Sequence-stratigraphic control on facies and karst in Europe’s largest geothermal carbonate reservoir: The Malm Reservoir of the South German Molasse Basin (greater Munich area)

The Upper Jurassic to Lower Cretaceous (Purbeck) sedimentary succession of the South German Molasse Basin, here referred as the Malm Reservoir, hosts one of the largest hydrothermal resources in continental Europe. It exhibits strong heterogeneity driven by depositional facies variability within a sequence stratigraphic framework, diagenetic overprint including karst horizon development, and structural elements typical for foreland sedimentary basins. The GIGA-M project aims to study the deep geothermal reservoir of the greater Munich area through integrated well data and large-scale 3D seismic interpretation, providing the geological basis for a reservoir management model enabling synergetic geothermal utilisation. Hydraulically active zones in the most productive geothermal wells are commonly observed within karstified intervals (Hörbrand et al., 2025, Schölderle et al., 2023) and are therefore commonly described as one of the main reasons for the exceptional productivity of the reservoir. Facies architecture and Mesozoic to Cenozoic faults further influence reservoir heterogeneity and fluid flow. Karst horizons are unevenly distributed throughout the reservoir, indicating a complex interplay of syn-depositional and diagenetic controls that is common in many karstified carbonate reservoirs worldwide.

This study evaluates how sequence stratigraphy, facies architecture, and karst development control flow zones and matrix porosity in the Malm Reservoir. The analysis focuses on stratigraphic and facies organisation and karst characterisation. Available well data and recent studies indicate that fault systems and fractures play only a minor role in the hydraulic behaviour of the Malm Reservoir; consequently, they are not a primary focus of this study. Our workflow integrates geophysical well logs, mud-log descriptions, and borehole image logs to identify and classify karst features in wells and, where flow data are available, to correlate karst categories with observed flow zones. This approach enables the recognition of karst horizons associated with enhanced porosity and permeability, directly relevant to reservoir quality and well-interference assessment.

A regional sequence stratigraphic framework (Wolpert et al, 2022; Wolpert, 2020) is used to link relative sea-level changes to facies distribution within the carbonate ramp system. Facies associations primarily control matrix porosity and storage properties, whereas sequence boundaries mark exposure surfaces and sedimentary gaps where karst can develop. While early diagenetic karst may initiate at sequence boundaries, the most extensive karst development is interpreted to result from prolonged subaerial exposure of the reservoir during the Cretaceous, highlighting the critical importance of identifying and differentiating sequence boundaries according to their timing and duration of exposure. This Cretaceous karst generation is considered the main candidate for the laterally extensive karst systems that cross-cut facies boundaries and form the main geothermal flow zones, as confirmed by flow observations in wells. These karst horizons exert a first-order control on transmissivity and hydraulic connectivity. Within the GIGA-M project, this stratigraphic and karst framework provides the geological basis for developing facies- and karst-probability maps calibrated with existing and future GIGA-M 3D seismic data, enabling the assessment of flow connectivity and well interference and supporting geothermal reservoir management at the greater Munich area scale.