Interactions Between Barrier Properties and Burial History of a Lower Jurassic Claystone Formation – Insights, Proceedings, and Initial Results from the Maturity-Project

Claystones are considered potential host-rocks for the long-term disposal of high-level radioactive waste (HLW). Their favorable barrier properties such as low permeability, self-sealing efficiency, potential for plastic deformation, and radionuclide sorption capacity mitigate the risk of radionuclide migration to the environment. However, these properties strongly depend on the burial history, defining effective stress and temperature conditions. This dependence complicates data transferability between sites and underscores the need to account for site-specific burial histories in assessing the formation`s barrier function.

The Maturity-project seeks to enhance our understanding on how variations in burial history systematically alter the barrier properties of claystone formations. The project focuses on a Lower Jurassic (Pliensbachian) claystone formation accessed through several shallow wells (~100 m depth) within the Hils and Sack Syncline, Lower Saxony (Germany). Previous studies from this region indicate a strong Southeast-Northwest directed burial gradient (from 1,300 m to 3,600 m) over a relatively short lateral distance (~50 km) (Littke et al., 1991; Gaus et al., 2022; Castro-Vera et al., 2024). A comprehensive research campaign aims to unravel the formations burial history and link it to alterations in its barrier properties. Moreover, a combination of in-situ and laboratory-based methods tackles open questions in the scale dependency of investigated properties.

In this contribution, we report on the general project proceedings and present initial results from various project steps. These initial results confirm a gradual increase in thermal maturity, documented by several parameters such as vitrinite reflectance and T_max from Rock-Eval pyrolysis data. X-ray diffraction (XRD) analysis reveals a mineralogical composition dominated by clay minerals (>50%), with minimal variation across the study area. Bulk densities derived from laboratory and well logging data show an increase with thermal maturity, rising from ~2.3 g/cm³ to ~2.5 g/cm³, while porosities decrease from ~14 % to ~9 %.

References

Castro-Vera, L., Amber, S. Gaus, G., Leu, K., Littke, R. (2024). 3D basin modeling of the Hils Syncline, Germany: reconstruction of burial and thermal history and implications for petrophysical properties of potential Mesozoic shale host rocks for nuclear waste storage. International Journal of Earth Sciences, Volume 113, pages 2131-2162.

Gaus, G., Hoyer, E.M., Seemann, T., Fink, R., Amann, F., Littke, R. (2022). Laboratory investigation of permeability, pore space and unconfined compressive strength of uplifted Jurassic mudstones: The role of burial depth and thermal maturation. Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, 173 (3), 469-489

Littke, R., Leythaeuser, D., Rullkötter, J., & Baker, D. R. (1991). Keys to the depositional history of the Posidonia Shale (Toarcian) in the Hils Syncline, northern Germany. Geological Society, London, Special Publications, 58(1), 311–333.