While France has set itself ambitious targets in terms of decarbonisation, heating needs account for more than 43% of the country's final energy demand. However, heat production is still heavily reliant on carbon-based fossil fuels.
In this context, 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. By improving our understanding of the subsurface, the results of the Géoscan Arc programme will enable local authorities and industrial companies to start developing deep geothermal-energy projects, with the support of the French State (via the Fonds Chaleur Renouvelable (Renewable Heat Fund), managed by Ademe), and the Provence-Alpes-Côte d’Azur Regional Council (through its renewable heating and cooling plan) and the Aix-Marseille-Provence Metropolitan Council (through its operating company SPL Énergies de Provence).
Deep geothermal energy in the syncline of the Arc: a potential resource to be explored and subsequently exploited
The syncline of the Arc covers a geological basin stretching from Miramas to Martigues and from Arles to Aix-en-Provence, with the Étang de Berre in the centre. This area has characteristics that suggest it would be suitable for the development of deep geothermal energy. Unfortunately, however, not enough was known about the area to warrant starting the first drilling operations.
To remedy this situation, the Géoscan Arc programme enabled surveys of the subsurface to be conducted beneath 29 municipalities, covering an area of almost 1,500 km².
Launched at the start of 2024, this two-year project began with a preparatory geophysical data-acquisition campaign on an unprecedented scale, involving 300 km of measurements across 29 local municipalities, using vibrator trucks to generate acoustic waves that propagate through the subsurface and allow images to be created, like an ultrasound scan. At the same time, a barge fitted with an acoustic wave generator was used to collect data beneath the Étang de Berre (data that were essential for the consistency of the overall results).
Following these data acquisition campaigns, BRGM analysed and interpreted the various data available to produce a 3D geological model of the area's subsurface, including the main targets as well as other geological layers and structural elements (faults, etc.).
This work enabled us to refine our understanding of the properties of the subsurface and assess its suitability for the development of deep geothermal energy.
Geothermal energy: a resource hidden beneath our feet – Géoscan Arc
A two-year project to uncover the region’s geothermal potential
The Géoscan study will help improve our knowledge of the subsurface in the syncline of the Arc, with a view to developing deep geothermal energy. It will provide new information about the two geological targets identified prior to the campaign.
- The Urgonian formations, which lie at depths of between 100 and 2,500 metres, are the project’s main focus. These formations have not yet been exploited, but are currently being studied with a view to developing a deep geothermal project in the municipality of Vitrolles.
- The Jurassic formations, located at depths of between 1,000 and 3,700 metres. Older and deeper than the Urgonian formations, they are currently being studied with a view to a potential development project to the south-west of the city of Aix-en-Provence.
Covering an area of nearly 1,500 km2 stretching from Aix-en-Provence to Port Saint-Louis, this study aims to answer the following question: which areas are most suitable for the development of deep geothermal energy?
The lack of data about the geometry of the deep targets and their properties meant that studies across the entire syncline of the Arc were needed to help local authorities and industrial companies identify the areas and targets most suitable for deep geothermal-energy projects.
New data and a new 3D model of the subsurface
By analysing historical data (existing surveys) and acquiring new data (300 km of 2D seismic profiles, plus field surveys), the project has made it possible to produce a geological model of the subsurface for the entire area surveyed. This model has helped improve our understanding of the geometry of the subsurface. In order to provide a summary view based on cross-referenced data, maps have been produced showing the suitability of different zones for the development of deep geothermal projects. These maps have been produced for both target formations, the Urgonian and the Jurassic.
The main findings of the Géoscan Arc project
- Precise depths and geometry of the two main geological targets (rocks that may contain hot water). The surface areas mapped are significantly more reliable and detailed than those from previous studies, providing a better understanding of the targets, their depth and the thermal distribution across the entire survey area.
- Identification of events that could have an impact on the properties of the reservoirs. The studies shed new light on the potential of these targets, particularly in terms of their capacity and the fluid-circulation mechanisms.
- The main target, consisting of Lower Cretaceous limestones (the Urgonian formations of Provence), is essentially located near Vitrolles at depths of just under 2,000 m, with estimated temperatures of between 50°C and 70°C. At greater depths, near Plan-de-Campagne (where the target is located at a depth of around 2,500 m), the temperatures could reach between 55°C and 85°C, depending on the geothermal gradient assumptions used.
- The secondary target, consisting of Upper Jurassic limestone, is generally found at greater depths. Indeed, this target is only found at depths of less than 2,500 m on the very edge of the survey area, notably to the south-west of Aix-en-Provence, with comparable potential temperatures (55°C to 85°C), if there are no surface fluid inflows.
- The suitability maps produced by cross-referencing various geological and thermal criteria identify the most suitable areas for the development of deep geothermal projects.
- The Géoscan Arc study marks a significant milestone in the characterisation of geothermal targets in Lower Provence. The initial predictive maps produced provide a better understanding of the most suitable areas and the associated risks. Nevertheless, the lack of drilling data from the survey area means uncertainties remain about certain key properties, in particular temperatures and flow rates. Boreholes therefore need to be drilled to confirm and refine these parameters.
The development of new projects
Some of the results and data produced as part of the project are already available on the Géothermies website, in the Géoscan Arc project results section, and further data will be added in the future to supplement the datasets already made available to professionals working in the geothermal-energy sector and to local authorities. These technical results will help to improve the preliminary surveys carried out on behalf of local authorities and reduce the risks associated with projects. Drilling techniques can be adapted accordingly in order to reach deep targets and develop the target reservoirs. Any additional investigations required for new projects can now be better defined and targeted, and further research programmes can be designed to take into account the current state of knowledge about the area.
This study therefore represents a decisive first step towards the development of deep geothermal energy in the Étang de Berre area and is fully in line with the objectives of both the regional council and the public authorities, aimed at boosting local renewable heat production.
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