Estimation of the deep geothermal potential within the Tertiary Limagne basin (French Massif Central): An integrated 3D geological and thermal approach

The geothermal potential of a deep sedimentary-rock reservoir, in a Tertiary graben, the Limagne d'Allier basin (Massif Central, France), is assessed. The most interesting geothermal target is identified as a thick basal Tertiary sandstone overlying crystalline Paleozoic basement. The total amount of recoverable energy in this clastic aquifer is estimated at over 500 PJ (500 × 1015 J) in the modelled area. The most promising zones appear along the north-western edges of the basin, where sediment infill is thickest. The methodology used for estimating geothermal potential starts from geological field data. The first step is to obtain a better understanding of the structure and geometry of the target zone, using various data such as field measurements, and borehole and geophysical data. These data are reinterpreted through the construction of a 3D geological model. Inconsistencies are checked and turned into a coherent 3D interpretation. The second step consists in meshing the geological model into an unstructured 3D finite-element mesh where realistic thermal boundary conditions are applied. The temperature field is computed in a third step. The thermal calculation is achieved by assuming a purely conductive behaviour and through comparison with existing borehole profiles. The computed temperatures fit the measurements in the deepest part of the Limagne d'Allier basin, while the potential role of fluid flow is highlighted in its upper part, either within more permeable formations, or around the boreholes. A fourth, final, step maps the geothermal potential (recoverable energy) in the deepest part of the Tertiary graben, where the total amount of geothermal energy available is calculated. The result of this work provides valuable guidelines for geothermal exploration in the area and our methodology can be replicated elsewhere.