Title: Insights from surface analogues of the Odenwald into the structural architecture of crystalline units in the Northern Upper Rhine Graben

Claire Bossennec1, Matthis Frey1, Lukas Seib1, Jeroen van der Vaart1, Kristian Bär1, Ingo Sass1,2

1Technical University of Darmstadt, Institute of Applied Geosciences, Department of Geothermal Science and Technology, Schnittspahnstraße 9, 64287 Darmstadt, Germany; 2Darmstadt Graduate School of Excellence Energy Science and Engineering, Otto-Berndt-Straße 3, 64287 Darmstadt, Germany

Event: GeoKarlsruhe 2021

Date: 2021

DOI: 10.48380/dggv-149g-je36

The Upper Rhine Graben (URG) is a target area for deep geothermal and heat storage projects, as petrophysical and hydraulic properties of the faulted crystalline basement rocks, and the temperature field comprise a high geothermal potential (Soultz-sous-Forêts, Landau, Insheim, Rittershoffen). However, there is a lack of knowledge on the multi-scale structural architecture of such rock units in fault zones. Therefore, a multi-scale structural analysis is performed on surface analogues to improve the conceptual crystalline reservoir model accuracy. The surface analogues selected are located in the Odenwald Massif, the largest outcropping section of the Mid German Crystalline High. Regional-scale lineament analysis and LIDAR and GIS interpretation of the fracture network on 21 profiles in 11 outcrops were analysed to quantify statistical parameters describing the fracture and fault network. The variability of crystalline rock lithologies (granite, granodiorite, ‘Flasergranitoid’, amphibolite and gabbro) and fault directions sampled allows for the construction of an extensive structural dataset with fracture network geometry, dimension, and connectivity features. Four significant lineament strikes dominate the structural trend of the NURG, being N000-N015°E, N050-N075°E, N100-N115°E and N150-N165°E. In the Odenwald itself, lineaments striking N100-N115°E and N055-N070°E are in a predominant proportion, compared to the N000-N015°E and N150-N165°E striking trends. Fracture length distribution follows a power law with an exponent varying from -2.2 to -1.8, depending on the background lithology. The connectivity of the fracture network is heterogeneous, with varying configurations (no fractal organisation), due to a fault control at hectometric scale and clustering marked by secondary faults. At the outcrop scale, this pattern is strongly enhanced in the vicinity of weathered fractures or fault corridors. These properties distribution can be implemented into sub-surface semi-artificial discrete fracture network models to quantify the flow properties of fractured reservoir rocks.

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