The influence of burial history on physical properties of claystones – Overview of a systematic research program across scales

Abstract. The search for a suitable host rock for the deep geological disposal of high-level radioactive waste is one of the major geological challenges of our time. In Germany, alongside rock salt and crystalline rock, claystones are considered a promising geomaterial and the subject of intensive research within the scope of the site selection process. Particular focus is placed on those rock properties that are intended to prevent the migration of radionuclides into the environment effectively, referred to as barrier properties. These primarily include low permeability, self-sealing efficiency with respect to fractures, sorption capacity, and mechanical properties for long-term stability of the underground infrastructure.

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 affects porosity, permeability, and mechanical properties. Within the framework of the MATURITY project, the impact of burial history on barrier-relevant properties of claystones is investigated in a detailed multidisciplinary investigation approach across scales. For this purpose, a Lower Jurassic claystone formation (the Amaltheenton-Formation (Fm)) was investigated which was subjected to variable maximum depth and subsequent uplift during its burial history. Eight shallow boreholes at the margin area of the Lower Saxony Basin (Germany) were drilled through the formation with varying degrees of maturation. Comprehensive field and laboratory investigations are aimed to analyze burial-induced alterations of claystone barrier properties, and thereby advance the current understanding of these processes. With this contribution, we aim to establish a framework for a series of detailed parametric studies that will systematically approach the dependencies between burial history and petrophysical (e.g. density, permeability), geochemical (e.g. cation exchange capacity), hydrogeological (e.g. transitivity, storativity) and mechanical (e.g. rock strength, elasticity) properties.

We present the first results of different project steps that show (a) a relatively homogeneous clay dominated mineralogical composition of the Amaltheenton-Fm across the boreholes, (b) an increase of max. burial temperatures (83 °C–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 under-compacted Amaltheenton-Fm sequence, and (e) hydraulic conductivity determined from in-situ hydraulic tests that span two orders of magnitude (10^-5 m/s to 10^-7 m/s).