Aretz, A. ; Bär, K. ; Götz, A. E. ; Sass, I. (2015)
Facies and Diagenesis of Permocarboniferous Geothermal Reservoir Formations (Upper Rhine Graben, SW Germany): Impact on Thermophysical and Hydraulic Properties.
World Geothermal Congress 2015. Melbourne (19.04.2015-25.04.2015)
Konferenzveröffentlichung, Bibliographie

Kurzbeschreibung (Abstract)

The Permocarboniferous is the largest hydrothermal reservoir in the northern Upper Rhine Graben in SW Germany and has so far been investigated in large scale studies only. The eastern part of the Permocarboniferous Saar Nahe Basin, a variscan intramontane molasse basin, is crossed by the northern end of the Cenozoic Upper Rhine Graben. Due to the subsidence in this graben structure the top of the up to 2 km thick Permocarboniferous is located at a depth of 1 to 3 km and is overlain by Tertiary and Quaternary sediments. At this depth the reservoir temperatures exceed 150 °C, which are sufficient for geothermal electricity generation with binary power plants. To further assess the potential of this geothermal reservoir knowledge of thermophysical and hydraulic properties of the different lithostratigraphical units and facies types is essential. In the present study a combination of outcrop analogue studies and drill core investigations was conducted. In total 850 outcrop samples were analyzed, measuring porosity, permeability, thermal conductivity and thermal diffusivity. Furthermore 60 plugs were taken from drillings that encountered or intersected the Permocarboniferous in the northern Upper Rhine Graben at depths between 1,800 to 2,900 m. Petrographic analysis of 90 thin sections of outcrop and reservoir samples were conducted to quantify the mineral composition, sorting and rounding of grains and the kind of cementation. This enables the determination of the diagenesis type and its influence on porosity, permeability and the degree of compaction. In early stage diagenesis the strongest influence on reservoir properties exhibits the Hematite-type. It is characterized by grain covering hematite coatings that inhibit cementation of pore space and compaction during diagenesis. In late stage diagenesis the Illit Meshwork-type and Bitumina-type illustrate CO2 rich acidic pore water conditions which are interpreted as the result of a hydraulic contact to an underlying Carboniferous oil source rock. Under these conditions the hematite coatings are dissolved and the protection of these sandstones against cementation and compaction is eliminated. During the formation of the Upper Rhine Graben this reaction caused a strong reduction of porosity and permeability. Of the encountered facies types the eolian sandstones of the Kreuznach Formation (Upper Nahe Subgroup) exhibits the best reservoir properties. Based on the combined investigation of facies and diagenetic processes, reservoir properties of the different Permocarboniferous formations within the northern Upper Rhine Graben and their changes with burial depth and temperature can be predicted with satisfactory accuracy. This leads to a better understanding of the reservoir and enables an adapted approach for exploration and exploitation of these geothermal resources.

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