Applying Variscan Metasediments as an EGS Reservoir at European Scale – Preliminary Stages of Outcrop Analogue Characterisation in the Western Harz Mountains, Germany

One of the aims of the EU-Horizon-2020-MEET-project (Multidisciplinary and multi-context demonstration of Enhanced Geothermal Systems exploration and Exploitation Techniques and potentials) is to explore and bring about this potential in Variscan crystalline and metasedimentary reservoirs. The Göttingen University Campus as one of the demo sites of the MEET-project focuses on the conversion of the University of Göttingen’s natural gas power plant, into a deep geothermal power plant to supply the district heating.

Through investigative desk studies the Western Harz Mountains has been chosen as a suitable analogue for the basement seen in Göttingen. The Harz Mountains is contained within the Rhenohercynian zone, a fold belt that runs across Western Europe. This zone was formed during the Variscan Orogeny and is characterised by NW-SE verging fold and thrust belts. After some erosion of the Variscan Mountains and subsequent Triassic and Cretaceous uplift, exhumation of the Variscan basement (meta-greywackes, slates and quartzites) occurred, the same basement we find below Göttingen is found in the Western Harz Mountains. To the western area of the Harz the boundary between the autochthonous and allochthonous zones can be found. This boundary is thought to continue SW along strike subsequently through Göttingen, rendering this area as a suitable analogue.

The main aim of this PhD research is to characterise the structural features of the Variscan fold and thrust belt seen in the Western Harz and how these features interact and change throughout the structural setting. To accomplish this, intensive field campaigns and photogrammetry sessions are undertaken for data collection and eventually a 3D structural model of the reservoir beneath Göttingen campus will be created. This model will contain detailed analysis of the fracture network and how parameters such as orientation, displacement, mineralisation, trace length and distribution change due to lithology and the primary fold and thrust structures. From this structural model, general conceptual reservoir models will be created which will be the basis to develop a reservoir model.

The preliminary research presented shows the first stages of the characterisation of one of the chosen outcrop areas located in the Western Harz and the planned future work flow for the building of the structural model. This will include the initial structural field data and preliminary interpretations of the fold structures of the outcrops through digital methods, as well as future plans for the characterisation of this Variscan metasediment reservoir.