Michael Drews, Indira Shatyrbayeva, Florian Duschl
Technische Universität München, Deutschland

Pore pressure and stress are key properties to understand geological processes in the subsurface, its mechanical integrity, fluid flow and advective/convective thermal transport. Pore pressure and stress are therefore of great relevance to successfully and safely extract geothermal energy from the deeper subsurface:

- Exploration risk critically depends on the knowledge of subsurface temperatures and hydraulic connectivity

- Deep geothermal drilling requires a model of the mechanical integrity of the subsurface to prevent well control situations such as kicks, losses, cavings, etc.

- Geothermal production risk related to fault reactivation (microseismicity) is also critically dependent on the stress field and pore pressure

The Bavarian Molasse Basin is part of the North Alpine Foreland Basin and can be divided into an undeformed foreland part (Foreland Molasse) and folded part (Subalpine Molasse). The Subalpine Molasse is part of the North Alpine Thrust Front. The Foreland Molasse in Bavaria is subject to high activity in deep geothermal (hydrothermal) exploration and production. Hot water is produced from a carbonate reservoir of Mesozoic (Upper Jurassic) age and requires drilling through up to 5 km thick Cenozoic basin infill sediments. The highest geothermal production activity is in the greater Munich area and recent exploration is moving towards the southern and southeastern part of the Foreland Molasse.

Pore pressure of the Bavarian Molasse Basin has been studied on a regional scale [1, 2] and few local case studies [3], but is generally found to be at hydrostatic levels in the reservoir [4] and in general in the area west-northwest of Munich and with significant overpressure magnitudes in the south and east of Munich. Although there have been multiple studies addressing the stress field in the reservoir so far [e.g. 5, 6], in particular in the greater Munich area, only few studies address the stress regime in the Cenozoic basin fill [7-9], but are either restricted to the greater Munich area [7] and/or associated with significant uncertainty due to a lack of high quality data [7, 9]. The stress regime is therefore still under debate and it is unclear to which extent the stress field and pore pressure distribution of the Foreland Molasse are influenced by the North Alpine Thrust Front.

In this study, we investigate the compaction, pore pressure and vertical effective stress profiles of several wells, which have been drilled just north (Foreland Molasse) or south (Subalpine Molasse) of the North Alpine Thrust Front in Bavaria. We will present new insights and qualitative models regarding the influence of the North Alpine Thrust Front on the stress and pressure distribution in the most southern part of the Bavarian Molasse Basin and its possible impact on the far field stress regime as well as the thermal and hydraulic fields in the undeformed Foreland Molasse.

References

1. Drews, M.C., et al., Marine and Petroleum Geology, 2018. 92: p. 37-50.

2. Müller, M., F. Nieberding, and A. Wanninger, Geol. Rundsch., 1988. 77(3): p. 787-796.

3. Drews, M.C., et al., Geoth. Energy, in press.

4. Lemcke, K., Bulletin der Vereinigung Schweiz. Petroleum-Geologen und -Ingenieure, 1976. 42(103): p. 9-18.

5. Seithel, R., et al., Geothermics, 2019. 82: p. 81-90.

6. Seithel, R., et al., Geoth. Energy, 2015. 3.

7. Drews, M.C., et al., Tectonophysics, 2019. 755: p. 1-9.

8. Reinecker, J., et al., Tectonophysics, 2010. 482(1-4): p. 129-138.

9. Ziegler, M.O., et al., Solid Earth, 2016. 7(5): p. 1365-1382.