Geothermal Prospection in NE Bavaria
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Geo Zentrum 77. Jahrestagung der DGG 27.-30. März 2017 in Potsdam Nordbayern Geothermal Prospection in NE Bavaria: 1 GeoZentrum Nordbayern Crustal heat supply by sub-sediment Variscan granites in the Franconian basin? FAU Erlangen-Nürnberg *[email protected] 1 1 1 1 1 2 2 Schaarschmidt, A. *, de Wall, H. , Dietl, C. , Scharfenberg, L. , Kämmlein, M. , Gabriel, G. Leibniz Institute for Applied Geophysics, Hannover Bouguer-Anomaly Introduction with interpretation Late-Variscan granites of different petrogenesis, composition, and age constitute main parts of the Variscan crust in Northern Bavaria. In recent years granites have gained attention for geothermal prospection. Enhanced thermal gradients can be realized, when granitic terrains are covered by rocks with low thermal conductivity which can act as barriers for heat transfer (Sandiford et al. 1998). The geothermal potential of such buried granites is primarily a matter of composition and related heat production of the granitoid and the volume of the magmatic body. The depth of such granitic bodies is critical for a reasonable economic exploration. In the Franconian foreland of exposed Variscan crust in N Bavaria, distinct gravity lows are indicated in the map of Bouguer anomalies of Germany (Gabriel et al. 2010). Judging from the geological context it is likely that the Variscan granitic terrain continues into the basement of the western foreland. The basement is buried under a cover of Permo- Mesozoic sediments (Franconian Basin). Sediments and underlying Saxothuringian basement (at depth > 1342 m) have been recovered in a 1390 m deep drilling located at the western margin of the most dominant gravity low (Obernsees 1) and subsequent borehole measurements have recorded an increased geothermal gradient of 3.8 °C/100 m. The boundary between exposed and buried basement rocks is marked by the Franconian fault system. Gravity Modelling Heat production of Variscan granites 4500000 4520000 Rehau For the interpretation of the Bouguer anomalies in the Franconian Basin, gravity modelling using Granites constitute a major part of the ex- Schwarzenbach an der Saale Aš Bad Brambach the software IGMAS+ (Interactive Geophysical Modelling Assistent; Transinsight GmbH, Dresden; posed Variscan basement in eastern Bavaria. Schönwald 0 Münchberg 0 "" " "" 00 " 00 " "" Selb " "" Skalná Schmidt et al. 2011) has been performed. The model consists of 18 sections with orientation The granites can be divided into the Older " Kornberg " 60 " 60 """ "" 5 Kirchenlamitz 5 " 5 " 5 perpendicular to the NW-SE-striking fault system of the Franconian Line. The underground model Intrusive Complex (OIC) and the Younger """ Waldstein Marktleuthen Františkovy Lázně """ """" " """" " "" "" "" Intrusive Complex (YIC). Heat production of Weißenstadt"" " is based on information from drillings, seismic 2D-profiles, and previous small scale gravity models. Gefrees "" Röslau"" "" the Variscan granites is calculated from K-, U-, """" Cheb " """ Arzberg Bischofsgrün" Th-concentrations measured with a portable " "" Wunsiedel "" "" " " " " Geologic Map Ochsenk""opf " " """ " " "" Tröstau gamma ray spectrometer (RS-230 BGO 0 " " " " 0 with model area and sections """" "" Waldsassen "" " Marktredwitz "" """ "" " 00 " "" " 00 """ Waldershof Super-Spec, RadiationSolutions and Geo- 40 "" 40 " " 5 """ 5 5 Kösseine 5 Radis). Calculations using the equation of "" Rybach (1988) rank the Variscan granites as " " " moderate heat producing. """ " " """ Tirschenreuth Kemnath """" " Falkenberg "" "" The impact of a heat producing body on the """" Erbendorf """" 0 Neustadt am Kulm 0 00 Bärnau 00 geothermal gradient in the Franconian Basin Franconian Line 20 Windischeschenbach 20 5 "" 5 is modelled with the software COMSOL 5 5 " Pressath " GR Measurements Granites Fichtelgebirge " Multiphysics. The geothermal gradient is Störungen Eschenbach in dGe3rK Oberpfalz Granites Oberpfalz G2K YIC-2 Neustadt an der Waldnaab " G2∗ "" "" calculated for a heat producing, 300 Ma old Flossenbürg-Granite Grafenwö hr " " """ G4 Steinwald-Granite granite beneath the Variscan Flysch and the Friedenfels-Granite YIC G3 YIC-1 Liebenstein-Granite G2 Weiden in der Oberpfalz Falkenberg-Granite Permo-Mesosoic sediments. The thermal G1Sm 0 Leuchtenberg-Granite G1S Pleystein 0 OIC 00 Zainhammer-Granite G1HS OIC " 00 conductivity of the stratigraphic units were "" Vohenstrauß 00 Redwitzite G1R 00 5 G1 5 measured on core samples from Obernsees . 5 5 Re "" 4500000 4520000 The underground model contains four granitic bodies in the upper Saxuthuringian crust. The depth of the granites was estimated by interpretation of high-passed filtered gravity anomalies and calculations using the half width and the slope of the gravity anomaly. The boundary between Saxothuringian and Moldanubian is marked by positive gravity anomalies caused by tectonically intercalated diabases with a higher density than the surrounding basement. The Moldanubian is characterised by granitic intrusions and therefore causes generally lower gravity values in the south of the modelling area. modelled depth Geothermal modelling with COMSOL Granite below m.s.l. Bayreuth 9 - 18 km Lichtenfels 9.5 - 14 km depth to top heat temperature in geothermal Erlangen 6 - 14 km of granite production 1000 m depth gradient Haßfurt 3 - 9 km m µW/m³ °C °C/100 m 0.0 39 2.9 6400 4.0 48 3.8 8.0 58 4.8 0.0 34 2.4 Examples of IGMAS+ model sections 9400 4.0 44 3.4 8.0 53 4.3 Results four granitic bodies have been identified in the subsurface of the Franconian basin Literature: good correlation of measured and modelled gravity anomaly Gabriel, G., Vogel, D., Scheibe, R., Wonik, T., Pucher, R., Krawczyk, C. & Lindner, H. (2010). Anomalien des erdmagnetischen Totalfeldes der Bundesrepublik Deutschland 1: 1.000.000. GeoCenter Scientic Cartography, Stuttgart. radiogene heat production of granite contributes Huston, D.L., Ayling, B., Connolly, D., Lewis, B., Mernagh, T.P., Schofield, A., Skirrow, R.S. & Van der Wielen, S.E (2010): An assessment of the uranium and geothermal potential significantly to increased geothermal gradient of north Queensland. – Geoscience Australia, Record 2010/14. Rybach, L. (1988): Determination of heat production rate. – In: Haenel, R., Rybach, L. & Stegena, L. (ed.): Handbook of terrestrial heat-flow density determination: 125–141, Dordrecht (Kluwer). Sandiford, M., Hand, M. & McLaren, S. (1998): High geothermal gradient metamorphism during thermal subsidence. Earth and Planetary Science Letters, 163(1), 149-165. Acknowledgements: We are thankful for financial support by the Ministry of Environment of State Schmidt, S., Plonka, C., Götze, H.-J. & Lahmeyer, B. (2011). Hybrid modelling of gravity, gravity gradients and magnetic fields. – Geophysical Prospecting, 59, 1046-1051. Bavaria and for permission to publish the data set..