Gravel formations, with thicknesses modelled using the GDM Multilayers tool (Crau Plain, 2023)
© BRGM
The need
SYMCRAU exploits the water resources of the Crau plain, which are used for a variety of purposes (drinking water, industrial and agricultural needs). To support its policy aimed at ensuring the sustainable management of the area's water resources, the SYMCRAU commissioned BRGM to carry out studies to help improve its knowledge of the geological formations in the area and to identify target zones in which to search for potential aquifer reservoirs. The aim was to gain a better understanding of the extension of the impermeable Piacenzian levels beneath the alluvial aquifer and identify the nature of the terrain beneath the Messinian incision surface (Middle Miocene formations), with a view to installing future boreholes for hydrogeological exploration in the underlying ancient permeable levels, which might hold preserved water resources at great depth.
The results
The results of the project were presented in the form of 3D mapping, with the identification of priority areas for investigation, in which reconnaissance drilling could be carried out. Eight preferential targets were established and prioritised thanks to the analysis of:
- 3D representations of the gravel formations of the CRAU plain and the underlying sedimentary layer;
- the geological interpretation of the older Miocene sedimentary formations, i.e. those beneath the alluvial aquifer, in the form of a “stripped-down diagram” representing the depths reached, relative thicknesses and estimated uncertainties.
The 3D geological modelling of the targeted formations was carried out using GDM Multi-layers, a digital tool developed by BRGM, based on a summary of the existing data available: geological maps, historical studies, borehole data from the French Subsurface database (BSS), old oil boreholes and geophysical seismic reflection profiles (from the 1986 Crau campaign carried out by TOTAL). This model incorporates data from 451 of the 906 digital logs of previous boreholes studied in this sector. As a result, the model was adapted to the inherent constraints of the data. The analysis of the seismic profiles enabled us to define the overall geometry of the sedimentary substratum beneath the Crau plain. The map of the gravel formations, in the form of a “stripped-down diagram”, was produced in part using the sketch proposed by Bailleux (2020), and from the analysis of additional data and knowledge acquired since then by various players.
All these results made it possible to prioritise sites for further investigations and to target zones that merit additional data acquisitions, notably to remove uncertainties and improve the model's resolution. Based on this, a new drilling campaign will then be carried out to assess the aquifer potential of these target zones.
We needed the expertise of BRGM's teams to produce a “3D pre-Pliocene” geological model. In addition to the knowledge and resources they provided, BRGM's data complemented our existing data, enabling us to obtain more precise and detailed information. When it is finished, this model will enable us to better manage and diversify our water supplies, and thus better protect existing resources. Today, we have a better understanding of our reservoirs and can now decide whether or not to exploit these resources, once again with the support of BRGM.
Using the results
The 3D model developed by BRGM made it possible to identify targeted zones in which to search for potential aquifer reservoirs. This will enable SYMCRAU to carry out a new exploration campaign to search for groundwater in deeper aquifers, in order to improve the management of the Crau gravel aquifer. BRGM will be able to improve the model as new investigations are carried out, notably by drilling, and enrich it with hydrological data acquired on new wells.
The partners
- SYMCRAU (Joint Syndicate in charge of managing groundwater in the Crau area)
