Heap et al. Geotherm Energy (2019) 7:27 https://doi.org/10.1186/s40517‑019‑0145‑4 RESEARCH Open Access Petrophysical properties of the Muschelkalk from the Soultz‑sous‑Forêts geothermal site (France), an important lithostratigraphic unit for geothermal exploitation in the Upper Rhine Graben Michael J. Heap1* , Alexandra R. L. Kushnir1, H. Albert Gilg2, Marie E. S. Violay3, Pauline Harlé4 and Patrick Baud1 *Correspondence: [email protected] Abstract 1 Géophysique The Muschelkalk, composed of Triassic limestones, marls, dolomites, and evaporites, Expérimentale, Institut de Physique de Globe de forms part of the Permo-Triassic cover of sedimentary rocks that directly overlies the Strasbourg (UMR 7516 CNRS, fractured granitic reservoir used for geothermal energy exploitation in the Upper Université de Strasbourg/ Rhine Graben. Petrophysical data for this lithostratigraphic unit are sparse, but are of EOST), 5 Rue René Descartes, 67084 Strasbourg Cedex, value for reservoir prospection, stimulation, and optimisation strategies at existing and France prospective geothermal sites throughout the Upper Rhine Graben. To this end, we Full list of author information present here a systematic microstructural, mineralogical, and petrophysical characteri- is available at the end of the article sation of the Muschelkalk core (from the Middle to Lower Muschelkalk; from a depth of ~ 930 to ~ 1001 m) from exploration borehole EPS-1 at Soultz-sous-Forêts (France). First, we assessed the microstructure and mineral content of samples from six depths that we consider represent the variability of the available core. The majority of the core is composed of fne-grained, interbedded dolomites and marls; however, anhydrite and a dolomitic sandstone bank were found in the Upper and Lower Muschelkalk core, respectively. A larger suite of samples (from ffteen depths, including the six depths chosen for microstructural and mineral content analysis) were then character- ised in terms of their petrophysical properties. The matrix porosity of the measured Muschelkalk samples is low, from ~ 0.01 to ~ 0.1, and their matrix permeability is below 18 2 the resolution of our permeameter (≪ 10− m ). P-wave velocity, thermal conductiv- ity, thermal difusivity, specifc heat capacity per unit volume, Young’s modulus, and uniaxial compressive strength range from 2.60 to 5.37 km/s, 2.42 to 5.72 W/mK, 1.19 to 2.46 mm2/s, 1.63 to 2.46 MJ/m3 K, 9.4 to 39.5 GPa, and 55.1 to 257.6 MPa, respectively. Therefore, and despite the narrow range of porosity, the petrophysical properties of the Muschelkalk are highly variable. We compare these new data with those recently acquired for the Buntsandstein unit (the Permo-Triassic unit immediately below the Muschelkalk) and thus provide an overview of the petrophysical properties of the two sedimentary units that directly overly the fractured granitic reservoir. Keywords: Muschelkalk, Porosity, P-wave velocity, Thermal properties, Uniaxial compressive strength, Microstructure, Geothermal, Upper Rhine Graben © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Heap et al. Geotherm Energy (2019) 7:27 Page 2 of 29 Introduction Geothermal energy projects within the Upper Rhine Graben, a 350-km-long and 50-km- wide Cenozoic rift valley, exploit anonymously high geothermal gradients (> 80 °C/km) that are attributed to crustal thinning and efcient large-scale hydrothermal convection (e.g., Ledésert et al. 1996; Pribnow and Schellschmidt 2000; Buchmann and Connolly 2007; Guillou-Frottier et al. 2013; Baillieux et al. 2013; Magnenet et al. 2014; Vallier et al. 2018, 2019). Indeed, more than ffteen geothermal wells have been drilled in the Upper Rhine Graben since the 1980s (Vidal and Genter 2018). Te geology of the region con- sists of a fractured granitic basement (e.g., Ledésert et al. 1993; Genter and Traineau 1996; Genter et al. 1997; Hooijkaas et al. 2006; Sausse et al. 2006; Dezayes et al. 2010; Genter et al. 2010; Villeneuve et al. 2018; Glaas et al. 2018) overlain by a sequence of Permian and Triassic sedimentary rocks (Buntsandstein, Muschelkalk, and Keuper; e.g., Hafen et al. 2013; Vidal et al. 2015; Grifths et al. 2016; Aichholzer et al. 2016, 2019; Heap et al. 2017; Kushnir et al. 2018a, b; Heap et al. 2018, 2019; Duringer et al. 2019; Harlé et al. 2019), Jurassic sedimentary rocks, and Tertiary to Quaternary graben fll (e.g., Berger et al. 2005; Hinsken et al. 2007, 2011; Aichholzer et al. 2016; Duringer et al. 2019) (Fig. 1c). Te Buntsandstein unit, a ~ 400-m-thick sequence of sandstones (Fig. 1c) (e.g., Aich- holzer et al. 2016; Heap et al. 2017), and the Muschelkalk unit, a ~ 100-m-thick sequence of Triassic limestones, marls, dolomites, and evaporites (Fig. 1c) (e.g., Aichholzer