The Influence of Burial History on the In-situ Petrophysical and Mechanical Characteristics of Pliensbachian Shale

Abstract

Characterizing the host rocks for high-level radioactive waste disposal requires considering various rock parameters such as petrophysical (e.g., porosity, permeability and storativity) as well as mechanical (e.g., Young’s modulus) properties. The burial history and thermal maturity of rock formations, particularly shale, significantly affect these properties. Understanding the relationship between the thermal maturity and the referred properties is essential for transferring data among various test sites. Pliensbachian shales (Lower Saxony Basin, Hils area) are characterized by a notably homogenous mineral composition. Previous studies on this formation suggest a highly variable burial history over a lateral distance of about 50 km, reaching maximum burial depths between 1300 m in the southeast to 3600 m in the northwest of this region (Gaus et al., 2022; Littke et al., 1987, 1991). In the context of the MATURITY project, eight boreholes were drilled at five different locations with varying thermal maturity along this axis. Geophysical well logging was carried out in all boreholes to investigate mineralogical (clay mineral content), petrophysical (porosity), and rock mechanical properties (e.g., dynamic Poisson's ratio). First results revealed a homogeneous clay mineralogy in all boreholes. However, significant differences were observed in both the dynamic-elastic behavior and porosity of the investigated shale formation at different locations. The obtained data can be used to establish correlations between burial history and various shale rock properties, contributing to a site-independent characterization of shale formations for long-term disposal of high-level nuclear waste.    References 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., Baker, D. R., & Leythaeuser, D. (1988). Microscopic and sedimentologic evidence for the generation and migration of hydrocarbons in Toarcian source rocks of different maturities. Advances in Organic Geochemistry, Vol.13(Nos 1-3), 549–559. 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.