3D Modelling of the Zeeland Formation in the Utrecht province
Summary
The Dinantian aged carbonates below the city of Utrecht have lately become of particular interest for ultra-deep geothermal purposes. It is expected that these carbonates, which belong to the Zeeland Formation, are located on a local high in this part of the Netherlands. This high runs in a northwest-southeast direction. According to the DGM-Deep V4 onshore model the Zeeland Formation is 5200 meters buried below the Uithof, East Utrecht. However, as this area is sampled by only a few old 2D seismic lines and lack of well data, use of the DGM model for interpretation of the Zeeland Formation is very limited. The purpose of this study is to construct a more accurate model of this formation in the proximity of Utrecht to have a better insight in the possibilities of a geothermal project in East Utrecht. This is done by making a detailed reinterpretation of the available seismic lines in order to obtain a 3D static depth model of the subsurface of a study area that runs from the city of Utrecht in a southeast direction, parallel to the strike of the presumed high. There are no wells available in the study area and the deepest well in the proximity to the Uithof reaches the shallow part of the Limburg Group. Therefore, the modelling is divided into two parts: a shallow and a deep domain. Interpretations of the shallow domain include the interval between the North Sea Group and Rotliegend Formation. These interpretations were extended from the well tops of the wells closely located to the Uithof, into the study area. The top and base of the Zeeland Formation were interpreted on the Deep-NAM 84 line only. The quality of the Amersfoort survey lines do not allow interpretation of this formation. The interpretations of the top and base of the Zeeland Formation on the Deep-NAM 84 line were extrapolated in a southeast direction on the Amersfoort survey lines, using the average thicknesses of the Limburg Group and Zeeland Formation calculated on the Deep-NAM 84 line in time domain. Time-depth conversion of the shallow domain was done using the regional velocity model Velmod-3 and using a local velocity model. The latter model consist of velocity grids created from velocity data of 10 wells in the proximity of the study area. These local velocity grids are more accurate and detailed than those of the regional model. The velocity grid of the Limburg Group has been modelled by Ten Veen (2014) and the velocities of the Zeeland Formation were obtained from three wells in the Netherlands that drilled the entire formation. The resulting depth model confirms the presence of a carbonate platform that runs in a southeast direction. Following this direction, the Zeeland Formation becomes shallower. The depth of the top of the Zeeland Formation below the Uithof is 5170 meters. Despite the shallower interpretation of the Zeeland Formation on the Deep-NAM 84 line, with respect to the interpretation in the DGM-deep V4 onshore model, the depths of both models are similar. Due to both poor seismic quality and absence of a seismic line through the Uithof, uncertainties exist on the approached depth of the Zeeland Formation.