Fault and natural fracture control on upward fluid migration: insights from a shale gas play in the St. Lawrence Platform, Canada

Environmental concerns have been raised with respect to shale gas exploration and production, especially in eastern Canada and northeastern United States. One of the major public concerns has been the contamination of freshwater resources. This paper focuses on the investigation of possible fluid upward migration through structural features in the intermediate zone (IZ), located between a deep shale-gas reservoir and shallow aquifers. The approach provides insights into how such an investigation can be done when few data are available at depth. The study area is located in the shale-dominated succession of the St. Lawrence Platform (eastern Canada), where the Utica Shale was explored for natural gas between 2006 and 2010. Detailed analyses were carried out on both shallow and deep geophysical log datasets providing the structural attributes and preliminary estimates of the hydraulic properties of faults and fractures. Results show that the active groundwater flow zone is located within the upper 60 m of bedrock, where fractures are well interconnected. Fractures from one set were found to be frequently open in the IZ and reservoir, providing a poorly connected network. The fault zones are here described as combined conduit-barrier systems with sealed cores and some open fractures in the damage zones. Although no direct hydraulic data were available at depth, the possibility that the fracture network or fault zones act as large-scale flow pathways seems very unlikely. A conceptual model of the fluid flow patterns, summarizing the current understanding of the system hydrodynamics, is also presented.