The Upper Ordovician black shales of southern Quebec (Canada) and their significance for naturally occurring hydrocarbons in shallow groundwater

Shale gas exploration in the St. Lawrence Platform of southern Quebec (eastern Canada) focussed on the Upper Ordovician Utica Shale from 2006 to 2010 during which 28 wells were drilled, 18 of which were fracked. The St. Lawrence Platform is thus considered as a pristine geological domain where potential environmental effects of fracking can be evaluated relative to the natural baseline conditions of the shallow aquifers. In the Saint-Edouard area southwest of Quebec City, it has been shown that groundwater carries variable and locally high levels of naturally occurring dissolved hydrocarbons in which thermogenic ethane and propane can be found. Fifteen shallow (30-147 m) wells were drilled into bedrock and sampled (cores and cuttings) with the purpose of characterizing the shallow bedrock in a shale gas pre-development context. The shallow bedrock geology is made of three Upper Ordovician clastic formations. The Lotbiniere and Les Fonds formations are time- and facies-correlative with the Utica Shale present at a depth of 1.5 to 2 km in this area. They are dominated by calcareous black shales with minor siltstone and micrite beds. The Nicolet Formation is the youngest unit of the area and consists of gray to dark gray shales with locally abundant thick siltstone and fine-grained sandstone beds. The organic matter in the Lotbiniere and Les Fonds formations is represented by solid bitumen with subordinate liptinite algae, graptolites and chitinozoans representing normal marine Type II kerogen. Both formations are at the post-peak hydrocarbon generation as indicated by the equivalent random vitrinite reflectance of 0.94 to 1.04%. Rock Eval data support the Type II nature of the kerogen and the late oil window maturation level. Hydrocarbon extracts from the three formations have yielded high to low concentrations of Cl to C6. For all units, an upward decrease in total volatiles (C1 + C2 + C3) together with an increase in the gas dryness ratio (C1/C2 + C3) is recorded, the transitions occurring at depths shallower than 50 m where the shales are more fractured. The upward increase in the gas dryness ratio results from the more significant reduction of ethane and propane concentrations compared to that of methane. Consistent with the dryness ratio trend, the delta C-13(VPDB) values of methane change from thermogenic values (approximate to-1-50 parts per thousand) for deeper samples, to more biogenic (negative) values (<-60 parts per thousand) atshallow depths. A similar gamma H-2(VSMOW) trend of more negative values at shallower depths is noted. The delta C-13(VPOS) and delta H-2(VSMOW) values of rock-hosted methane indicate that samples at shallow depth recorded a microbial influence. It is proposed that diffusion and some microbial degradation of hydrocarbons are responsible for the decrease of rock volatiles and the in situ generation of biogenic methane in the shales at shallow depths to mix with the in situ thermogenic methane. The Utica Shale is a very good source rock with high generation potential. However, thermogenic volatiles can also originate from shallower units with much shorter migration pathways. The mixed thermogenic and biogenic methane in the groundwater results from fracture-enhanced diffusion and biodegradation of volatiles at shallow depths. Crown Copyright (C) 2016 Published by Elsevier B.V. All rights reserved.