3D subsurface characterization of the Netherlands: Results from stochastic modeling

The Geological Survey of the Netherlands aims at building a 3D geological model of the upper 30 m of the subsurface of the Netherlands in order to provide a sound basis for subsurface related questions on groundwater extraction and infrastructural issues. The Province of Zeeland (SW Netherlands, covering an area of approximately 70*75 km) was chosen as the starting point for this model due to an excellent dataset of 23,000 stratigraphically interpreted well descriptions.
The model area is positioned at the southern rim of the North Sea rift basin. The shallow subsurface geology mainly consists of gently N-S dipping Tertiary and Quaternary strata that were formed under shallow marine, estuarine and fluvial sedimentary settings. The upper sequence in the area is formed by Holocene deposits consisting of tidal channel, tidal flat and lagoonal sediments that alternate with peat beds and coastal dune deposits.
The Zeeland modelling procedure involved a number of (iterative) steps. During the first model step, 2D bounding surfaces were constructed that allowed placing each 3D grid cell (100*100*0.5 m) within a correct lithostratigraphical context at formation and member level. Secondly, the lithological data from each core was transferred into a lithofacies code using newly developed, Python-based software. Examples of lithofacies zones include tidal channels, tidal flats and coastal dune sands. The combination of the lithostratigraphical model and the lithofacies zonations, allowed us to perform a final interpolation procedure in which a 3D, 50 million cell lithofacies model was constructed. In addition, the use of stochastic techniques such as Sequential Gaussian Simulation and Sequential Indicator Simulation, allowed us to compute probabilities for both lithostratigraphy and lithofacies for each grid cell, providing a measure of model uncertainty.
The procedures described above resulted in the first fully 3D regional-scale facies model of Tertiary, Quaternary and Holocene strata in the Netherlands. The model provides important new insights on spatial connectivity of sediment units like for example Early Pleistocene floodplain clay layers and patterns of sandy Holocene tidal channel systems. Our results represent a major step forward towards a cell based, 3D model of the Netherlands that should eventually replace the existing 2D models.