NOBLE GASES GEOCHEMISTRY OF THE ST-LAWRENCE LOWLANDS BRINES – IMPLICATIONS FOR HYDROCARBON EXPLORATION

Carbonate platform sequences of Potsdam-Beekmantown and Black River-Trenton formations of the St. Lawrence Lowlands contain Ca-Na-Cl type brines with very high salinities, ranging from 110 to 340 g/L. The origin and ages of these brines and their relation to natural gas and oil reservoirs are unclear. The chemistry of major dissolved ions, stable isotopes (δH, δ¹⁸O) as well as ⁸⁷Sr/⁸⁶Sr suggest an origin from ancient evaporated seawater likely modified by water-rock interactions with the crystalline Precambrian basement of the St. Lawrence Lowlands. A geochemical survey of these brines was initiated in the Bécancour region, Southern Québec. Sampled wells, with depths ranging from 960 to 1390 m show salinities between 200 and 340 g/L. Noble gases (He, Ne, Ar, Kr, Xe) were analyzed to constrain the origin of these brines. Measured ³He/⁴He ratios range from 0.13 to 0.25 R_a and present a positive correlation both with depth and salinity, indicating the presence (1.9 to 3.7%) of subcontinental mantle helium. Similar ³He/⁴He ratios were found in brines circulating in the Trenton-Black River reservoirs in New York and Pennsylvania (Laughrey et Kostelnik, 2006). A ³He/⁴He ratio of 1.2 R_a was also measured at the Mirror Lake Basin, New Hampshire (Torgersen et al., 1995). The origin of mantle helium in Eastern North-America is still unclear, but previous work suggested an ageing magmatic source either from the Monteregian Hills (~120 Ma) or from magmatism related to late stages of the Acadian orogeny (~340 Ma) (Torgersen et al., 1995). One of the sampled wells, at the proximity of a fault, has a particularly high ⁴⁰Ar/³⁶Ar ratio (2459), supporting also the contribution of magmatic/mantle volatiles to some of these brines.

Laughrey, C. D. and J. Kostelnik (2006), in Patchen, D. G., et al. (eds.) A geologic play book for Trenton-Black River Appalachian basin exploration, U.S. Dept of Energy, DE-FC26-03NT41856, 632 pp.

Torgersen, T., S. Drenkard, M. Stute, P. Schlosser, and A. Shapiro (1995), Geology, 23, 675-678.