Characterising the Heterogeneity of Transmissibility and Hydraulically Active Zones in the Deep Geothermal Reservoir in Bavaria

The Upper Jurassic-Lower Cretaceous (Malm-Purbeck) reservoir of the North Alpine Foreland Basin (NAFB) in Bavaria represents one of Europe’s most important deep hydrothermal reservoirs for sustainable heat supply. Reservoir transmissibility shows strong spatial variability and remains insufficiently characterized. In particular, the linkage between basin-scale transmissibility, vertically resolved hydraulically active zones, and their sequence stratigraphic context has not yet been systematically investigated. This gap is addressed by integrating transmissibility, hydraulically active zones, and a sequence-stratigraphic framework to provide a comprehensive characterisation of the reservoir.

Transmissibility values were derived from pressure transient analyses of geothermal and research well tests, resulting in a harmonised dataset of 57 high-quality measurements across the NAFB. These data were used to generate a basin-wide probabilistic transmissibility map using Empirical Bayesian Indicator Kriging (EBIK), a geostatistical approach that explicitly accounts for spatial uncertainty and is well-suited for sparse datasets. The resulting map confirms a general decrease in transmissibility with increasing burial depth from north to south, while also revealing regional deviations from this trend.

To resolve reservoir heterogeneity at the vertical scale, flowmeter measurements from 14 wells were analysed to identify hydraulically active zones and quantify their relative contribution to total flow. By distributing total well transmissibility according to flow contribution and zone thickness, transmissibility values were converted into permeability for individual hydraulically active zones. This approach reveals a systematic decrease in permeability with depth, characterized by distinct regional reservoir types previously identified by multivariate statistical analyses.

Hydraulically active zones were further positioned within a sequence-stratigraphic framework, enabling basin-scale correlation. The results demonstrate that hydraulically active zones occur predominantly within specific sequence-stratigraphic intervals, while deeper units contribute progressively less to flow. Although sequence-stratigraphy does not directly control permeability magnitude, it provides a consistent framework for understanding the vertical distribution of flow zones. Overall, this study provides the first integrated basin-scale assessment linking transmissibility, hydraulically active zones, and sequence stratigraphy in the NAFB. The results significantly improve reservoir characterisation, form a robust basis for static and dynamic modelling, and will be a key component of a decision support model for deep geothermal energy in the future.