Abstract
For the successful realization and productivity prediction of new hydrothermal projects in the South German Molasse Basin, the hydraulic matrix properties of the Upper Jurassic Malm reservoir have to be determined as accurately as possible. To obtain specific information on the distribution of the petrophysical parameters (e.g., rock density, porosity, and permeability) 363 samples of rare drilling cores from the reservoir northeast of Munich (wells Moosburg SC4 and Dingolfing FB) were investigated using different experimental methods. Additionally, porosity was calculated by a downhole resistivity log of a nearby borehole close to Munich for comparison and the attempt of transferability of the data set to other locations within the Central Molasse Basin. Core data were divided into groups of different stratigraphic and petrographic units to cover the heterogeneity of the carbonate aquifer and provide data ranges to improve reservoir and prediction models. Data for effective porosity show a high variance from 0.3 to 19.2% throughout this heterogeneous aquifer. Permeability measured on core samples is scattered over several orders of magnitude (10−4–102 mD). Permeability models based on the porosity–permeability relationship were used to estimate permeability for the whole aquifer section and identify possible flow zones. A newly developed empirical model based on distinct lithofacies types allows a permeability estimation with a deviation < 10 mD. However, fractured, karstified, and vuggy zones occurring in this typically karstified, fractured, and porous reservoir cannot yet be taken into account by the model and result in an underestimation of permeability on reservoir scale. Overall, the dominant permeability trends can be mapped well using this model. For the regional transfer and the correlation of the results, a core-related porosity/permeability log for the reservoir was compiled for a well close to Munich showing similarities to the core investigations. The validation of the regional transferability of the parameter set to other locations in the Molasse Basin was carried out by correlation with the interpreted log data of a well near Munich.
Highlights
Due to its relatively high temperature and promising hydraulic conditions, the Upper Jurassic Malm aquifer buried within the Molasse Basin is the main target formation for geothermal exploration in South Germany (Steiner et al 2014)
A conservative approach was chosen to interpret the hydraulic properties of the rock matrix, using the minimum possible effective porosity to avoid an overestimation of the hydraulic properties
A detailed comparison between helium gas expansion porosimeter (HEP) and Water Immersion Porosimetry (WIP) methods with possible deviation of effective porosity is provided in Appendix 2
Summary
Due to its relatively high temperature and promising hydraulic conditions, the Upper Jurassic Malm aquifer buried within the Molasse Basin is the main target formation for geothermal (hydrothermal) exploration in South Germany (Steiner et al 2014). In the proximity of Munich, the basin offers ideal conditions for the use of geothermal (hydrothermal) energy to cover the increasing demand for renewable heat sources in Germany. Upper Jurassic carbonate deposits (Meyer and Schmidt-Kaler 1996). These carbonates in turn directly rest on Permo-Carboniferous troughs and a Variscan crystalline complex (Lemcke 1988). The Upper Jurassic is composed of alternating shallow marine sequences of limestones, marls, and dolostones that can be separated into a massive and a bedded facies (Meyer and Schmidt-Kaler 1990b) (Fig. 1b). The deposits can reach a thickness of up to 400 m, with the depth increasing south to about 5 km below the fringe of the Alps (Goldbrunner and Vasvári 2016; Mraz 2019)
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