DEEP CIRCULATION AND UPWARD MIGRATION OF BRINES IN THE ST. LAWRENCE LOWLANDS (QUÉBEC) AS TRACED BY HE, NE, AR AND XE ISOTOPIC ANOMALIES

The Canadian Precambrian shield and North-American sedimentary basins hold brines with salinities as high as 10 times that of seawater. Although these brines have been studied for several decades, their origin, residence time as well as migration pattern remain unknown. Considering the common association of such brines with gas/oil reservoirs, these scientific issues are very important to investigate.

Hypersaline brines (120 to 340 g/L) have been found in the Cambrian-Ordovician passive margin sequence of the St. Lawrence Platform in the Trois-Rivières area (Québec). The brines and associated gas reservoirs are hosted by fractured rocks of the lower and intermediate parts (Potsdam, Beekmantown and Trenton groups) of the platform sequence, more precisely in tilted domal anticlines occurring in the hanging-wall of normal faults. The brines' chemistry is either Na-Ca-Cl or Ca-Na-Cl type. Their stable isotopes (δH and δ¹⁸O) are located well above the global meteoric water line and their ⁸⁷Sr/⁸⁶Sr ratios are higher than seawater. These results indicate protracted water-rock interactions between evaporated seawater and silicates, possibly within the Grenvillian crystalline basement. This implies that the brines circulated first in this crystalline basement and then migrated upward, possibly through normal faults related to the St. Lawrence rift system. However, the exact timing of migration of these brines remains unknown.

Noble gases (He, Ne, Ar, Kr and Xe) isotopes are good tracers of the origin and migration of crustal fluids. Eight gas and/or brine wells have been sampled in Bécancour, near Trois-Rivières. The ³He/⁴He (0.02-0.07 R/RA), ²⁰Ne/²²Ne (9.66 to 10.20), ²¹Ne/²²Ne (0.0292 to 0.0340), ⁴⁰Ar/³⁶Ar (288 to 1500) and finally ¹²⁹Xe/¹³⁰Xe (6.494 to 6.611) ratios suggest a mixing between fractionated atmospheric gases and mantle-derived gases (5-10%). This finding strongly supports the hypothesis that the Bécancour brines circulated in the Grenville basement, leaching magmatic intrusive rocks and being enriched by mantle-derived noble gases, prior to the upward migration into the overlaying sedimentary reservoirs. U-Th-⁴He and U-¹³⁶Xe age models are underway to help quantify the residence time of brines in both the Grenvillian basement and the hosting platform sequence.