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TitleA multi-isotope approach to determine the origin of methane and higher alkanes in groundwater of the St. Lawrence Platform, Saint-Édouard area, eastern Canada
AuthorBordeleau, G; Rivard, CORCID logo; Lavoie, DORCID logo; Lefebvre, R; Ahad, J M EORCID logo; Xu, X; Mort, A
SourceEnvironmental Geosciences vol. 25, no. 3, 2018 p. 75-100, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20170284
PublisherAmerican Association of Petroleum Geologists
Mediapaper; on-line; digital
File formatpdf
Lat/Long WENS -72.0000 -71.5000 46.7500 46.5000
Subjectshydrogeology; environmental geology; fossil fuels; geochemistry; groundwater resources; aquifers; groundwater geochemistry; groundwater pollution; petroleum resources; hydrocarbons; gas; methane; bedrock geology; lithology; sedimentary rocks; structural features; faults; folds; isotopic studies; carbon isotopes; hydrogen isotopes; radioisotopes; hydrocarbon migration; flow regimes; fluid flow; well samples; analytical methods; St. Lawrence Platform; Lorraine Group; Pontgravé Formation; Nicolet Formation; Sainte-Rosalie Group; Lotbinière Formation; Utica Shale; Les Fonds Formation; Bourret Formation; Trenton Group; Potsdam Group; Beekmatown Group; Chazy Group; Black River Group; Aston Fault; Jacques-Cartier River Fault; Logan's Line; Phanerozoic; Paleozoic; Ordovician
Illustrationslocation maps; geoscientific sketch maps; cross-sections; tables; graphs
ProgramEnvironmental Geoscience Shale Gas - groundwater
Released2018 09 01
AbstractIn the last decade, production of shale gas has tremendously increased, and the need for local pre-exploitation baseline data on dissolved natural gas in aquifers has been stressed. This study investigated the origin of hydrocarbons naturally present in shallow aquifers of the Saint-Édouard area (Québec, eastern Canada), where the underlying Utica Shale is known to contain important gas resources that have not yet been exploited. Groundwater and shallow bedrock gas samples were collected and analyzed for isotopic composition of alkanes (delta-13C and delta-2HC1-C3), dissolved inorganic carbon (delta-13CDIC), and radiocarbon in methane and DIC (14CDIC, 14CCH4). This multi-isotope approach proved enlightening, and results revealed that (1) most of the methane in the region is of microbial origin; (2) partial contribution of thermogenic gas occurs in 15% of the wells; (3) processes such as late-stage methanogenesis and methane oxidation are responsible for ambiguous methane isotopic compositions; and (4) both microbial and thermogenic gas originate from the shallow bedrock aquifer, with the exception of one sample likely coming from deeper units. The thick succession of shales overlying the Utica Shale thus appears to act as an effective migration barrier for the shallow aquifers. However, evidence of upward migration of old brines near major fault zones indicates that these may serve as a preferential migration pathway over a certain depth but most likely no more than approximately 200-500 m (~650-1640 ft). The geochemical framework presented here will hopefully be useful in other research projects, especially when conventional indicators of natural gas origin provide ambiguous results.
Summary(Plain Language Summary, not published)
A multidisciplinary project was carried out to evaluate shallow aquifer vulnerability to eventual shale gas development in the Saint-Édouard area located in the St. Lawrence Lowlands, where the Utica Shale was the target of shale gas exploration between 2006 and 2010. This paper presents the multi-isotope approach that was used to identify the origin of dissolved hydrocarbons in aquifers. Both groundwater and core samples were analysed for a wide variety of compounds, including 11 types of isotopes. Results allowed us to draw the following conclusions: 1) most of the methane in shallow groundwater is of microbial origin that was mainly produced in the distant geological past; 2) microbial processes are responsible for ambiguous methane isotopic signatures; 3) a contribution of thermogenic gas occurs in 15% of the wells; and 4) both types of methane come from the aquifer. Therefore, it appears that the aquifer vulnerability to deep industrial activities is very low in this region.

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