The Bavarian Molasse Basin represents a peripheral foreland basin hosting abundant hydrothermal resources in 3-5 km deep Upper Jurassic carbonate rocks. Faults and facies play a major role in targeting production wells; however the kinematic evolution of fault zones and the classification of carbonate facies of the Upper Jurassic are still debated. At the geothermal prospect Mauerstetten in the Western Bavarian Molasse Basin, a geothermal well and a side track are drilled along and about 650 m off an ENE-WSW striking normal fault. A stratigraphy related fault throw analysis of six 2D seismic sections crossing this fault evidences multiphase normal faulting from Cretaceous to Upper Miocene with a major activity phase in the Oligocene. This fault, inactive since Upper Miocene, is presumably a fossil normal fault in the present-day stress field that has a maximum horizontal stress direction in N-S. Analysis of carbonate facies by thin section petrography of drill cuttings and geophysical borehole logs lead to two major conclusions: (i) the reservoir rock represents low permeable platform limestones, reef detritus and dolostones of the Franconian facies, and (ii) the fault consists of multiple normal faulting steps with higher permeability than in intact rock. This observation suggests a fracture controlled reservoir with permeable damage zones in a tight rock mass along reactivated normal faults.

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<p>Devonian and Carboniferous carbonate rocks are present in the subsurface of the Weisweiler lignite-fired power plant near Aachen, Germany. The utilisation of these rocks for deep geothermal energy extraction is currently being explored within the scope of the transnational EU-INTERREG-funded “Roll-out of Deep Geothermal Energy in North-West Europe (DGE-ROLLOUT)” project, which aims to provide solutions to reduce carbon-dioxide emissions using a variety of geoscientific approaches.</p><p>Marine transgressive-regressive cycles during mid-Palaeozoic times enabled the formation of extensive reef complexes on the southerly continental shelf of the Laurussian palaeocontinent. Supported by favourable climatic conditions including warm, clear and shallow waters, the Givetian to Frasnian Massenkalk facies and the Dinantian Kohlenkalk Group, each several hundred meters thick, were deposited in North-West Europe.</p><p>In the Weisweiler area, these Palaeozoic carbonate rocks were covered by voluminous paralic sedimentary rocks and deformed to large-scale, generally northeast-southwest-trending, syncline-anticline structures during the Variscan Orogeny. Alpine (post-)orogenic processes further induced faulting, resulting in fault-block tectonics in the Lower Rhine Embayment area of tectonic subsidence. Significant multiphase karstification of the Palaeozoic carbonate rocks, which can be observed in nearby exposed counterparts, supports their enhanced geothermal exploitation potential.</p><p>3D-modelling of the depths and dimensions of the Weisweiler subsurface carbonate reservoirs is carried out using the commercial software Move [v2019.1.0; Petroleum Experts Ltd], and is constrained by lithostratigraphic data obtained from drilling operations, geological mapping, and interpretation of seismic profiles. The 3D-model exhibits a complex geotectonic environment, including the development of both parasitic folds and thrust faults prior to the generation of Tertiary fault-block tectonics. The depths of the tops of the reservoirs are estimated to c. 1,200 m for the Carboniferous and to c. 2,000 m for the Devonian carbonate rocks, taking into account typical thicknesses of the overlying and underlying strata. Considering possible tectonic repetition below the thrust faults, the reservoir rocks may also occur significantly deeper in the subsurface. The 3D-model is currently being transformed into a HeatFlow3D [DMT GmbH & Co. KG] / Petrel [v2017; Schlumberger N.V.] model in order to approximate the fluid circulation and pathways within the carbonate reservoirs.</p><p>Based on the current model, a target area for 2D-seismic surveys and a c. 1,000 to 1,500 m deep exploration borehole have been selected. These investigations will commence in the summer of 2020, and will then enable geochemical and petrophysical investigations of the Palaeozoic rocks. The possibility of deep geothermal energy extraction from the Weisweiler subsurface and subsequent evaluation of the transition of the conventional lignite-fired power plant towards its utilisation of renewable “green” energy is carried out in close collaboration with DMT GmbH & Co. KG, Fraunhofer Institute for Energy Infrastructures and Geothermal Energy and RWE Power AG, all partners within the DGE-ROLLOUT project. The successful realisation of this project may serve as a pilot for similar projects considering the forthcoming fossil fuel phase-out.</p>

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