Multiphase fossil normal faults as geothermal exploration targets in the Western Bavarian Molasse Basin: Case study Mauerstetten

Abstract

The Bavarian Molasse Basin represents a peripheral foreland basin hosting abundant hydrothermal resources in 3-5 km deep Upper Jurassic carbonate rocks. Faults and facies play a major role in targeting production wells; however the kinematic evolution of fault zones and the classification of carbonate facies of the Upper Jurassic are still debated. At the geothermal prospect Mauerstetten in the Western Bavarian Molasse Basin, a geothermal well and a side track are drilled along and about 650 m off an ENE-WSW striking normal fault. A stratigraphy related fault throw analysis of six 2D seismic sections crossing this fault evidences multiphase normal faulting from Cretaceous to Upper Miocene with a major activity phase in the Oligocene. This fault, inactive since Upper Miocene, is presumably a fossil normal fault in the present-day stress field that has a maximum horizontal stress direction in N-S. Analysis of carbonate facies by thin section petrography of drill cuttings and geophysical borehole logs lead to two major conclusions: (i) the reservoir rock represents low permeable platform limestones, reef detritus and dolostones of the Franconian facies, and (ii) the fault consists of multiple normal faulting steps with higher permeability than in intact rock. This observation suggests a fracture controlled reservoir with permeable damage zones in a tight rock mass along reactivated normal faults.

Detailed Fluid Inclusion and Stable Isotope Analysis on Deep Carbonates from the North Alpine Foreland Basin to Constrain Paleofluid Evolution

“…High-temperature formation waters of the Upper Jurassic in deeper zones of the foreland basin ( Figure 2) are unlikely, as the Upper Jurassic carbonate rocks in the deeper zone are low to no permeable and thus formation fluids could only migrate parallel to stylolites. However, there is still the problem of the nearly east-west-striking fault structures [78] which the migrating fluids had to overcome to migrate from the deeper zone to the sampled area in the North Alpine Foreland Basin. The higher geothermal gradient seemed to be the most relevant cause of the temperature increase, as a varying geothermal gradient would explain the high temperatures and might not cause a change in fluid chemistry.…”

Geothermal play typing in Germany, case study Molasse Basin: a modern concept to categorise geothermal resources related to crustal permeability

“…Lüschen et al, 2014;Hartmann et al, 2017), well log data, hydraulic test data (e.g. Birner et al, 2012;Stober et al, 2014;Dussel et al, 2016), drill core and cutting analysis (Mraz et al, 2018(Mraz et al, , 2019 and hydrochemistry studies (e.g. Birner et al, 2011;Stober et al, 2013aStober et al, , 2014Stober, 2014).…”

“…Besides the primary geologic controls such as deposition or faults, also compaction and diagenesis, and hence episodes of basin subsidence and burial, have a strong effect on reservoir productivity (Drivet & Mountjoy, 1997;Mountjoy et al, 2001;Mraz et al, 2019). This is especially relevant in foreland basins, where one play level is located at increasing depth towards the orogenic belt (Sinclair & Allen, 1992;Mraz et al, 2018). We consider these depth-related different porosity and permeability domains as play elements (Table 1).…”

Multiphase, decoupled faulting in the southern German Molasse Basin – evidence from 3-D seismic data

“…Formation tops in depth were taken from the GEN-1 well. Qualitative mechanical stratigraphy (Fischer, 1960;Müller, 1970;Budach et al, 2017) and the location of inferred detachments (Bachmann et al, 1982;Müller et al, 1988;Ortner et al, 2015;von Hartmann et al, 2016) are also indicated. Mapped seismic horizons, and the stratal units they bound, are depicted in the right column.…”

“…The latter is difficult to assess for rocks at the time they were deformed (Ferrill et al, 2017). The stratigraphic units are specified as either competent or incompetent, based on the published interpretation of their mechanical behaviour from outcrop and well data (Fischer, 1960;Müller, 1970;Budach et al, 2017) and on the mechanical properties from literature, e.g. von Hartmann et al (2016).…”