Sulphur and oxygen isotopes of dissolved sulphur species in formation waters from the Dogger geothermal aquifer, Paris Basin, France

Abstract

Isotopic analyses of the S in sulphides and sulphates from formation waters were made at 35 geothermal wells exploiting the Dogger geothermal aquifer of the Paris Basin. Isotopic analyses of the O of the dissolved sulphates were also made on 12 of these. The results show a distribution of the δ 34S of the sulphates according to geographic zones that also correspond to variations in the geochemistry of the fluids, in particular the sulphide content. Samples taken in the Seine-St-Denis area and in the areas north and west of Paris show high δ 34S values ranging from +26.3 to +48.9%. Values of δ 34S for sulphides range from −8.5 to + 5%, and are interpreted as reflecting the effect of bacterial reduction in a confined part of the basin. Modelling this reduction by a Rayleigh distillation mechanism made it possible to calculate a value of 1.038 ± 0.003 for the coefficient of fractionation between the sulphates and sulphides. We show that an important part of the sulphides formed by bacterial reduction has been subtracted from the fluid by deposition. South of Paris and in the Val de Marne, the δ 34S values for the sulphates are more uniform, ranging between +22.4 and +26.6%, despite the existence of bacterial activity. This is most probably due to a higher rate of flow of the waters, entailing renewal of the sulphate stock. The isotopic composition of the O of the sulphates for the Seine-St-Denis area and the areas west and north of Paris is very uniform, between +16.1 and +17.1%, despite the wide range in δ 34S (+26.3 to +48.9%) for the same samples. The calculated isotopic temperatures are close to those measured in situ. These results suggest either that sulphate-water isotopic equilibrium was established after the reduction, or that the coefficient of O fractionation is very low (<1.003) during bacterial reduction.