European Geothermal Congress 2019 Den Haag, The Netherlands, 11-14 June 2019 Refinement of the geothermal play type concept by comparison of two foreland basins Tom V. Schintgen1, Manfred W. Wuttke1, Thorsten Agemar1, Inga S. Moeck1, 2 1 Leibniz Institute for Applied Geophysics (LIAG), Stilleweg 2 30655 Hannover, Germany 2 Georg-August-Universität, Goldschmidtstraße 3 37077 Göttingen, Germany Keywords: Late Glacial Maximum, North Alpine findings and eventually updated. The work performed Foreland Basin, Molasse Basin, Upper Jurassic, Malm, within the PlayType project will lead to the first coupled heat and fluid flow, gravity-driven national play type map in GeotIS and supplement the groundwater flow, Play Type new e-learning platform, which is (like GeotIS) run by the Leibniz Institute for Applied Geophysics. ABSTRACT Geothermal resources are traditionally classified either 1. INTRODUCTION in terms of shallow, intermediate and deep geothermal The classical approach in geothermal exploration energy based on resource depth or in terms of low-, consists in cataloguing geothermal resources according medium- and high-enthalpy resources according to to temperature or depth mainly for technical and reservoir temperature zones. The traditional definition economic reasons. The recent Play-Type concept is thereby primarily adapted to site development and (Moeck 2014) aims at classifying geothermal resources drilling costs, and not to the geothermal resource itself. according to geological criteria and heat transport Particularly in Europe where the thermally conduction- mechanisms to assess reservoir quality. Internationally dominated play types are widespread, this classical applicable criteria allow worldwide comparisons scheme is neither constructive for the assessment of between resources of similar geological settings and geothermal resources nor for the comparison of structures (play type). learning curves within a same reservoir type. Among the three main German geothermal provinces The play type concept introduced by Moeck (2014) of Upper Rhine Graben, North German Basin and offers a classification scheme of geothermal resources Molasse Basin, the latter shows the highest geothermal based on geological criteria, notably on the potential. The Molasse Basin has been studied for characterization by heat transport mechanisms and decades, for its geology, structures and stratigraphy geological parameters on reservoir type and quality. (Meyer and Schmidt-Kaler 1996, Schwerd et al. 1996), The PlayType project aims at comparing the three for oil & gas (Bachmann et al. 1982, Bachmann et al. major geothermal provinces in Germany, namely the 1987, Bachmann et al. 1992, Brink et al. 1992, North German Basin, Upper Rhine Graben and Reischenbacher and Sachsenhofer 2011, Sachsenhofer Molasse Basin with similar geological structures and et al. 2006), and more recently for geothermal energy settings worldwide. The applied methods consist of a (Birner et al. 2012, Böhm 2012, GeoMol Team 2015, combination of seismics, quantitative structural Jodocy and Stober 2009). Many boreholes tap the geology, reservoir geology, geothermics and numerical Upper Jurassic (Malm) carbonate aquifer mainly for thermal-hydraulic modelling. district heating but also for electricity production. The In particular, the Molasse Basin in southern Germany Malm carbonate aquifer is considered to be the largest as part of the North Alpine Foreland Basin is compared thermal water resource not only in Germany but also in to the Alberta Basin in western Canada, thus taking Central Europe (Goldschneider et al. 2010). Indeed, advantage of findings and conclusions in both plays. By carbonate aquifers frequently present a high secondary contrast to several existing studies, we focus on the porosity and permeability due to karstification. assessment of the main fluid and heat transport Therefore they probably constitute the most important processes for a better understanding of thermal thermal water resources outside of volcanic areas anomalies induced by gravity-driven groundwater flow (Goldschneider et al. 2010). and paleoclimatic conditions. The selection of The Molasse basin has been studied hydrogeologically, reference plays like the Molasse Basin ensures that the and recently by coupled heat and fluid flow. However, assessment of geothermal resources is based on the geostatistical assessment of the 3D thermal geological criteria. structure based on temperature measurements in Furthermore, the existing play type definitions and boreholes to date represents the most reliable prognosis criteria are critically evaluated, verified by the new of subsurface temperature in the Molasse Basin. 