3D basin modeling of the Lower Saxony Basin, Germany: the role of overpressure in Mesozoic claystones with implications for nuclear waste storage

Abstract

Jurassic and Cretaceous organic-lean claystone formations in the Lower Saxony Basin (LSB), Germany, are considered for nuclear waste storage due to their favorable physical properties. Understanding the burial and thermal history of these formations, including overpressure generation and its impact, is crucial for site selection. Past undetected overpressures may result in erroneous estimation of present-day petrophysical properties. Therefore, this study investigates the evolution and spatial distribution of overpressure in claystones in northern Germany on a rather large scale, focusing on Jurassic and Lower Cretaceous units. Utilizing 3D numerical basin modeling, this study: (i) reconstructs the geodynamic evolution of the LSB, (ii) identifies key mechanisms driving overpressure during burial, (iii) detects areas of high pore-to-lithostatic pressure ratios susceptible to fracturing, and (iv) assesses overpressure’s influence on the evolution of petrophysical properties. Results reveal that during the fastest burial phase, overpressure generation began, primarily driven by disequilibrium compaction coupled with gas generation, peaking around 99 Ma at the basin’s depocenter. During the Late Cretaceous uplift, thousands of meters of sediment were eroded within the basin’s depocenter, and overpressure dissipated. According to the model, absolute overpressures were higher for Jurassic than Cretaceous claystones, but dissipation was slower for Cretaceous claystones leading to relatively higher pore-to-lithostatic pressure ratios during uplift. During overpressure buildup, the porosity reduction slowed due to undercompaction effects, underscoring overpressure’s influence on petrophysical properties. While Pleistocene glaciations caused localized overpressure, they did not impact the petrophysical properties of the assessed units. Glacial-induced erosion, however, is projected to reduce remaining overpressure substantially. Graphical abstract

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