Triassic Geothermal Reservoirs of the North German Basin Matthias Franz1, Markus Wolfgramm2, Gregor Barth1, Kerstin Rauppach2, Jens Zimmermann1 1 TU Bergakademie Freiberg, Institut für Geologie, Bernhard-von-Cotta-Straße 2, D-09599 Freiberg, [email protected] 2 Geothermie Neubrandenburg GmbH, Seestraße 7a, D-17033 Neubrandenburg, [email protected] The Mesozoic of the North German Basin do not comprise any potential as geothermal (NGB) comprises numerous sandstone reservoirs in the NGB. horizons of considerable potential for The Muschelkalk/Keuper facies shift favoured geothermal use. Based on boundary conditions the southwards progradation of the mainly for commercial exploitation of geothermal continental Keuper that is dominated by shaly reservoirs (>20 m thickness, >20 % effective lithologies without any reservoir potential. porosity, >500 mD permeability) four main Fluvial sandstones of the Lower Keuper Erfurt geothermal reservoir complexes have been Formation and the Middle Keuper Stuttgart recognised: (i) Middle Buntsandstein, (ii) Formation occur intercalated and may be Rhaeto-Liassic, (iii) Middle Jurassic and (iv) locally of interest for geothermal use. Lower Cretaceous (Wolfgramm et al. 2004). Especially the Stuttgart Formation, if Among those the Rhaeto-Liassic comprises the developed in sandy facies, can achieve most promising geothermal reservoirs of the valuable reservoir properties. NGB since geothermal heating plants are operating with Rhaetian sandstones at Waren, The Rhaetian transgression triggered the Neubrandenburg und Neustadt-Glewe. progradation of large fluvial systems from sources to the North and South. The Upper The Lower and Middle Buntsandstein Keuper Exter Formation comprises deposits of palaeogeography was controlled by a large a larger terminal fluvial fan (Lower Exter Fm) lake that occupied larger parts of the Central and large fluvial delta (Middle to Upper Exter European Basin (CEB). In the Lower Fm). In both, thick sandstones are limited to Buntsandstein the lake and its marginal facies rather narrow distributaries that are laterally stretched from the North Sea area up to Central associated with overbank environments (Franz Poland and thus, prevented the progradation of & Wolfgramm 2008). Sandstones of fluvial fan fluvial sandstones into the NGB. Therefore the, distributaries have porosities between 9 – 37 % in parts, more than 400 m thick Lower and permeabilities between 29 – 3000 mD Buntsandstein is dominated by shaly (average: ca. 750 mD). Sandstones of deltaic lithologies that do not bear any potential as a distributaries have porosities of 8 – 35 % and geothermal reservoir. In the Middle permeabilities of 20 – 5814 mD (average: 2000 Buntsandstein the lake generally diminished in mD). In general, sandstones of fluvial and size and long-term fluctuations of its level deltaic distributaries comprise high geothermal resulted in the characteristic potentials, whereas the potentials of the progradational-retrogradational pattern of the respective overbank environments are rather Middle Buntsandstein formations with low. prominent sandstones at their bases that are fining-up to alternations of siltstones and claystones. The basal sandstones of the References Volpriehausen, Detfurth and Hardegsen Wolfgramm, M., Seibt, P. & Lenz, G. (2004): Neue formations are up to 50 m thick and can locally Aspekte der Speicherbewertung für die attain porosities of 10 to 30% and moderate to geothermische Stromerzeugung. – GTV-Tagung good permeabilities and therefore comprise Landau: 120-130. Franz, M. & Wolfgramm, M. (2008): some potential as geothermal reservoir. Sedimentologie, Petrologie und Fazies The predominantly shaly lithologies of the geothermischer Aquifere des Norddeutschen Upper Buntsandstein and the carbonatic to Beckens am Beispiel der Exter-Formation marly and shaly lithologies of the Muschelkalk (Oberer Keuper, Rhaetium) NE-Deutschlands. – Z. Geol. Wiss., 36 (6): 223-248..