Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)

Geothermal projects in the Upper Rhine Graben aim to harness thermal anomalies that have arisen due to hydrothermal circulation within the granitic basement and the overlying Permo-Triassic sedimentary units. We present here a systematic microstructural, mineralogical, and petrophysical characterisation of the lowermost unit of this Permo-Triassic sedimentary succession—the Buntsandstein—sampled from exploration well EPS-1 at the Soultz-sous-Forêts geothermal site (France). Twelve depths were sampled (from 1008 to 1414 m) and cylindrical cores were prepared perpendicular and parallel to bedding. These cores were described in terms of their microstructure, grain size and shape, specific surface area, pore size and pore throat size distribution, mineral content, porosity, P-wave velocity, and permeability. The Buntsandstein sandstones are predominantly feldspathic sandstones, often characterised by pores filled or partially filled (with clays (R3 illite–smectite), dolomite, siderite, and barite) as a consequence of diagenesis, tectonics, and the circulation of hydrothermal fluids. The porosity, dry P-wave velocity, and permeability of these sandstones vary from ~ 0.03 to 0.2, ~ 2.5 to 4.5 km s−1, and ~ 10−18 to 10−13 m, respectively. Our data show that P-wave velocity decreases and permeability increases as porosity increases. P-wave velocities are significantly higher when measured parallel to bedding (by about 10 to 25%), and that saturation with water increases P-wave velocity (by about 5 to 50%, depending on sample orientation). The pervasive pore-filling precipitation has significantly reduced the permeabilities of the Buntsandstein sandstones, which are orders of magnitude less permeable than similarly porous unaltered sandstone. We also find that their permeability can be up to an order of magnitude more permeable when measured parallel to bedding than perpendicular to bedding. Although Buntsandstein units with low matrix permeabilities (as low as ~ 10−18 m) require macroscopic fractures to attain the high permeability required to sustain regional hydrothermal circulation, matrix permeability is important for units with low fracture densities and high matrix permeabilities. We anticipate that these data will aid future fluid flow modelling and seismic investigations at geothermal sites within the Upper Rhine Graben. Open Access © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/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. RESEARCH Heap et al. Geotherm Energy (2017) 5:26 DOI 10.1186/s40517‐017‐0085‐9 *Correspondence: [email protected] 1 Géophysique Expérimentale, Institut de Physique de Globe de Strasbourg (UMR 7516, CNRS, Université de Strasbourg/ EOST), 5 rue René Descartes, 67084 Strasbourg Cedex, France Full list of author information is available at the end of the article Page 2 of 37 Heap et al. Geotherm Energy (2017) 5:26 Background Thermal anomalies (~ 100 °C/km) in the Upper Rhine Graben—a rift valley that straddles the border between eastern France and western Germany—are attributed to hydrothermal circulation within the fractured Palaeozoic granitic basement and the overlying Permian and Triassic sedimentary units (e.g. Pribnow and Schellschmidt 2000; Buchmann and Connolly 2007; Guillou-Frottier et al. 2013; Baillieux et al. 2013; Magnenet et al. 2014). The granitic basement, and the interface between the Permo-Triassic sedimentary units and the granite, has consequently received considerable attention for geothermal energy exploitation. Notable examples include the enhanced geothermal system (EGS) sites at Soultz-sous-Forêts (France) (e.g. Gérard and Kappelmeyer 1987; Kappelmeyer et al. 1991; Baria et al. 1999; Gérard et al. 2006), Rittershoffen (France) (e.g. Baujard et al. 2017), Brühl (Germany), Landau (Germany), Insheim (Germany), Bruchsal (Germany), and Riehen (Switzerland). Two sites are currently in development close to Strasbourg, in Illkirch and Vendenheim (both in France). Despite the importance of the Permo-Triassic sedimentary units (namely the Buntsandstein, the Muschelkalk, and the Keuper; Aichholzer et al. 2016) for regional hydrothermal convection (e.g. Ledésert et al. 1996; Aquilina et al. 1997) and as a heat-exchanger, the majority of studies aimed at assessing or quantifying the permeability at the geothermal sites at Soultz-sous-Forêts and Rittershoffen, for example, have focussed on the granite basement (e.g. Genter and Traineau 1996; Shapiro et al. 1999; Sausse et al. 2006; Dezayes et al. 2010; Ledésert et al. 2010; Vogt et al. 2012a, b; Vidal et al. 2017). Few studies, especially laboratory studies that offer values of porosity and permeability, have targeted the overlying Permo-Triassic sedimentary units (e.g. Haffen et al. 2013; Stober and Bucher 2015; Vidal et al. 2015; Griffiths et al. 2016). For example, Haffen et al. (2013) estimated the permeability of the Buntsandstein sandstones of EPS-1 using a portable TinyPerm II permeameter and found that their permeability ranges from 10−15 to 10−13 m2. However, the laboratory measurements of Griffiths et al. (2016) highlight that the permeability of Buntsandstein sandstones from EPS-1 can be lower than 10−18 m2. We present here a systematic microstructural (grain size and shape, specific surface area, pore size, and pore throat size distribution), mineralogical, and petrophysical (porosity, P-wave velocity, permeability) characterisation of the Buntsandstein sandstones, sampled from exploration well EPS-1 at the Soultz-sous-Forêts geothermal site (Fig. 1). EPS-1 was drilled vertically to a depth of 2227 m between 1990 and 1991 and continuous core was retrieved between the measured depths of 930 and 2227 m (all depths reported in this study are measured depths). The cored interval sampled the Triassic Muschelkalk unit (930–1000 m depth) and the Permo-Triassic Buntsandstein unit (1000–1417 m; Fig. 1b) directly overlying the granite basement (encountered at a depth of 1417 m). The Buntsandstein unit—the focus of this study—extends over large parts of west and central Europe (McCann 2008), and the formations specific to those found at Soultz-sous-Forêts can be readily identified in the wells drilled at Rittershoffen (Aichholzer et al. 2016), located ~ 6.5 km from the Soultz-sous-Forêts site (Fig. 1c). Therefore, although our study uses core material from Soultz-sous-Forêts, the data presented herein are likely relevant for current and future geothermal projects within the Upper Page 3 of 37 Heap et al. Geotherm Energy (2017) 5:26 Rhine Graben. The continued development of geothermal energy is important to mitigate anthropogenic carbon emissions and therefore climate change (e.g. Fuss et al. 2014; Smith et al. 2016). 100 km Paris