Hydrogeological assessment of western anticosti island related to shale oil exploration

Morgan Peel, René Lefebvre, Erwan Gloaguen, John Molson, Jean-Marc Lauzon

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Mineral and Gas I

ABSTRACT: Anticosti Island, Quebec, Canada, is located in the Gulf of the St. Lawrence and is part of the St. Lawrence Platform sedimentary basin. Sections of the carbonate-dominated succession on Anticosti Island have been the target of hydrocarbon exploration . Although potential conventional plays have long been identified in hydrothermal dolostones, technological advances in unconventional hydrocarbon extraction have increasingly refocused the attention to an organic-rich shale unit. Indeed, results stemming from recent exploration on Anticosti Island have established the presence of vast volumes of potentially recoverable source rock oil in the buried Ordovician Macasty shales (lateral equivalent of the Utica shale in the St. Lawrence Lowlands). Upcoming hydrocarbon exploitation on Anticosti is henceforth a tangible possibility. In this context, an ongoing regional hydrogeological assessment is being carried out on the western part of Anticosti Island. The study aims to achieve two main objectives: 1) a detailed description of the baseline hydrogeological and hydrochemical conditions, prior to any shale oil operations, and 2) development of a better understanding of the potential hydraulic links connecting the shale oil unit and the near-surface groundwater, which would provide an indication of the risk to groundwater quality related to shale oil operations. In order to shed light on these central issues, a suite of hydrogeological, hydrochemical and geophysical tests were carried out on fourteen vertical observation wells, drilled in the autumn of 2012 along a 20 km long N-S transect (Figure 1). These wells, ranging from 24 to 102 m in depth, were drilled at 10 different sites at intervals of 1 km to 3.5 km (Figure 1b). At four different locations, two adjacent wells were drilled at different depths, in order to quantify vertical hydraulic gradients. Short duration pumping tests (18-154 minutes) were conducted on all but two wells. Groundwater samples were collected from most wells and analysis of physical parameters (pH, temperature, conductivity, volatile gas detection), major and minor ions, metals, TOC, VOCs, petroleum hydrocarbons (C10 C50) and PAHs was conducted on all samples. Finally, 7 wells were subject to geophysical logging (normal resistivity, spectral gamma-ray, DT, sonic) and optical borehole televiewer (BHTV) imaging. Piezometric and pumping test results show a flow system strongly dependent on topography (Figure 1c), characterized by rather low hydraulic conductivity values, ranging from 2x10-8 to 5x10-6 m/s, with a median K of 5x10-7 m/s. Near-surface hydrochemistry mainly follows a pattern of carbonate dissolution and sodium-calcium ion exchange, all samples being of Ca-HCO3 to Na-HCO3 water types. Hydrochemical signatures provide information on groundwater evolution and the associated local hydrogeological context, such as the delimitation of main recharge and discharge areas and the presence of both regional and more localized flow patterns. These localized flow patterns are mainly controlled by irregular topographic features (cuestas and incised river valleys). No significant concentrations of VOCs, petroleum hydrocarbons or PAHs were detected in any of the groundwater samples. The presence of gas has been detected in 6 wells, although measurements were made with a non-specific gas detector. Interpretation of borehole televiewer data essentially shows the existence of a fractured carbonate system exhibiting a family of subhorizontal, bedding-parallel fractures, characterized by an overall decrease in frequency with depth. However, no link has yet been established between observable fracture properties (spacing, aperture, infilling), measured hydraulic properties of the wells, and groundwater flow conditions. Furthermore, the wells being vertical, an important bias of vertical joint sampling exists. As these joints are known to occur pervasively across all formations on the island, their role in providing fracture network connectivity, perhaps at depth, may be of importance. Preliminary findings bring to light the broad features of a fractured, topographically-driven groundwater flow system. Supplementary geophysical and geochemical data and analysis, expected to be finalized in the summer of 2013, are needed to complement the current findings. This would include additional borehole measurements, including the use of a flowmeter to assess fracture hydraulics. The drilling and well-logging of a set of inclined boreholes is likely, and would offer key information on the geometry and the hydraulic properties of vertical joints that have not been intercepted in the first set of vertical wells.

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