Presence and Origin of Dissolved Gas in Groundwater in the St-Edouard Area (Quebec, Canada)

The Upper Ordovician Utica Shale, located in the St. Lawrence Lowlands, is a potential shale gas producer. The industry targeted this formation between 2007 and 2010, until a moratorium was imposed. A total of 29 wells were drilled in this shale, of which 18 were subjected to hydraulic fracture stimulation. The Geological Survey of Canada began a project in 2012 in the St-Edouard area, 65 km southwest of Quebec City, to study the potential impacts of shale gas development on near-surface aquifers. This study includes four components—geochemical, geomechanical, geophysical, and hydrogeological—so as to collect and integrate multi-source data in order to better understand the behavior of the hydrogeological system as a whole.

Results of geochemical analyses show the presence of methane in groundwater throughout the region, but with widely variable concentrations. Some samples also show the presence of propane, indicating a thermogenic component. Results of geochemical analyses of soil gas provide similar indications. Available isotopic analyses suggest that all groundwater samples have a biogenic signature, but that 20% to 40% of samples have a mixed origin (i.e., containing both biogenic and thermogenic gas). Geochemical analyses of core samples from shallow wells (50-60 meters depth) show that the near-surface bedrock contains hydrocarbons (C1-C20) that could constitute the source for both biogenic and thermogenic gas. Additional analyses are planned to identify more precisely the origin of dissolved natural gas and the possibility for fluids to migrate from deep shale units to the surface aquifers.