NATURAL HYDROCARBON MIGRATION PATHWAYS INFERRED FROM INTEGRATED CASE STUDIES IN TWO UNCONVENTIONAL HYDROCARBON PLAYS IN EASTERN CANADA: ESTABLISHING BASE LINES FOR THE FUTURE

In eastern Canada, the development of unconventional hydrocarbons is hampered by the perceived contamination risk to groundwater from hydraulic fracturing. Two projects studying potential hydrocarbon migration natural pathways from deep (~2 km) shale or tight sand units to shallow aquifers are ongoing in southern Quebec (St-Édouard area, St. Lawrence Lowlands) and in southern New Brunswick (Sussex area, McCully gas field). Their context is, however, very different. Bedrock shallow aquifers in St-Édouard are mainly found in organic matter-rich Ordovician black shale, while they are hosted by organic matter-lean Carboniferous sandstone in the Sussex area. While there has not been shale gas production the St. Lawrence Lowlands, the McCully gas field has been in production since 2001. Because the intermediate zone between reservoir units and shallow aquifers is poorly characterized, these projects are based on the integration of multi-source direct and indirect data, and include field geology, geophysics, geomechanics, hydrogeology and geochemistry for both water and rock.

In the St-Édouard area, the presence of dissolved methane in the regional bedrock aquifer is ubiquitous. Groundwater from the 45 sampled wells (including 15 research wells) contains biogenic methane and 15% of the wells show a thermogenic component. Results obtained from core samples suggest that both types of gas likely come from the shallow bedrock. Monitoring of 7 wells (2 residential and 5 research wells) over 2.5 years show significant time variations in methane concentrations and, for some of the wells, in δ¹³C ratios of methane. There is no evidence that fluids are migrating from the deep Utica Shale (at a depth of 2 km), although brines were discovered in a few shallow wells along a normal fault, indicating a hydraulic link with deeper units, likely from the intermediate zone. The project in New Brunswick started in 2015 with the drilling of 6 hydrogeological wells. Preliminary results of water chemistry show that only one well contained dissolved methane in low concentration and its isotopic signature was thermogenic. These studies allow the acquisition of scientific data that will help provincial authorities take informed decisions on hydrocarbon development.