Comment on se-2022-18

Thomas Voigt

doi:10.5194/se-2022-18-rc1

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.

Highlights

1.1 Palaeogeographic frameworkThe Franconian Alb east, south and north of the city of Nuremberg (Figure 1) is well known for its impressive 35 exposures of Jurassic carbonates and reef structures in an area extending for ~120 km east-west and ~160 km north-south
From various locations distributed over the Franconian Alb a large number of mudstone density and porosity measurements were performed and complemented by vitrinite reflectance and both new and published in-situ p-wave velocity data from seismic surveys and and downhole logging
These datasets were subsequently related to a compaction-depth-trend that was calibrated on 530 mudstones of the same stratigraphic unit in the North Alpine Foreland Basin (NAFB) to the south of our study area

Summary

Introduction

1.1 Palaeogeographic frameworkThe Franconian Alb east, south and north of the city of Nuremberg (Figure 1) is well known for its impressive 35 exposures of Jurassic carbonates and reef structures in an area extending for ~120 km east-west and ~160 km north-south. Following dominantly terrestrial deposition during the Upper Triassic Keuper, marine environments returned during the Early Jurassic (Liassic), when the South German Basin was flooded by the 40 Tethys Ocean, depositing mostly clays and clayey marls (Figure 2) (Piénkowski et al, 2008). Uplift of the Bohemian Massif likely amounted up to 1-1.5 km (Peterek et al, 1996; Peterek and Schröder, 2010; Reicherter et al, 2008; Schröder, 1987; Wagner et al, 1997), probably related to far-field compression (Scheck-Wenderoth et al, 2008) and a wrench-dominated tectonic regime at the 50 southern end of the North Sea rift system (Pharaoh et al, 2010). A likely second major uplift phase was induced by the Alpine continental collision between the latest Late Cretaceous and Palaeocene (Peterek et al, 1997; Reicherter et al, 2008; Schröder, 1987; Wagner et al, 1997; Ziegler, 1987). Subsequent and tilting-related differential erosion in turn resulted in the characteristic scarpland morphology (Meschede, 2018; Schröder, 1968; Walter, 2007), leaving only local erosional remnants and residual weathering products (e.g. Kallmünz boulders, Alblehm) witnessing former Cretaceous overburden (Glaser et al, 2001; Schirmer, 2015)

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Conclusion