Aquifer Thermal Energy Storage (ATES) is a promising solution for reducing the time mismatch between energy production and demand in urban environments, and recent successful experiences suggest that technical issues can be overcome. The Paris area is a priori a favorable region, since there is locally a surplus of heat production during the summer, an appropriate geological reservoir and both existing and projected district heating networks. This article focuses on a remaining issue: estimating the geothermal contribution to the energy mix of a district heating network over time when using an ATES. This result would then enable estimating the fuel cost savings obtained by avoiding the consumption of expensive energies during the winter retrieval. This work considers an ATES made of two reversible wells reaching the Dogger aquifer and providing energy to a new low-temperature district heating network heating 7500 housing-equivalents. Non-geothermal energy sources with fluctuating prices over time are used for winter peak demand and for summer heat storage. The temperature of brine unloading at the hot and cold wells is simulated and the adequacy of this geothermal system to meet the load is studied in order to evaluate the time dependent energy mix of the network. Results suggest that in average over the 30 years of operation, the ATES delivers 54 GWh per year to the heating system, i.e. a power of 9.5 MW during the 34 unloading winter weeks. The geothermal energy share in the energy mix is 70%, higher than the 50% possible with a conventional geothermal doublet. The ratio of energy delivered by the ATES divided by energy spent for storage reaches 143%, and is only slightly reduced to 137% when the cold storage is located on an existing cold plume created by past geothermal energy operations. (c) 2013 Elsevier Ltd. All rights reserved.