HIGH SALT CONTENT IN SHALLOW GROUNDWATER OF THE MCCULLY GAS FIELD (NEW BRUNSWICK, CANADA): HYDROCARBON-RELATED CONTAMINATION, OR PURELY NATURAL MIGRATION?

Unconventional hydrocarbon production through hydraulic fracturing has sparked public concerns for several years, especially regarding potential potable groundwater contamination from deep-seated brines and chemicals used during these operations. Upward migration of such fluids could only occur in presence of preferential migration pathways, such as permeable geological faults or leaky hydrocarbon wellbores. In New Brunswick, the McCully gas field has been host to natural gas production from tight sandstone and shale for nearly 20 years. At the heart of the gas field, recent sampling of shallow monitoring wells, located less than 60 m away from production wellheads, has highlighted the local presence of an upward flow of water with high salinity and sporadic, low hydrocarbon concentrations. The origin of these particular fluids could indicate the presence within the shallow aquifer of either: 1) deep basin brines from the hydrocarbon reservoir (Albert Formation, Tournaisian, Early Mississipian); or 2) brines coming from evaporitic units located higher in the stratigraphic column, i.e. within the so-called intermediate zone (Windsor Group, Visean, Late Mississipian). Identifying the origin of these fluids is key in determining whether their presence is related to hydrocarbon production or rather to natural migration occurring through regional groundwater flow. To achieve this, multiple geochemical lines of evidence are being used, including salinity-related ion ratios, and stable isotopic composition of water (δ²H, δ¹⁸O) and strontium (⁸⁷Sr/⁸⁶Sr). These various analyses were conducted, when relevant, on shallow groundwater samples (< 50 m depth) and intermediate-zone Windsor Group rocks (173-332 m); they are then compared with previously published values for deep basin brines (1940-3168 m) from the natural gas bearing Albert Formation. This suite of indicators and sample matrices allowed identifying the true origin of the brackish fluids in the shallow aquifer.