Heat storage in the Danish subsurface is gaining increasing interest for optimizing the use of energy resources, but no deep heat storage facilities have yet been established. As an analogue we study the Gassum Formation in the Stenlille structure that is presently used for gas storage. This allows us to discuss geological and technical characteristics of an aquifer relevant for heat storage in Denmark. We develop a 3D model for a high-temperature aquifer thermal energy storage system using analysis of geological core data, sedimentological description, geophysical data including well logs and seismic lines, as well as a finite difference model to calculate the recovery efficiency, heat storage capacity and thermal breakthrough time. Based on geostatistical methods we made three realisations and found similar results for the three cases. In accordance with results from published simplified models we found a high recovery efficiency of 70% after 4 years and 69% after 20 years, a high heat storage capacity of 1.8x10(18) J, and a long thermal breakthrough time of 66-77 years. These results reflect the excellent reservoir properties of the Gassum Formation in Stenlille, characterised by a uniformly layered sand/shale sedimentology, a high average porosity of 25% and a high permeability of 1000 to 10 000 mD of sandstone intervals.