The influence of burial history on physical rock properties: a case study of Lower Jurassic claystones from the Hils and Sack Synclines (Germany)

The search for a suitable host rock for the deep geological disposal of high-level radioactive waste is a major societal challenge of our time. In Germany, clay-bearing formations are under investigation to potentially host a repository for high-level radioactive waste (alongside rock salt and crystalline rock). Their intrinsic properties such as low permeability, self-sealing efficiency with respect to fractures, and sorption capacity provide promising conditions for long-term waste containment. However, these properties are dependent on numerous factors such as mineralogical composition, temperature and stress conditions, and water content. Among these factors, the burial history and thus compaction affect mineralogy, porosity, permeability, and mechanical properties. Within the framework of the MATURITY project, the impact of the burial history on these properties is investigated based on a combination of different laboratory and field methods. For this purpose, a Lower Jurassic claystone formation (the Amaltheenton-Formation, Fm) which was subjected to variable maximum depth and subsequent uplift during its burial history was chosen as target formation. At five locations, in the margin area of the Lower Saxony Basin (Germany) shallow boreholes were drilled through the formation where varying degrees of maturation indicate substantial differences in maximum burial depth. In this contribution, we present the first results of initial project steps that show (a) a similar clay-dominated mineralogical composition of the Amaltheenton-Fm across the borehole locations, (b) an increase of max. burial temperatures (83–169 °C) over a lateral distance of ∼ 50 km within the investigation area, (c) a gradual increase in bulk density accompanied by a reduction in porosity and permeability for normally-compacted Amaltheenton-Fm sequences along increasing max. burial temperatures, (d) a reverse trend of those parameters for a potentially undercompacted Amaltheenton-Fm sequence, and (e) hydraulic conductivity determined from in-situ hydraulic tests that significantly differs from laboratory derived equivalents and span two orders of magnitude (10⁻⁵ to 10⁻⁷ ms^-1).