When a geological Group consists of a stack of aquifers separated by relatively thin clay layers, it may be possible to produce from them using a production well that penetrates all aquifers. An example is the Triassic, where in south-western part of the Netherlands the shale layers between the Formations of Hardegsen, Detfurth and/or Volpriehausen may be thin or even absent. Per Group, the geothermal potential of the combined ('stacked') aquifers within that Group was therefore calculated (see the table in Aquifers). For the combined aquifers, maps are calculated based on the property maps of the underlying aquifers. The net thicknesses are summed. The net to gross ratio of these merged layers is therefore always equal to 1. The top is taken as that of the shallowest layer of the 'stack', for the temperature that of the centre of the merged layer. A weighted average of the permeability is calculated using the net thicknesses as weight. To calculate the uncertainty in thickness and permeability of the merged layers, a correlation factor of zero is assumed. This means it is assumed that the properties of the underlying aquifers vary independently.
Apart from the calculated properties of the merged layers mentioned above, summary maps are shown in ThermoGIS of the power, the in situ heat, the potentially recoverable heat and the economic potential (see the ThermoGIS maps). Except for the latter, the underlying maps of the merged layers are summed for this purpose.