et al. 2016; Duringer et al. 2019), are considered to form the top of the regional convection zone (e.g., Vidal et al. 2015; Baujard et al. 2017). Both units are known to be laterally extensive in the Upper Rhine Graben (e.g., Sittler 1969; Brun and Wenzel 1991). Te per- meability required to support large-scale hydrothermal convection in the Buntsandstein Quaternary to Pliocene b c Oligo- Wissembourg cene Soultz-sous-Forêts Rittershoffen EPS-1 a GPK-1-4 Eocene Paris Haguenau Strasbourg Saverne Jurassic Nantes Keuper STRASBOURG Musch- elkalk Lyon Molsheim N Bordeaux Marseille Obernai Natzwiller Buntsandstein 100 km N Kintzheim granite town m wells 20 km 200 GPK-2 Fig. 1 a Map of France showing the location of Strasbourg. b Map of the Bas-Rhin department showing the location of several towns and cities, including Soultz-sous-Forêts and Rittershofen, and EPS-1 exploration well and the GPK-1-4 wells at Soultz-sous-Forêts (blue circle). c Stratigraphic column showing the stratigraphy at the Soultz-sous-Forêts geothermal site (for GPK-2; Aichholzer et al. 2016). The reservoir granite was encountered down to the fnal drilling depth of 5080 m (not shown here) Heap et al. Geotherm Energy (2019) 7:27 Page 3 of 29 and Muschelkalk units is provided by faults and fractures (e.g., Vidal et al. 2015; Kushnir et al. 2018a). In the Muschelkalk unit, the focus of the present study, there is ample evi- dence for permeable fractured zones (e.g., Reyer et al. 2012; Meier et al. 2015; Vidal et al. 2015; Aichholzer et al. 2016). For example, Reyer et al. (2012) and Meier et al. (2015) analysed fault zones within the Muschelkalk unit with displacements ranging from a few centimetres to a few tens of metres in northwest and southeast Germany, respectively. Many of the faults described in these studies appear related to the regional stress feld associated with the Upper Rhine Graben, in which intra-graben faults strike dominantly NNW (with subsidiary NNE and NW sets) and faults on the border of the graben strike between NW and NE (Peters and van Balen 2007; see also Schumacher 2002; Cardozo and Behrmann 2006; Meixner et al. 2016). Te faults studied by Reyer et al. (2012) and Meier et al. (2015) are also associated with damage zones containing fractures preferen- tially orientated parallel to the fault strike. Within France, the correlation of stratigraphic logs from Soultz-sous-Forêts and Rittershofen (locations given in Fig. 1b) suggests the presence of faults within the Muschelkalk (Aichholzer et al. 2016). Te two permeable zones in the Muschelkalk identifed in the GPK-2 and GPK-3 wells at Soultz-sous-Forêts have apparent thicknesses between 5 and 20 m; the orientation of these faults, however, is challenging to interpret in the absence of imaging logging (Vidal et al. 2015). Te Bunt- sandstein and Muschelkalk units are not only important for regional hydrothermal con- vection, but recent and forthcoming geothermal projects have also targeted the interface between the fractured granitic basement and these overlying Permo-Triassic sedimen- tary rocks. Te Muschelkalk unit, for example, is also used as a hot water aquifer at the geothermal plant at Riehen in Switzerland (in the Upper Rhine Graben; e.g., Link et al. 2015). As a result, the petrophysical properties of the rocks forming the Buntsandstein and Muschelkalk units form essential input parameters in thermo-hydro-mechanical modelling designed to better understand large-scale fuid circulation, heat fow calcula- tions, and temperature estimations and can be used to guide reservoir stimulation strat- egies and assessments of borehole stability. Recent experimental studies have explored the microstructural and petrophysical properties of the Permo-Triassic Buntsandstein lithostratigraphic unit (Fig. 1c). Heap et al. (2017), for example, measured the porosity, permeability, and P-wave velocity of sandstones from exploration borehole EPS-1 at the Soultz-sous-Forêts geothermal site (France) (Fig. 1a, b). Tese authors found that the porosity, P-wave velocity, and perme- ability of these sandstones vary from ~ 0.03 to 0.2, ~ 2.5 to 4.5 km/s, and ~ 10−18 to 10−13 m2, respectively. Te low porosity, high P-wave velocity, and the low permeability of the sandstones comprising the top (Grès à Voltzia and Couches Intermédiaires formations) and bottom (Grès d’Annweiler and Grès anté-Annweiler formations) were considered by these authors to be a result of pore-flling alteration (Heap et al. 2017). Te uniaxial com- pressive strength and Young’s modulus of Buntsandstein sandstones from EPS-1 were found to range from ~ 50 MPa and ~ 10 GPa, respectively, for a porosity of ~ 0.25 and up to ~ 250 MPa and ~ 40 GPa, respectively, for a porosity of ~ 0.04 (Heap et al.