WärmeGut: Stratigraphic Significance of Tertiary Sandstones as Potential Shallow to Medium-Depth Geothermal Reservoirs in the North German Basin

The increasing demand for renewable energy highlights a compelling necessity to exploit and use of geothermal energy in Germany. A notable research initiative in this regard is the WärmeGut project, funded by the Federal Ministry of energy and climate protection (BMWK) and aims on reconnaissance and re-evaluation of subsurface data to extend the geothermal information system of Germany, GeotIS. Specifically, Tertiary successions are hitherto underexplored because the focus of past geothermal exploration went to deeper levels below the Tertiary. One of the research goals places therefore significant emphasis on evaluating the potential of shallow to medium-depth geothermal resources of the Tertiary sandstones in the North German Basin (NGB).

In this study, we conducted a high-resolution sequence stratigraphy applied to reservoir characterization of the Tertiary sandstones in the NGB. Sequence stratigraphy holds considerable importance in geological reconstruction, significantly enhancing the precision of predictions within the domain of underground geological exploration. This aspect has received comparatively less attention, especially in the context of the Tertiary interval in the NGB. We conducted analysis of well logs from 15 sites, which included gamma ray, density, sonic, resistivity and photoelectric factor data. We investigate the temporal and spatial distribution of sandstone facies within the sequence stratigraphic framework of the Tertiary sequence in the NGB Basin. Primarily, the focus of the study revolves around the examination of four sandstone units located in the uppermost portion of the Lower Eocene, the Middle Eocene (Brüsselsand), the lower Oligocene (Neuegammer Sand), and the Upper Miocene.

The primary findings suggest that sedimentary sequences in the NGB during the Tertiary period display repetitive episodes of progradation, interspersed with periods of flooding on the depositional platform. These dynamics are notably marked by a significant increase in gamma-ray signals, which record maximum marine flooding over the basin margin. This observation underlines the cyclical nature of sedimentary processes during the Tertiary in the NGB. Each sequence is characterized by progradational and retrogradational facies, with shale predominating during the transgression phase, followed by an alternation of sand and shale. Sand and silt content gradually increases during the regression phase, with four major sandstone layers occurring at the maximum regression phases. On a spatial scale, the increasing shale content within alternating sand and shale intervals indicates the accommodation depth of the depositional system.

Additionally, we have conducted a petrophysical analysis to evaluate the quality of sandstones as a geothermal reservoir. Obtained results suggest that the porosity exceeds 5%, with variations influenced by the thickness, shale content, and depth of the respective sandstone units. However, both the thickness and petrophysical characteristics of the sandstone units demonstrate variations across different spatial scales within the NGB. The results indicate a general applicability of Tertiary sandstones for hydrothermal direct use in medium –depth level, but more detailed well log analysis is required to delineate shale successions. The study helps assess the geothermal potential of Tertiary sandstones in the NGB.