1 Schintgen, Wuttke, Agemar and Moeck Strikingly, a prominent cold temperature anomaly to Huber 2000). However, the massive carbonates of the the east and northeast of Munich could not yet be reef facies being most prone to karstification are satisfactorily explained. located in the central and eastern parts of the Molasse Basin, so that the magnitude of karstification in these Cross-formational gravity-flow of groundwater is long parts was greatest in the depth realm of 150 to 200 m known to act on geological timescales. Similar to below the top of the Malm (Frisch and Huber 2000). studies in the Alberta, North German and Paris basins, The duration of karstification is about 45 Ma. By we perform thermal-hydraulic modelling of the glacial contrast to karst systems in the west, the rapid burial by influence on the present-day thermal and flow regime Cretaceous sediments in the east appears to have caused in the Molasse Basin. The result is compared with the a better preservation of these karst systems which are geostatistically evaluated 3D thermal field. The presently characterized by extraordinary high calculation of the Rayleigh number for the permeabilities (Frisch and Huber 2000). heterogeneous carbonate aquifer permits to differentiate zones with a higher versus lower The pre-Tertiary sediments and thus the foreland basin geothermal potential. began to subside during the Late Eocene (Schmid et al. 2004), followed by the deposition of Lower Oligocene flysch units and of orogen-derived continental clastics during the late Oligocene to late Miocene (Roeder and 2. GEOLOGICAL SETTING Bachmann 1996). The post-flysch (Oligocene - 2.1 Molasse Basin Miocene) palaeogeographic evolution of the entire North Alpine Foreland Basin (NAFB) and the facies The Molasse Basin in southern Germany and Upper distribution in the NAFB was driven by two major Austria is part of the North Alpine Foreland Basin types of processes, which are related to the tectonic (NAFB). The Molasse basin is up 130 km wide evolution of the Alpine orogen (Kuhlemann and perpendicular to strike and 400 km long in Germany. Kempf, 2002, Schmid et al 2004): (1) a direct influence The depth of the Upper Jurassic aquifer reaches more by tectonic processes at the thrust front, and (2) an than 4000 m close to the Alpine front. indirect impact of Alpine uplift and tectonics, In southern Germany, marine transgression at the transformed by varying sediment discharge. beginning of the Jurassic led to progressive flooding of Today, the Upper Jurassic (Malm) limestones are the Variscan basement. In the Upper Jurassic (Malm), cropping out to the north of the Danube River where a carbonate platform formed under varying conditions they are called Swabian and Franconian Jura west and (Meyer and Schmidt-Kaler 1990, Birner et al. 2012, east of the Ries impact crater, respectively. To the Frisch and Huber, 2000). On the deeper shelf, in the south, they are dipping beneath the thick Tertiary southern and southwestern part of the Molasse Basin, molasse sediments (Frisch and Huber 2000). Lying at a rather dark carbonates rich in clay and organic material depth of about 2000 m below Munich, they reach a accumulated pertaining to the so-called Helvetic facies, depth of more than 4000 m at the Alpine front (Brink et being characterized by a generally very low hydraulic al. 1992). The molasse sediments are composed of conductivity. In a northerly direction and towards the cyclically deposited clayey and sandy series (from northwest, on the higher shelf, and in between reef bottom to top Lower Marine Molasse, Lower complexes, carbonate sedimentation led to the Freshwater Molasse, Upper Marine Molasse and Upper formation of the stratified facies called ‘Schichtfazies’ Freshwater Molasse) (GeoMol Team 2015). with interbedded marls. The latter is also characterized by a relatively low hydraulic conductivity. Towards the The Molasse Basin contains numerous faults which are east and northeast, mainly in the Bavarian and Austrian related to thrusting and uplift of the Alps and the part of the Molasse Basin, carbonate accumulation contemporaneous downbending of the European plate formed the reef facies on top of submarine barriers in a and formation of the Molasse Basin (Schmid et al. higher energy environment (Meyer and Schmidt-Kaler 2004). In the western and central parts, the W-E striking 1990, Frisch and Huber 2000). These massive reef faults parallel to the basin axis and to the Alps were carbonates have the highest hydraulic conductivities formed as synthetic and antithetic faults due to and are prone to karstification (Birner et al. 2012, downbending and extension of the top of the European Birner 2013). After the maximum extension of sponge crust (Bachmann et al. 1982, Brink et al. 1992). In the reefs in the Malm Delta, the shallowing sea caused the eastern