A good understanding of coastal aquifers is particularly important for their management and preservation, as well as for specific implementations (artificial recharge). A detailed knowledge of the lithology and geometry of the aquitards and aquicludes is as important as that of the aquifers, particularly for seawater encroachment dynamics. A new methodology, based on reservoir geology, is used to contribute to highresolution lithological and geometrical knowledge of coastal sedimentary aquifers. The proposed methodology is being applied on the Roussillon Basin, located along the southernmost part of the French Mediterranean coast, near the Spanish border. This basin covers an area of 700 km2 and comprises two Pliocene aquifers whose characteristics are linked to the Messinian event, and is overlain by the Quaternary aquifer whose geometry is linked to eustatic variations. All these aquifers consist of alluvial and coastal deposits. A detailed lithology and geometry model of this basin has been made mainly on the basis of reservoir geology methods: (i) on-shore, through genetic stratigraphy and (ii) off-shore, through seismic stratigraphy. A detailed mapping of the elevation of the top and base of the aquifers and aquitards/aquicludes is thus available and will be used in a hydrogeological finite difference model. This model will be used for various applications such as aquifer management, location of the outflow areas of the aquifers and sensitivity analysis on geometry (on-shore-off-shore).