STABLE-ISOTOPE CONSERVATION AND ORIGIN OF SALINE WATERS FROM THE MIDDLE JURASSIC AQUIFER OF THE PARIS BASIN, FRANCE

Seventy-nine samples of formation water from geothermal boreholes, oil -field and groundwater wells, tapping the Dogger (Middle Jurassic) lim estone reservoir of the Paris Basin, have been analyzed for their H-2, O-18, TDS, bromide and chloride contents. Geostatistical mapping of H -2 and of O-18 contents and their variations with chloride content ind icate that both isotopes have a common origin and a similar behaviour within the aquifer. The deltaH-2 and deltaO-18 values increase progres sively from the outcrops toward a zone where temperature, Cl- and TDS values are high. The deltaH-2 and deltaO-18 values are very uniform in this zone and do not show any relation with formation temperature, in contrast to what is generally observed in other sedimentary basins. A discussion of isotope exchange with the carbonate matrix for oxygen, and with H2S for deuterium, shows that such processes are not controll ing the heavy isotope contents of groundwaters. This is attributed to the relatively short residence time of groundwaters in the aquifer. Bo th H-2 and O-18 contents of groundwater may thus be considered as cons ervative tracers in the Dogger aquifer of the Paris Basin and their in itial contents are modified only by fluid mixings. Complex mixing proc esses account for stable isotope composition, TDS, Cl- and Br- values of the Dogger fluids. All mixtures include a brine component. Meteoric water percolated and dissolved halite in the Triassic aquifer (second ary brine). This saline solution mixed with some residual primary brin e, then migrated via vertical faults into the Dogger aquifer. Such mig ration is found to be in agreement with data from fluid inclusions in late carbonate cements from the Liassic and Dogger strata. The require ment that these units were exposed implies that meteoric invasion coul d not have occurred before the Tertiary. Successive mixings of the bri ne with several types of meteoric waters depleted in heavy isotopes ac count for the present isotope and chemical compositions of the Dogger fluids.