France has set itself ambitious targets for decarbonisation in Île-de-France where heating needs account for almost 45% of final energy consumption and heat production is still heavily reliant on carbon-based fossil fuels.
Against this backdrop, geothermal energy is a strategic lever for accelerating the energy transition and reducing our dependence on fossil fuels, by offering renewable, local energy at a controlled cost.
The results of the Géoscan Île-de-France programme, by providing a better view of the Île-de-France subsurface, will enable local authorities to develop deep geothermal energy projects as early as this year.
Deep geothermal energy in Île-de-France: potential to be exploited
The Île-de-France region is currently home to the largest number of deep geothermal installations in France and Europe, with 54 sites already in operation. On the other hand, geothermal energy is less developed in the west and south of the region, mainly because our knowledge of the subsurface in these areas is still incomplete.
To remedy this situation, the GéoscanÎle-de-France programme has enabled surveys of the subsurface in almost 300 communes (municipalities), covering almost 2,000 km² of the region.
Launched in November 2023 for a two-year period, the project began with a preparatory geophysical data acquisition campaign on an unprecedented scale, involving 280 km of measurements across the region's 6 départements and almost 100 communes. Carried out at night between March and April 2024, this operation, which used vibrator lorries to generate acoustic waves that propagate through the subsurface – in order to produce images in the same way as an ultrasound scan – aimed to improve our knowledge of it and thus facilitate geothermal exploration.
Following these acquisitions, BRGM analysed and interpreted the various data available to produce a 3D geological model of the area's subsurface and a “reservoir model” for the Oxfordian and Dogger geological layers concerned. This work has enabled us to refine our understanding of the properties of the subsurface and assess its suitability for the development of deep geothermal energy.
The initial results of the Géoscan programme (detailed in the press pack) are accessible to elected representatives, decision-makers and stakeholders in the ecological transition via the dedicated website. Further results will be available on-line by the summer.
Two years on: what are the results?
The Géoscan study represents a major step forward in our knowledge of the subsurface in the west and south of Île-de-France, with a view to developing deep geothermal energy. Covering an area of almost 2,000 km² stretching from the south of the Val d'Oise to the north of the Essonne and from the east of the Yvelines to the Hauts de Seine, it attempts to answer the key issue of determining those areas which are most favourable for the development of deep geothermal energy.
The Géoscan Île-de-France study provides new information on three key geological reservoirs (in order of increasing depth) :
The Oxfordian, between 700 and 1600 metres deep
Although geothermal energy has never before been exploited in Île-de-France, it appears to be a very promising resource over a large part of the area surveyed. The recent Alfortville borehole in the western Val-de-Marne also confirmed this potential, with tests giving very encouraging flow rates. Although its temperatures are lower than those of the Dogger, between 40°C and 65°C, it could become a major new resource for regional geothermal energy, particularly in combination with heat pumps or as an extension to existing heating networks. BRGM intends to continue exploring this reservoir through research projects. Additional measurements, such as those taken in the recent Alfortville borehole during drilling for the Dogger project, will provide essential new information for characterising this resource.
The Dogger, between 1000 and 2000 m deep
Currently exploited by 54 facilities, with nearly 1.97 TWh produced in 2025, its potential remains uncertain in the west and south of Île-de-France (the existing plants are concentrated in Val-de-Marne and Seine-Saint-Denis). Although less efficient than to the east of Paris, this reservoir has geothermal potential within the area. The study made it possible to better locate the geological formations characterised by impermeable marl facies, less favourable to geothermal energy, and the porous limestone formations to be targeted for the development of new projects. The temperature of this resource varies from 50°C to almost 80°C from west to east.
The Triassic, between 1200 and 2500 m deep
The Triassic formations, which have also been unexploited in the Île-de-France region, are now better understood, and three reservoirs of interest have been identified. However, further targeted investigations are needed to improve our understanding of the complex architecture of these reservoirs and to ensure that they can be exploited while considerably limiting the risk of failure.
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