The interplay of Malm carbonate permeability, gravity-driven groundwater flow, and paleoclimate\xa0\u2013 implications for the geothermal field and potential in the Molasse Basin (southern Germany), a foreland-basin play

Tom Vincent Schintgen,Inga Sigrun Moeck

doi:10.1186/s40517-021-00207-x

Abstract

The Molasse Basin in Southern Germany is part of the North Alpine Foreland Basin and hosts the largest accumulation of deep geothermal production fields in Central Europe. Despite the vast development of geothermal energy utilization projects especially in the Munich metropolitan region, the evolution of and control factors on the natural geothermal field, more specifically the time-varying recharge and discharge governing groundwater and heat flow, are still debated. Within the Upper Jurassic (Malm) carbonate aquifer as the main geothermal reservoir in the Molasse Basin, temperature anomalies such as the Wasserburg Trough anomaly to the east of Munich and their underlying fluid and heat transport processes are yet poorly understood. We delineate the two end members of thermal–hydraulic regimes in the Molasse Basin by calculating two contrasting permeability scenarios of the heterogeneously karstified Malm carbonate aquifer along a model section through the Wasserburg Trough anomaly by means of two-dimensional numerical thermal-hydraulic modelling. We test the sensitivity of the thermal-hydraulic regime with regard to paleoclimate by computing the two Malm permeability scenarios both with a constant surface temperature of 9 °C and with the impact of paleo-temperature changes during the last 130 ka including the Würm Glaciation. Accordingly, we consider the hydraulic and thermal effects of periglacial conditions like permafrost formation and the impact of the numerous glacial advances onto the Molasse Basin. Thermal-hydraulic modelling reveals the effect of recurrent glacial periods on the subsurface targets of geothermal interest, which is minor compared to the effect of permeability-related, continuous gravity-driven groundwater flow as a major heat transport mechanism.

Highlights

We use the COMSOL Multiphysics® software to perform two-dimensional numerical thermal-hydraulic modelling of the main fluid and heat transport processes in the Upper Jurassic (Malm) carbonate aquifer in the Southern German Molasse Basin (MB) for a better understanding of thermal anomalies induced by the interplay of high aquifer permeability (k), gravity-driven groundwater flow, and paleoclimatic conditions in foreland basins
We focus on two endmember scenarios of Malm k, thereby taking into account that the Malm carbonate aquifer presents a large range of permeabilities as shown in a map by Birner et al (2012) and Fritzer et al (2018)
No numerical thermal-hydraulic model of the subsurface taking into account the effect of paleoclimate, permafrost and glaciers exists for the MB, but subglacial infiltration of meltwater during glaciations has repeatedly been considered as the main recharge mechanism of the Malm aquifer in the deeper MB (Bertleff et al 1993; Bertleff and Watzel 2002; Heidinger et al 2019)

Summary

Introduction

We use the COMSOL Multiphysics® software to perform two-dimensional numerical thermal-hydraulic modelling of the main fluid and heat transport processes in the Upper Jurassic (Malm) carbonate aquifer in the Southern German Molasse Basin (MB) for a better understanding of thermal anomalies induced by the interplay of high aquifer permeability (k), gravity-driven groundwater flow, and paleoclimatic conditions in foreland basins. No numerical thermal-hydraulic model of the subsurface taking into account the effect of paleoclimate, permafrost and glaciers exists for the MB, but subglacial infiltration of meltwater during glaciations has repeatedly been considered as the main recharge mechanism of the Malm aquifer in the deeper MB (Bertleff et al 1993; Bertleff and Watzel 2002; Heidinger et al 2019) If this hypothesis is true, we expect that the Wasserburg Trough anomaly occurring in the MB and reaching as deep as the Malm aquifer (Agemar and Tribbensee 2018) is a direct consequence of subglacial recharge by the Inn Glacier during the last glaciations(s). In view of the existing dedicated studies of the hydraulic properties of the Malm aquifer by Birner et al (2012) as well as of the Top Malm T (Agemar and Tribbensee 2018) extracted from the 3D thermal field of the MB (Agemar et al 2012) to which we refer, a renewed borehole data analysis was not justified and beyond the scope of this work

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