Lithologically constrained velocity–density relationships and vertical stress gradients in the North Alpine Foreland Basin, SE Germany

Abstract

Geophysical properties of the subsurface and the vertical stress acting within are key prerequisites to understanding fundamental geological processes and mitigating risks associated with the economic usage of the subsurface. In SE Germany, the North Alpine Foreland Basin (NAFB) is a well-studied sedimentary basin, which was extensively explored for oil and gas in the last century and which is currently explored and exploited for deep geothermal energy. The up to 5 km thick Cenozoic basin fill comprises mostly shales, marls, sandstones, carbonates, and coarse-grained clastics; in particular, Oligocene–Miocene age sediments display significant lateral lithological variability due to two marine transgressions. In addition, Cenozoic marine sediments in the eastern part of the basin are significantly overpressured. The basin sediments overlay Mesozoic passive margin sediments. Here, karstified Upper Jurassic carbonates represent the main target for deep geothermal exploration and production. Even though the North Alpine Foreland Basin has been well studied during its economic development, the relationships between basic geophysical parameters, such as bulk density and seismic velocity, both of which are key for seismic imaging and the prediction of physical rock properties, have not yet been systematically investigated. The same is true for the distribution of vertical stress gradients, a key input parameter for geomechanical modelling and the prediction of natural and induced seismicity. To improve the understanding of density–velocity relationships and the distribution of vertical stress gradients, we systematically analysed 78 deep wells with total depths of 650–4800 m below ground level, which form two overlapping datasets: bulk density and sonic velocity data from 41 deep boreholes were used to establish velocity–density relationships for the main lithological units in the North Alpine Foreland Basin in SE Germany. We applied these newly derived relationships to velocity data of a second set of 55 wells, which at least penetrated the Cenozoic basin fill section in the study area and spliced resulting bulk densities with measured but scarcer measured bulk density data. We integrated these spliced bulk density profiles to vertical stress to investigate the spatial distribution of vertical stress gradients. Thereby, we observed an eastward decrease in vertical stress gradients, which correlates well with the geological configuration of the North Alpine Foreland Basin in SE Germany. In addition, we investigated the distribution of vertical stress gradients at the top of the economically important Upper Jurassic carbonates. As a practical result, we provide lithologically constrained velocity–bulk density relationships and depth-dependent vertical stress gradient models, which can be used as an improved input for future geophysical, geomechanical, geological, and rock physics studies in the North Alpine Foreland Basin, both in fundamental and applied research contexts.