Geneviève Bordeleau

In 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 (δ13C and δ2HC1–C3), dissolved inorganic carbon (δ13CDIC), 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.

Methane concentrations and isotopic composition in groundwater are the focus of a growing number of studies. However, concerns are often expressed regarding the integrity of samples, as methane is very volatile and may partially exsolve during sample lifting in the well and transfer to sampling containers. While issues concerning bottle-filling techniques have already been documented, this paper documents a comparison of methane concentration and isotopic composition obtained with three devices commonly used to retrieve water samples from dedicated observation wells. This work lies within the framework of a larger project carried out in the Saint-Édouard area (southern Québec, Canada), whose objective was to assess the risk to shallow groundwater quality related to potential shale gas exploitation. The selected sampling devices, which were tested on ten wells during three sampling campaigns, consist of an impeller pump, a bladder pump, and disposable sampling bags (HydraSleeve). The sampling bags were used both before and after pumping, to verify the appropriateness of a no-purge approach, compared to the low-flow approach involving pumping until stabilization of field physicochemical parameters. Results show that methane concentrations obtained with the selected sampling techniques are usually similar and that there is no systematic bias related to a specific technique. Nonetheless, concentrations can sometimes vary quite significantly (up to 3.5 times) for a given well and sampling event. Methane isotopic composition obtained with all sampling techniques is very similar, except in some cases where sampling bags were used before pumping (no-purge approach), in wells where multiple groundwater sources enter the borehole.

Ce dossier public vise à décrire les travaux de caractérisation hydrogéologique qui ont été réalisés dans la région de Saint-Édouard-de-Lotbinière, dans le sud du Québec, dans le cadre d'un projet sur les impacts potentiels de l'exploration et de l'exploitation des gaz de shale sur les aquifères. La région d'étude couvre environ 500 km2 dans la portion nord-est de la plateforme sédimentaire du Saint-Laurent, entre le fleuve Saint-Laurent et le piémont appalachien. Quinze puits d'observation ont été forés au roc lors du projet. La majorité des puits ont environ 50 mètres de profondeur (150 mètres pour le plus profond). L'épaisseur et les propriétés hydrauliques des dépôts quaternaires ont été investiguées au droit de chacun des forages. Suite à ces investigations, quatre hydrofaciès ont été définis pour représenter l'hétérogénéité des dépôts à l'échelle de la région d'étude. De plus, les variations piézométriques et barométriques ont été suivies sur l'ensemble des puits sur une période allant d'un à deux ans. Ceci a permis de caractériser le comportement de l'aquifère régional et de déterminer les conditions de confinement des nappes recoupées par les puits. Il a ainsi été démontré que le substrat rocheux de la région renferme des nappes généralement captives. Enfin, les conductivités hydrauliques du socle rocheux ont également été estimées grâce à des essais in situ de perméabilité à charge variable. Celles-ci s'étalent sur quatre ordres de grandeur et sont généralement plus faibles à proximité du piémont appalachien. Ces travaux contribuent à améliorer la connaissance des aquifères dans une région où les enjeux de quantité et de qualité de la ressource en eaux souterraines sont importants.