Aquitaine Basin Case Study; Development of a geological MOdel for the Cretaceous reservoirs of the northern Aquitaine Basin: application to geothermal Resources (MOKA)

The project MOKA proposes to complete and improve the geological model of the Cretaceous of the northern Aquitaine Basin, with prospects of future applications on deep aquifers and geothermal energy in Nouvelle-Aquitaine. The demand for « low energy » geothermal energy for heating networks has increased significantly in recent years, and the Aquitaine Basin has the second largest geothermal potential in mainland France after the Paris Basin. In the Bordeaux Metropole area, six heating networks currently exploit the Cenomanian aquifer located at depth between ca. 800 and 1200 meters. One example is the Plaine de Garonne Energies project, which heats 28,000 homes on the right bank of the Garonne river. In a context of a reduction of the use of fossil fuels, and considering the high geothermal potential of the Cretaceous series of the Aquitaine Basin, it is likely that geothermal projects at intermediate depths (from 500 m to about 2 km) will multiply in the decades to come. One example is the MET-SUD project, a heating network supported by Bordeaux Metropole on the university campus. The success of these geothermal drillings implies, among other things, an extensive knowledge of the complex geology of the subsurface. Existing reports and publications show a relatively fragmented and disparate knowledge of the Cretaceous reservoirs targeted for geothermal energy (Platel, 1996; Platel et al., 2008; Labat et al., 2021). These studies also highlight the tectonic and stratigraphic complexity of the Cretaceous formations respectively due to (i) the opening of the Bay of Biscay followed by the Pyrenean orogeny and (ii) the lateral and vertical sedimentary heterogeneities found in the siliciclastic and carbonate reservoirs. Although much data (deep wells, seismic lines) has been acquired historically and recently for hydrocarbon exploration (Esso-Rep, Elf, Vermilion) or aquifer monitoring and exploitation (Smegreg, BRGM), no synthetic geological database was built up to date. First of all, this project proposes to synthesize the geological knowledge of the North Aquitaine Basin using available data: banque du sous-sol (BSS), petroleum data, existing (hydro-)geological models (e.g., MONA, GAIA) and even outcrops. These data consist of seismic profiles, logs that will be digitized (gamma ray, resistivity, etc.), cores, petrophysics (porosity, permeability). These data will be computed in a digital geological model, for example under Petrel. This model will aim to reconstruct the sedimentological and tectonic heterogeneities of the subsurface and to help predict the location and extension of geothermal reservoirs. Ultimately, this 3D geological model will be made available in open access, with the principal aim to inform the decisions of geothermal project developers in the years to come.