Multi-isotope systematics on the Dogger brines of the Paris Basin – Preliminary results.

The franco-canadian ANR-NSERC project G-BASELINE aims at constraining the potential effects of exploration and exploitation of unconventional hydrocarbon resources by defining a geochemical baseline for impact assessment. In France, the Lower Jurassic of the Paris Basin is investigated as potential shale oil reservoir. The parent rock are the black shales of Lower Jurassic. The oil migrates either upwards in the Dogger (Middle Jurassic) limestones, or downwards to the Upper Triassic sandstones. These conventional reservoirs have been exploited for centuries, and extraction is still active in some places. Associated high temperature Dogger brines are also exploited for low-enthalpy geothermal energy. They are investigated in the aim of identifying their migration or leakage, potentially triggered by non-conventional hydrocarbon exploitation, on the background of existing salinity anomalies in overlying freshwater aquifers, particularly the Albian aquifer. Several studies that were carried out during the 90’s have emphasized the complex origin of both Triassic and Dogger brines. In addition, it is not clear whether the Dogger brines could originate from the upward migration of the Triassic brines (in Millot et al., 2011). These latter have a heavy B isotopic composition (Millot et al., 2011), that is identical to the heaviest boron isotopic signature of the dilute Dogger formation waters from the eastern Paris Basin (Innocent et al., in prep.). Strontium isotopic compositions obtained on some Dogger brines (in Rojas et al., 1989) cluster at 0.7078-0.7079, slightly higher than dilute Dogger formation waters of the eastern Paris Basin (0.7073-0.7074, Rebeix et al., 2011). Such values reflect the isotopic composition of the aquifer material, but are largely lower than the isotopic compositions of oil-bearing Triassic brines (0.7104 to 0.7112, Millot et al., 2011). Thus, on the basis of Sr isotopes, the different brines do not seem to be completely geochemically homogenous. A multi-isotopic is underway on new samples of Dogger brines. The preliminary results are discussed.