3D basin modeling of the Hils Syncline, Germany: reconstruction of burial and thermal history and evaluation of their influence on the present-day petrophysical properties of potential host rocks for nuclear waste storage

The Hils Syncline is located in the south of the Lower Saxony Basin, Lower Cretaceous and Jurassic rocks crop out in its center. The Cretaceous and Jurassic sedimentary sequences in northern and southern Germany are known to be largely composed of clay-rich shales suitable for nuclear waste storage. Highly variable thermal maturity increasing towards the northwest characterizes the Hils Syncline, which makes it a natural laboratory to study the effects of burial and thermal maturation on petrophysical properties of shales at different levels of thermal maturation, which is our focus.

We reconstructed the burial and thermal history of the Hils Syncline resulting from a 3D-thermally-calibrated numerical model to better understand its geodynamic evolution, constrain maximum paleo-depths, –temperatures and erosion amounts, and evaluate their influence on the present-day petrophysical properties of Cretaceous and Jurassic units.

The basin experienced continuous subsidence interrupted by the Upper Cretaceous major phase of erosion. During the latest Early Cretaceous, deepest burial and maximum temperatures of Jurassic and Cretaceous rocks occurred showing an increasing northward trend. These units' porosity and vertical permeability decreased while the vertical thermal conductivity increased during burial. The Late Cretaceous inversion caused severe uplift and erosion that was stronger towards the northwest where up to 3,200 m of sediment was eroded compared to only about 1,200 m in the south. The petrophysical properties were not affected by the inversion according to the model. Calibration is currently performed based on new cores of Pliensbachian shales; thus, model results and experimental data will be compared.