Preprints
https://doi.org/10.5194/se-2022-18
https://doi.org/10.5194/se-2022-18
 
15 Feb 2022
15 Feb 2022
Status: a revised version of this preprint is currently under review for the journal SE.

Reconstructing post-Jurassic overburden in Central Europe: New insights from mudstone compaction and thermal history analyses of the Franconian Alb, SE Germany

Simon Freitag1, Michael Drews2, Wolfgang Bauer1, Florian Duschl2, David Misch3, and Harald Stollhofen1 Simon Freitag et al.
  • 1GeoZentrum Nordbayern, Friedrich-Alexander University (FAU) Erlangen-Nürnberg, Schlossgarten 5, 91054 Erlangen, Germany
  • 2Geothermal Technologies, Technical University of Munich (TUM), Arcisstraße 21, 80333 Munich, Germany
  • 3Department für Angewandte Geowissenschaften und Geophysik, Montanuniversität Leoben, Peter-Tunner-Straße 5, 8700 Leoben, Austria

Abstract. The Franconian Alb of SE Germany is characterized by large-scale exposures of Jurassic shallow marine limestones and dolostones which are frequently considered as outcrop analogues for deep geothermal reservoir rocks in the North Alpine Foreland Basin farther south. However, the burial history of the Franconian Alb Jurassic strata is not well known as they were affected by emersion, leading to extensive erosion and karstification with only remnants of the original Cretaceous and Cenozoic cover rocks preserved. To estimate the original thicknesses of the post-Jurassic overburden we investigated the petrophysical properties and the thermal history of Lower and Middle Jurassic mudstones to constrain their burial history in the Franconian Alb area. We measured mudstone porosities, densities, and maturities of organic material and collected interval velocities from seismic refraction and logging data in shallow mudstone-rich strata. Mudstone porosities and P-wave velocities vertical to bedding were then related to a normal compaction trend that was calibrated on stratigraphic equivalent units in the North Alpine Foreland Basin. Our results suggest maximum burial depths of 900–1700 m of which 300–1100 m are attributed to Cretaceous and younger sedimentary rocks overlying the Franconian Alb Jurassic units. Compared to previous considerations this implies a more widespread distribution and increased thicknesses of up to ~900 m for Cretaceous and up to ~200 m for Cenozoic units in SE Germany. Maximum overburden is critical to understand mechanical and diagentical compaction of the dolostones and limestones of the Upper Jurassic of the Franconian Alb. The results of this study therefore help to better correlate the deep geothermal reservoir properties of the Upper Jurassic from outcrop to reservoir conditions below the North Alpine Foreland Basin. Here, the Upper Jurassic geothermal reservoir can be found at depths of up to 5000 m.

Simon Freitag et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on se-2022-18', Thomas Voigt, 07 Mar 2022
    • AC1: 'Reply on RC1', Simon Freitag, 12 May 2022
  • RC2: 'Comment on se-2022-18', Hilmar von Eynatten, 06 Apr 2022
    • AC2: 'Reply on RC2', Simon Freitag, 12 May 2022

Simon Freitag et al.

Simon Freitag et al.

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Short summary
The carbonates of the Malm are the main reservoir rocks for hydrothermal heat and power generation in Southern Germany. To better understand these buried rocks, these carbonates, exposed in northern Bavaria, are often investigated. As the petrophysical properties of carbonates strongly depend on their subsidence history and maximum burial depth, we will investigate this issue by analyzing mudstones, which indirectly store this type of information and are found just below the Malm carbonates.