High-Temperature Aquifer Thermal Energy Storage (HT-ATES) for short, is a technology that can help bridge the gap between energy supply and energy demand, which is particularly interesting to use in combination with renewable energy sources. The impact of subsurface heterogeneity on energy efficiency and thermal plume extent is not yet fully understood. In this paper, subsurface heterogeneity is linked to energy efficiency and thermal plume extent. A case study at the TU-Delft is considered, in which the Pleistocene deltaic deposits of the Maassluis formation have been characterized. Insights are gained from performing well correlations with high-resolution Gamma-Ray logs, which translates to an increase in understanding the possible architecture of the deposit and which adds a level of heterogeneity to current subsurface models that depict the Maassluis formation as a homogeneous medium. Stochastic and process-based geomodels have been constructed to represent the subsurface, using sequential indicator simulation with sequential Gaussian simulation and PyBarSimPseudo3D, respectively. Due to the difference in the underlying principles of the geomodeling techniques, the created geomodels show contrasting distributions in hydraulic properties. HT-ATES simulations have been performed with these heterogeneous geomodels using open-DARTS (Delft Advanced Research Terra Simulator). The results of the simulations show that heterogeneity has a negative impact on energy efficiency. The negative impact varies between 0.4 and 9.5 percent in the final cycle of simulation. The thermal plume distortion as a result of subsurface heterogeneity and potential for buoyancy flow to develop show to be the controlling mechanisms for the long term efficiency. The impact of heterogeneity also decreases over time as the thermal plume distortion is reduced as a result of conduction. The thermal plume extend, becomes highly unpredictable when aquifers have a low net-to-gross ratios. For stratified and high net-to-gross aquifers, the predictability is higher, as the thermal plume is more circular.