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Baseline Geological and Geochemical Near-Surface Data: The Case of Upper Ordovician Shales in Eastern Canada

Abstract

In eastern Canada, the Utica Shale is a potential shale gas producer. Between 2007 and 2010, 29 wells (vertical and horizontal) were drilled and 18 of them fracked. In 2010, the exploration phase was suspended pending the outcome of an evaluation of all environmental aspects related to shale gas development. As part of its Environmental Geoscience program, the Geological Survey of Canada and its research partners have initiated a groundwater-focussed research project in the area where the well with the best IP and 30-day test was drilled. 25 private water wells have been sampled and 4 shallow (50 m) wells were drilled, cored and sampled for water chemistry and rock organic geochemistry. This presentation focuses on the preliminary data and interpretation based on detailed geochemical characterization of dissolved hydrocarbons in water wells, organic geochemistry of extracts from core samples from shallow Upper Ordovician black shales and soil gas survey results. The presence of naturally-occurring dissolved hydrocarbons in groundwater is fairly well established in southern Quebec, although the biogenic versus thermogenic source of the hydrocarbons remains ambiguous. Well water samples have significant concentrations of dissolved hydrocarbons including mostly methane, as well as ethane and propane in a few wells. High concentrations are found mainly in the northern part of the study area, 10 km north of the shut-in Utica Shale well. Methane concentrations in groundwater can be as high as 40 mg/l. The presence of dissolved ethane and propane in groundwater indicates that some of the hydrocarbons are thermogenic in origin. Gas wetness versus methane δ13C values suggests mixing of thermogenic and biogenic sources, a conclusion also supported by δ2H and δ13C values of methane. The shallow bedrock geology is dominated by shales and sandstones of the cap rock unit overlying the Utica Shale. Rock-Eval Tmax results suggest that core samples from the northern shallow wells are presently in the oil window, with maturity increasing south of the study area into the gas-condensate zone. GC and GC-MS analyses of these core extracts document the presence of C1 to C20 hydrocarbons. In addition, 250 soil sites were sampled for pore-space radon, CO2 and hydrocarbons. Areas with radon, methane, ethane and butane soil gas anomalies are associated with the tectonized Appalachian deformation front, an area with high concentrations of hydrocarbons dissolved in groundwater.