Exploring the Relationship of Burial History, Mineral Composition, Geomechanics and Petrophysical Properties of Pliensbachian Claystones

Petrophysical properties of claystones, including hydraulic, geomechanical, and pore-network characteristics, and their modification by diagenesis, are critical for the integrity and performance of geological barrier systems and thus relevant to the long-term safety of high-level radioactive waste deposits. Claystones are regarded as suitable rocks for nuclear waste storage due to their low permeability, self-sealing-, and radionuclide sorption capacity. All these properties are strongly dependent on mineral composition and evolve with diagenesis and burial history.

This contribution presents the MATURITY project, which targets the Pliensbachian (Lower Jurassic) Amalthean Clay in the Hils and Sack synclines (Lower Saxony, Germany). Eight 100-m boreholes were drilled at five sites along a SE–NW thermal maturity axis (0.48–1.45% vitrinite reflectance), corresponding to maximum burial depths of roughly 1400–3300 m as derived from basin modeling (Castro-Vera et al., 2024).

The project aims to (i) quantify compositional trends, (ii) characterize the evolution of geomechanical and pore-network properties, and (iii) assess the influence of thermal maturity on repository-relevant properties, with implications for fluid transport under repository conditions.

Compositional data (XRD, XRF, CEC, TOC) combined with multivariate inferential statistics suggest that the Amalthean Clay is compositionally homogeneous irrespective of burial history. Detailed clay mineral analyses show that mixed-layered illite/smectite—with an illite fraction of ~80% independent of burial—dominates and that the clay content is generally high (>0.55 g/g). Pore-network descriptors (e.g., pore-size mode, total pore volume, porosity, water suction), geomechanical (e.g., uniaxial compressive strength, friction angle and cohesion), and hydraulic parameters, such as permeability, display a consistent, method-independent trend: properties follow a trend up to intermediate thermal maturity and exhibit a rebound (>10%) at the highest maturity, a feature previously noted by Gaus et al. (2022) for the Amalthean Clay. Though micropore-controlled parameters (BET area and micropore volume) do not exhibit this rebound.

The results indicate that the Amalthean Clay in the Hils syncline constitutes an ideal natural laboratory for deriving transfer functions between thermal maturity (or maximum burial depth) and repository-relevant rock properties. These findings provide essential insight into the coupling of transport processes, hydro-mechanical behavior, and pore-network characteristics in clay-rich rocks as a function of burial history, forming a foundation for assessing the transferability of key parameters to potential host formations and guiding site evaluation efforts.

 

References

Castro-Vera, Leidy, Sebastian Amberg, Garri Gaus, Katharina Leu, and Ralf Littke. 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 113, no. 8 (2024): 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.