Abstract
The Muschelkalk, composed of Triassic limestones, marls, dolomites, and evaporites, forms part of the Permo-Triassic cover of sedimentary rocks that directly overlies the fractured granitic reservoir used for geothermal energy exploitation in the Upper Rhine Graben. Petrophysical data for this lithostratigraphic unit are sparse, but are of value for reservoir prospection, stimulation, and optimisation strategies at existing and prospective geothermal sites throughout the Upper Rhine Graben. To this end, we present here a systematic microstructural, mineralogical, and petrophysical characterisation 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 fine-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 fifteen depths, including the six depths chosen for microstructural and mineral content analysis) were then characterised 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 the resolution of our permeameter (≪ 10−18 m2). P-wave velocity, thermal conductivity, thermal diffusivity, specific 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.
Highlights
IntroductionGeothermal energy projects within the Upper Rhine Graben, a 350-km-long and 50-kmwide Cenozoic rift valley, exploit anonymously high geothermal gradients (> 80 °C/km) that are attributed to crustal thinning and efficient 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)
Petrophysical properties Our data show that the P-wave velocity (Fig. 6), thermal conductivity (Fig. 7a), thermal diffusivity (Fig. 7b), specific heat capacity (Fig. 7c), and Young’s modulus (Fig. 8c) of the measured Muschelkalk samples decrease as a function of increasing porosity
We note that our values of uniaxial compressive strength and Young’s modulus of the Muschelkalk unit compare well with similar measurements provided by Reyer et al (2012)
Summary
Geothermal energy projects within the Upper Rhine Graben, a 350-km-long and 50-kmwide Cenozoic rift valley, exploit anonymously high geothermal gradients (> 80 °C/km) that are attributed to crustal thinning and efficient 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). The Buntsandstein unit, a ~ 400-m-thick sequence of sandstones (Fig. 1c) (e.g., Aichholzer 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). The permeability required to support large-scale hydrothermal convection in the Buntsandstein
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