Fracture networks in the Muschelkalk aquifer of the external northen foreland of the Central Alps (CH/DE); implications for permeability, CO2 storage and geothermal potential

Switzerland aims to reach net-zero CO₂ emissions by 2050 (BAFU, 2021). Implementation of geological storage of CO₂ and geothermal heat mining to help achieve this aim requires reservoir properties to be assessed. The structural and permeability architectures of the target reservoirs are essential input for numerical models used to assess storage and production potential, minimize fluid injection and extraction uncertainties, and reduce exploration risks. The so-called Muschelkalk aquifer (Triassic) including the Schinznach Formation is regarded as one of the key potential aquifers for gas storage and hydrothermal geothermal systems in Switzerland (Chevalier et al., 2010). While the matrix permeability of the Schinznach Formation is relatively well known (Diamond et al., 2019), magnitude and distribution of its fracture permeability and structural controls on these fractures are poorly understood.

This study aims to assess the style and intensity of natural fracture networks in the Muschelkalk aquifer at sub-seismic scale and explain their regional variability. Outcrop analogues in the Wutach Gorge of southern Germany are used to improve understanding of lateral and vertical variability of fracture networks, including how they are influenced by regional structures. The Wutach Gorge is within the Tabular Jura and provides cliff exposures along 5–12 km-long E–W and N–S transects, aligning with and crossing fault strands of the Freiburg–Bonndorf–Bodensee Fault zone.

Insights from field observations contribute to ongoing work that supports the proposed pilot CO₂ injection test into the Schinznach Formation via an existing exploration borehole at Trüllikon in northern canton Zurich. A feasibility study (Diamond et al., 2023) assessed the reservoir properties at Trüllikon by building discrete fracture network models and computing their permeabilities from a combination of rock-matrix properties, vertical drill hole fracture logs, a horizontal fracture log from another nearby drill hole, and results from hydraulic tests. This multidisciplinary approach should provide a more robust basis for exploration for CO₂ storage sites and geothermal energy.

 

REFERENCES 

Chevalier, G., Diamond, L. W., & Leu, W. (2010). Potential for deep geological sequestration of CO2 in Switzerland: a first appraisal. Swiss Journal of Geosciences, 103, 427-455.

Diamond, L. W., Alt-Epping, P., Brett, A.C., Aschwanden, L. and Wanner, C. (2023) Geochemical–hydrogeological study of a proposed CO2 injection pilot at Trüllikon, Switzerland. Report 2023-7 submitted to the Swiss Geological Survey (swisstopo). Rock Water Interaction, University of Bern, 87 pp. https://doi.org/10.5281/zenodo.10938102

Diamond L.W., Aschwanden, L., Adams, A., and Egli, D. (2019) Revised potential of the Upper Muschelkalk Formation (Central Swiss Plateau) for CO2 storage and geothermal electricity. Slides of an oral presentation at the SCCER-SoE Annual Conference at EPFL-Lausanne, 4th Sept. 2019. 13 pp. http://static.seismo.ethz.ch/sccer-soe/Annual_Conference_2019/AC19_S3a_08_Diamond.pdf

BAFU (2021) Switzerland Long-Term Climate Strategy. 4 pp. https://www.bafu.admin.ch/dam/bafu/en/dokumente/klima/fachinfo-daten/langfristige-klimastrategie-der-schweiz.pdf.download.pdf/Switzerland's%20Long-Term%20Climate%20Strategy.pdf