Geoscientific characterization of claystone as part of the German Repository Site Selection Act – “Sub-areas of the Lower Marine Molasse”

According to the Repository Site Selection Act (Standortauswahlgesetz – StandAG), the German Site Selection Procedure is an iterative process and consists of three phases with increasing levels of detail in which the assessed area is continuously reduced during the process. Starting with an empty, so-called “white map of Germany,” BGE (implementer of the German Site Selection Procedure) completed Step 1 of Phase I in September 2020 with the submission of the Sub-areas Interim Report (BGE 2020/88), where BGE identified 90 individual sub-areas.

In Step 2 of Phase I, BGE aims to localize siting regions within the 90 sub-areas by applying preliminary safety assessments and other scientific criteria. Within the preliminary safety assessments, subsurface data are interpreted and the results documented. The workflow comprises the detailed characterization and interpretation with a focus on the host rock formation that acts as the main geological barrier but also considers the surrounding rock formations.

The claystone successions of the “younger” and “older” Lower Marine Molasse have been designated as claystone sub-areas. Based on mainly borehole data, the sequences were characterized lithologically in order to define a succession with a barrier function within the stratigraphic sequence. Also, the spatial geometry of the barrier (such as the thickness of the barrier and maximum depth of the top of the barrier) are important features that are used for the assessment and categorization of the sub-areas. The results for both sub-areas were already published as a preliminary working status of the preliminary representative safety analyses in 2024.