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TitleThe Upper Ordovician black shales of southern Quebec (Canada) and their significance for naturally occurring hydrocarbons in shallow groundwater
AuthorLavoie, DORCID logo; Pinet, N; Bordeleau, G; Haeri-Ardakani, OORCID logo; Ladevèze, P; Duchesne, M JORCID logo; Mort, A; Brake, V; Sanei, HORCID logo; Malet, X
SourceInternational Journal of Coal Geology vol. 158, 2016 p. 44-64,
Alt SeriesEarth Sciences Sector, Contribution Series 20150416
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf
AreaSt Lawrence Platform
Lat/Long WENS -74.0000 -71.0000 46.7500 45.7500
Subjectsfossil fuels; hydrogeology; coal; coal analyses; hydrocarbons; hydrocarbon potential; shales; black shales; groundwater; groundwater geochemistry; groundwater resources; Utica Shale; Paleozoic; Ordovician
Illustrationslocation maps; photographs; stratigraphic columns; photomicrographs; histograms; plots
ProgramEnvironmental Geoscience, Shale Gas - groundwater
ProgramEnvironmental Geoscience, Shale Gas - seismicity
AbstractShale 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 SaintÉdouard 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 Lotbinière and Les Fonds formations are time- and facies-correlativewith the Utica Shale present at a depth of 1.5 to 2 kmin 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 Lotbinière and Les Fonds formations is represented by solid bitumenwith 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 lowconcentrations of C1 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 50mwhere the shales aremore 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 ?13CVPDB values of methane change from thermogenic values (??50‰) for deeper samples, tomore biogenic (negative) values (b?60‰) at shallow depths. A similar ?2HVSMOWtrend ofmore negative values at shallower depths is noted. The ?13CVPDB and ?2HVSMOWvalues of rock-hostedmethane indicate that samples at shallow depth recorded a microbial influence. It is proposed that diffusion and somemicrobial 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 fromshallower unitswithmuch shorter migration pathways. Themixed thermogenic and biogenic methane in the groundwater results fromfracture-enhanced diffusion and biodegradation of volatiles at shallow depths.
Summary(Plain Language Summary, not published)
The St. Lawrence Lowland is considered a pristine area where evaluation of potential risks to groundwater from shale gas development fracking can be studied. In the Saint-Édouard area, the geochemistry of dissolved hydrocarbons in groundwater consist of a mixture of biogenic and thermogenic components. We present the detailed analyses of Upper Ordovician shallow buried shales of the area and contrast their geological and geochemical characteristics with those of the deeply buried Utica Shale. The shallow shale units are less rich in organic matter than the Utica Shale but they reached the thermal conditions where liquid hydrocarbons are produced, methane and longer chain hydrocarbons are present in these shales providing a shallow thermogenic source for dissolved hydrocarbons in the groundwater. Moreover, isotopic evidence suggests that biogenic methane in groundwater originates from physical diffusion and bacterial degradation of ethane and propane in the shales. The study concludes that there is no need for significant upward migration of hydrocarbon volatiles from the Utica Shale.

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