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2021
Conference Paper
Titel
Assessment of the Reservoir Potential of Devonian Carbonates in the Rhine-Ruhr Area, Germany
Abstract
Devonian carbonates in the Rhine-Ruhr area potentially depict reservoir formations for deep geothermal applications. Their thickness of up to 1,300 m and expected depth of more than 4,400 m underneath the overlaying sediments in the study area provide suitable circumstances. The Devonian carbonates in the underground of the Rhine-Ruhr area are still widely unexplored. Besides their depth, the reason is the scarcity of hydrocarbon exploration activities in the past resulting in a lack of seismic and borehole data or core sample material. Therefore an outcrop analogue study is carried out as a first step for evaluating the reservoir potential. In the course of the project, field and laboratory data is collected and the findings will be extrapolated to reservoir conditions in the later stages. As the relevant formations crop out along the northern flank of the Remscheid-Altena Anticline, three carbonate quarries along this range were chosen for field measurements and sampling purposes: Wuppertal, Hagen-Hohenlimburg and Hönne-Valley. The most important field observations are macroscopic facies determinations for being able to make facies-related statements about the reservoir potential of different sub-formations within the Devonian carbonates and the scanline surveys for analyzing spatial and facies-dependent fracture properties. In the laboratories microscopic facies determinations and petrophysical experiments are conducted focusing on thermal and hydraulic properties in the earlier stages of the project. The combination of the facies analyses in the course of the field work and in the laboratories resulted in the detection of two main depositional facies types in the studied quarries along the carbonate range. The Schwelm-facies represents the older units deposited on an outer shelf platform subjected to relatively slow, steady subsidence. Wave-resistant, relief-forming reef bodies are not existent in this facies type. This changed towards the end of the Middle Devonian. With increased subsidence, rigid reef bodies locally rose above the carbonate platform. This so called Dorp-facies represents the second main facies type and can be subdivided into fore-reef, reef-core and back-reef. Within the Dorp-facies, only fore- or back-reef sub-facies types are on hand. Epigenetic dolomitization occurred locally within the Devonian carbonate formations. Dolomitized parts are exceptionally detected in the Hagen-Hohenlimburg quarry so far. The results of the scanline survey show that the most obvious variation in fracture properties is detectable comparing dolomitized and non-dolomitized carbonates with the latter exhibiting a higher fracture density, portion of opened fractures and length of the discontinuities. Petrophysically the difference between the non-dolomitized Schwelm and Dorp-facies is less pronounced than the difference between those units and dolomitized sections, as well, especially from a hydraulic standpoint. Total and effective porosities, as well as permeabilities, are all higher in the dolomitized parts. For the thermal conductivity values representing the thermal properties, no clear-cut spatial or facies-related trend could be recognized. The comparison of the gained values with findings of outcrop analogue studies in the Munich area, where several deep geothermal applications are already in operation, offers a promising outlook regarding the reservoir potential of the Devonian carbonates in the Rhine-Ruhr area in general. Regarding the hydraulic properties, the dolomitized sections seem to hold the highest reservoir potential, while the fracture analysis points towards the calcitic units as the favorable targets for deep geothermal operations.
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