In eastern Canada and northeastern U.S.A, several jurisdictions have decreed moratoria on hydraulic fracturing due to public concerns regarding its potential environmental impacts, particularly on shallow groundwater resources. The Geological Survey of Canada has carried out two projects to assess the potential for fluid migration from deep (~2 km) hydrocarbon-rich shale or tight sandstone units to shallow aquifers (Saint-Édouard area, Quebec and Sussex area, New Brunswick). In both areas, regional faults are present and dissolved hydrocarbons gas have been found in shallow groundwater. However, the geological and hydrogeological contexts of the two study areas are very different. Also, while the gas field near Sussex has been in production since 2001, no gas well has been in production in southern Quebec.
In these two study areas, as in most regions around the world, the intermediate zone (IZ) located between shallow aquifers and deep hydrocarbon reservoirs has been overlooked, despite the fact that this IZ controls the vulnerability of shallow aquifers to industrial activities at depth. Due to this lack of data, these projects had to rely mainly on multi-source indirect data for the IZ characterization.
Data collected and their interpretation provided no evidence for the presence of large-scale connections between gas reservoirs and shallow aquifers in these study areas. On the contrary, in both project areas, the IZ seems to provide an efficient barrier protecting aquifers from the potential upward migration of fluids originating from deep hydrocarbon reservoirs. This conclusion integrates results from geomechanical studies, from geophysical and geological interpretations of the structural context and from geochemical baseline studies. Nonetheless, some old saline groundwater was found locally in some of the shallow (~50 m) observation wells, but this groundwater did not contain large concentrations of thermogenic methane. This water is inferred to come from a few hundred meters below the surface and is linked with the surficial discharge of regional groundwater flow pathways.
Regular monitoring in several wells in these projects proved very helpful to ascertain the source of methane, as individual samples commonly provided ambiguous results. Monitoring has also shown that large variations in both methane concentration and isotopic composition can occur naturally. These findings demonstrate that a single sampling campaign, which is what regulations require in most cases, may lead to erroneous interpretations of gas origin and imply that monitoring should be carried out over at least 1 or 2 years prior to hydrocarbon development to establish the natural variations of both methane concentration and isotopic composition. The methodology developed during these projects should help support the regulatory process for the protection of shallow groundwater in the context of unconventional resources development.