Stable isotope conservation and origin of saline waters from the Middle Jurassic aquifer of the Paris Basin, France

Abstract

Seventy-nine samples of formation water from geothermal boreholes, oil-field and ground-water wells, tapping the Dogger (Middle Jurassic) limestone reservoir of the Paris Basin, have been analyzed for their 2H, 18O, TDS, bromide and chloride contents. Geostatistical mapping of 2H and of 18O contents and their variations with chloride content indicate that both isotopes have a common origin and a similar behaviour within the aquifer. The σ 2H and σ 18O values increase progressively from the outcrops toward a zone where temperature, Cl − and TDS values are high. The σ 2H and σ 18O 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 isotopes exchange with the carbonate matrix for oxygen, and with H 2S for deuterium, shows that such processes are not controlling the heavy isotope contents of groundwaters. This is attributed to the relatively short residence time of groundwaters in the aquifer. Both 2H and 18O contents of groundwater may thus be considered as conservative tracers in the Dogger aquifer of the Paris Basin and their initial contents are modified only by fluid mixings. Complex mixing processes 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 (secondary brine). This saline solution mixed with some residual primary brine, then migrated via vertical faults into the Dogger aquifer. Such migration is found to be in agreement with data from fluid inclusions in late carbonate cements from the Liassic and Dogger strata. The requirement that these units were exposed implies that meteoric invasion could not have occurred before the Tertiary. Successive mixings of the brine with several types of meteoric waters depleted in heavy isotopes account for the present isotope and chemical compositions of the Dogger fluids.