Geothermal energy will certainly be one of the most important renewable energy sources in the future. Unlike other forms of renewable sources (e.g., wind energy, solar energy, wave energy), geothermal energy is a stable energy source, which can cover base loads. The aim of this paper was to optimize the exploitation of geothermal water and offer technological solutions that will enable the introduction of renewable energy sources in the energy system of the City of Ivanić Grad. The novelty of this research is that there is a number of oil wells which are located within the city of Ivanić-Grad itself. By conversion to geothermal wells rather than abandonment, they can be used for heating public buildings, which certainly represents an excellent example of the energy transition achieved in the center of the city. Extensive oil well database was interpreted, consisting of drilling reports, casing equipment specifications, geological interpretation of the layers and well production testing, which were used to locate isolated aquifer layers above oil reservoir, as well as hydraulically connected bottom-type aquifer below oil saturated layers, both with favourable temperature and permeability. Obtainable yield from sandstone brine layers was modelled with Prosper®, petroleum engineering software for production operations, quantifying available heat energy from brine was discussed trough four technological variants of direct utilization of geothermal brine, but also with using heat pumps in the system. The calculations showed that the variants where the brine is produced from both geothermal field and oilfield bottom aquifer meet the thermal loads of all considered facilities, over a significant period of time.