| Title | Facies and Diagenesis of Permocarboniferous Geothermal Reservoir Formations (Upper Rhine Graben, SW Germany): Impact on Thermophysical and Hydraulic Properties |
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| Authors | Achim ARETZ, Kristian BÄR, Annette GÖTZ, Ingo SASS |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Geothermal reservoir characterization, outcrop analogue study, Permocarboniferous, diagenesis, porosity, permeability |
| 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. Where the Cenozoic Upper Rhine Graben crosses the Permocarboniferous molasse basin, the top of the up to 2 km thick formation 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, making it suitable for geothermal power generation. To further assess the potential of this geothermal reservoir knowledge of the different lithostratigraphical units and facies types within this Variscan intramontane basin and their thermophysical and hydraulic properties is essential. In the present study, a combined outcrop analogue study and drill core investigation 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, the kind of cementation to determine the type of diagenesis and its influence on porosity, permeability and the degree of compaction. The strongest influence on reservoir properties exhibit the Hematite-type occurring in early diagenesis and being 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 being the result of a hydraulic contact to an underlying Carbonifereous oil source rock. Under these conditions the hematite coatings are dissolved and the protection of those sandstones against cementation and compaction eliminated which led to a strong reduction of porosity and permeability during the formation of the Upper Rhine Graben. The sandstone´s mineral composition was determined to detect the kind of bedrocks in order to evaluate them in terms of controlling factor for original sand compositions and its influence on diagenetic processes. Source rock analysis revealed that the highest contents of quartz being least vulnerable against diagenetic processes compared to feldspar and mica exhibit granitic source rocks. Best reservoir properties exhibit eolian sandstones of the Kreuznach Formation (Upper Nahe Subgroup). Based on these results, 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 higher accuracy. This leads to a better understanding of the reservoir and enables an adapted approach for exploration and exploitation of these geothermal resources. |