Fossil multiphase normal faults – prime targets for geothermal drilling in the Bavarian Molasse Basin?
Foreland basins with their increasing depth towards the orogenic belt are ideal geologic systems as host for geothermal resources. The Bavarian Molasse Basin is the only basin worldwide where geothermal energy is being successfully developed by industry in conduction dominated heat transport environment. However, the predicted productivity is not achieved in all projects because either temperature or flow rate or both are lower than expected. The utilized reservoir rock consists of Upper Jurassic fractured carbonates, and reservoir quality is governed by fracture and fault frequency, dolomitisation degree, karstification and facies type. From these controlling factors only fracture density can be reliably assessed before drilling because generally highest fracture density is found in the vicinity of faults which can be in turn clearly identified in reflection seismic. In contrast facies and dolomitic domains can be reliably detected only after drilling and seismic-well log correlation. From this perspective faults seem to be the most reliable target in geothermal exploration in the Molasse Basin. A particular fault system, however, might have experienced a complex kinematic history from Jurassic to the present-day state, eliciting the question which fault among many detected faults might be the best drilling target. The geothermal site Mauerstetten in the Western Bavarian Molasse Basin is an example where drilling into a fault zone did not result in high flow rates. This article tries to identify the effects on the lacking reservoir permeability by detailed geological well data and advanced structural geological interpretation. The results are set into context with successful geothermal drilling projects in the Molasse basin where high flow rates were achieved by fault zone drilling.