Sub-areas in steep rock salt formations – what do we know about the geological composition of North German salt structures?
Authors: Pollok Lukas , Henneberg Mareike 
Published: November 2021

Abstract. Suitable host rocks for a repository for high-level radioactive waste (HLW) in
Germany include not only clay and crystalline rocks but also rock salt
formations in so-called flat and steep bedding (StandAG, 2017). Favourable
repository relevant properties of rock salt are, e.g. the high heat
conductivity, low porosity and permeability, and its viscoplastic deformation
behaviour. Thicker salt deposits can be attributed to the formation of
approx. 700 salt structures that have formed under various geological
conditions in the North German Basin (NGB) over the last 250 million
years. According to their shape and genesis, salt structures are classified as
salt pillows (considered as flat bedding) or salt diapirs (steep bedding). Out
of a total of 74 sub-areas in rock salt, 60 sub-areas in salt diapirs
consisting of Permian evaporates were identified within the first phase of the
German site selection procedure (BGE, 2020). At the current stage of the site selection process, a conservative approach
has been adopted and the internal structure of the salt structures have not
yet been taken into account for further classification of the identified
sub-areas. However, the interior of the salt structures not only consists of
rock salt but also of varying proportions of clay, carbonate and anhydrite
rocks, as well as potassium salts formed by progressive evaporation of marine
brines. Multi-phase salt tectonics has led to the folding of these differently
composed layers and to complex internal structures. Therefore, detailed
knowledge of the salt structure compositions is necessary to identify suitable
rock salt areas for the designation of the containment providing rock zone. As a result of decades of research through extensive salt and potash mining,
cavern storage and exploration for final waste disposal, Permian salt rocks
represent a well-studied host rock in Germany. The use of different
exploration methods and multidisciplinary data evaluation have led to a
comprehensive understanding of the internal composition of some well-studied
salt structures. Systematic studies have shown that several factors have
influenced the formation of salt structures as well as their external shapes,
sizes, and spatial distribution (e.g., Pollok et al., 2020). Furthermore, the
amount and distribution of suitable host rocks varies greatly in different
salt structures and is closely related to their internal structure. Since the interior of salt structures has not been considered in the site
selection process so far, a classification of salt structures (or sub-areas)
into certain types with varying internal composition and complexity is
presented. By examining their lithofacial composition, genesis, and structural
geological position within the basin, it is possible to narrow down these
types to specific areas in the NGB. Without the acquisition of additional
exploration data in this phase of the site selection process, this salt
structure classification provides important data for the legally demanded
assessment of the overall favourable geological situation.