Evolution of interstitial waters along the passive margin of the Southeast Basin of France: WELCOM (Well Chemical On-line Monitoring) applied to Balazuc-1 well (Ardèche)

Abstract

As part of the Géologie Profonde de la France [GPF (Deep Geology of France)] Programme investigating the passive margin of the Southeast Basin of France, studies are being made on the possibility of modelling mass transfer. In the summer of 1990, when the first hole in the area (Balazuc-1) was being drilled along a basement-rooted fault, chemical monitoring of the drilling fluids (WELCOM) enabled the composition of the interstitial fluids contained within the intersected rocks to be reconstructed. The computations were compared to on-core leaching carried out during the drilling, following which the WELCOM results were calibrated. The interstitial fluids from the drilled formations show very contrasted compositions. From the top to the bottom of the hole it has been possible to distinguish: diagenetically evolved seawater within the Hettangian limestone, modified continental fluids within the upper part of the Triassic sandstone reservoirs, and evaporated seawater within the Middle Triassic evaporites. Several zones of intensive fluid circulation are also indicated by: dolomitization of the basal Hettangian limestone, circulation of fluids expelled from the evaporites within the upper part of Triassic sandstone reservoirs, and continental-derived hydrothermal fluid flow within the Lower Triassic sandstone reservoir and Carboniferous silty shale. The results show that chemical logging of drilling fluids can provide valuable data concerning interstitial fluids where other techniques are not available.