Malm, spatially related to faults and thick bedded to massive dolomites.

During the Lower Cretaceous, several transgressions from the alpine helvetic shelf reached the area of the later basin. The extent of transgressions in the Upper Vanginian/Hauterivian, Upper Aptian/Albian, Upper Cenomanian/Turonian, Priabonian, Lower Kiscell, Upper Kiscell, and Eger were evaluated to propose a karstification history by dividing Southern Germany in at least ten exposition zones, lasting from about 7 Ma to more than 100 Ma. The karstification is characterized by signatures from well and seismic data for the central zone. This zone comprises the area around the city of Munich and is recently the most explored and exploited.