The influence of burial and temperature history on hydraulic and hydro-mechanical properties of a Lower Jurassic (Pliensbachian) clay rock formation – First results of the MATURITY project

The characterization of potential host rocks for the final disposal of high-level radioactive waste is of great importance in the site selection procedure. For clay-rich host rocks, hydraulic and geomechanical characteristics often depend on the burial history determining the maximum effective stress and temperature (i.e., maturity), and the associated compaction as well as transformations during diagenesis. Therefore, the direct transferability of host rock properties between different regional-geological sites is challenging and requires detailed quantitative knowledge about host rock property variations at different maturity levels.

We present the first results of the “Maturity” research project that deals with a Lower Jurassic clay rock via field and laboratory investigations. The target formation of Pliensbachian age (182–190 Ma) is located in the Lower Saxony Hils Syncline and the neighboring Sack Syncline. Whereas the mineralogical composition can be considered similar, the clay rock is characterized by a large thermal maturation gradient over a relatively short lateral distance. The project involves shallow core drillings at five locations along the sequence of changing maturities and an extensive testing program. We explore the influence of thermal maturity on the i) hydraulic rock mass properties using in-situ single-hole and cross-hole hydraulic tests within fixed packer intervals, and ii) petrophysical and hydromechanical properties by conducting a variety of laboratory tests on core material. In this contribution, we present preliminary results of our field and lab testing campaigns including thermal maturity-dependent hydraulic rock mass and geochemical characteristics as well as porosity and mechanical strength determinations of the clay rock.