Chemical, multi-isotopic (Li–B–Sr–U–H–O) and thermal characterization of Triassic formation waters from the Paris Basin

This work reports chemical and isotope data and temperature estimates for seven water samples collected from Triassic formations in the Paris Basin in France. Four samples were collected in the central part of the Basin (saline waters) and three were collected at the edge of the Basin near the recharge zone (dilute waters). The saline waters collected from the Chaunoy and Champotran boreholes have high salinities (around 120 g/L) and very similar chemical and isotopic compositions. The saline water sample from the La Torche borehole has much higher salinity (168 g/L) and significantly different isotope characteristics. The chemical geothermometers applied to these fluids from the centre of the Basin give temperature values ranging between 80 and 100 °C. The fresh water samples collected at the edge of the Basin have very different chemical and isotopic compositions. These dilute water samples (Santenay and Châteauroux) are shallow, with colder temperatures of around 45 to 50 °C. The study of uranium activity ratios for these Triassic formation waters allows us, as a first approximation, to estimate a mean apparent fluid circulation velocity of around 0.2 m/yr, which corresponds to a “mean age” slightly higher than 1 My for waters sampled in the Chaunoy, Champotran and La Torche fields in the centre of the Basin. The multi-isotope characterization of Triassic formation waters shows that our data are in agreement with literature values as concerns “traditional” isotope systematics (δ18O, δD, 87Sr/86Sr). Li and B isotope signatures in the centre of the Paris Basin are in a good agreement with a fluid signature derived mainly from water/rock interactions involving clastic rocks with water essentially resulting from a seawater-derived brine endmember diluted by meteoric waters. The data reported in the present work for Li and B isotopes could be used as a reference for future studies to characterize sandstone formation waters.

Research Highlights
► The Triassic formation waters from the centre of the Paris basin have high salinity and temperature ranging between 80 and 100°C.
► We estimate a mean apparent fluid circulation velocity of around 0.2 m/yr, which corresponds to a mean age slightly higher than 1 My.
► Li and B isotope signatures are in a good agreement with a fluid signature involving clastic rocks interactions with water essentially resulting from a seawater-derived brine endmember diluted by meteoric waters.