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TitleOrigin, distribution and hydrogeochemical controls on methane occurrences in shallow aquifers, southwestern Ontario, Canada
AuthorMcIntosh, J C; Grasby, S E; Hamilton, S M; Osborn, S G
SourceApplied Geochemistry vol. 50, (2014), 2014 p. 37-52,
Alt SeriesEarth Sciences Sector, Contribution Series 20130429
Mediapaper; digital; on-line
File formatpdf
NTS30L; 30M; 31D; 40I; 40J; 40O; 40P; 41A; 41H
AreaSouthwestern Ontatio; Georgian Bay
Lat/Long WENS -83.5000 -79.0000 45.5000 42.0000
Subjectsgeochemistry; fossil fuels; hydrogeochemistry; aquifers; methane; gas; isotope geochemistry
Illustrationslocation maps; tables; plots; ternary diagrams
ProgramShale-hosted petroleum ressource assesment, Geoscience for New Energy Supply (GNES)
AbstractNatural gas reservoirs in organic-rich shales in the Appalachian and Michigan basins in the United States are currently being produced via hydraulic fracturing. Stratigraphically-equivalent shales occur in the Canadian portion of the basins in southwestern Ontario with anecdotal evidence of gas shows, yet there has been no commercial shale gas production to date. In case of future hydraulic fracturing and shale gas production, it is prudent to acquire baseline data on water quality and natural gas occurrences in shallow aquifers to evaluate potential contamination issues, such as leakage of natural gas, brine, or hydraulic fracturing fluids. To address this issue, this study utilizes an extensive geochemical database of 862 recently sampled domestic groundwater wells, screened in bedrock and overburden aquifers, throughout southwestern Ontario, in addition to new analyses of select wells for gas molecular and isotopic composition. Results from this study are used to investigate the origin, distribution, and hydrogeochemical controls on natural gas accumulations along the margins of a glaciated sedimentary basin, prior to any shale gas development, which can be applied to other gas-bearing regions.
Dissolved gas samples collected from bedrock and overburden wells were composed primarily of CH4 (29.69-98.61 mole %), N2 (0.8-66.18 mole %), Ar+O2 (0.17-3.43 mole %), and CO2 (0-1.24 mole %). Ethane was detected, but in low concentrations (<0.81 mole %). The highest methane concentrations (up to 414 in situ % saturation; 248 ppm CH4) were found in bedrock wells completed in the Upper Devonian Kettle Point Formation, Middle Devonian Hamilton Group and Dundee Formation, and in surficial aquifers overlying these organic-rich shale-bearing formations, indicating that bedrock geology is the primary control on methane occurrences. One hundred and four out of 1010 water samples contained greater than 28 ppm CH4 (explosion hazard limit). The relatively low d13C values of CH4 (<-57.28¿), covariance of dD values of CH4 and H2O, positive correlation between d13C values of CH4 and CO2, and lack of higher chain hydrocarbons (C3+) in dissolved gas samples indicates the natural gas in the groundwater throughout the study area is microbial in origin. In addition, a comparison of the spatial distribution of CH4 and its isotopic composition between bedrock and overburden wells, suggests that microbial gas was generated within organic-rich bedrock formations and migrated into overlying surficial aquifers. Besides the availability of organic substrates, the presence or absence of alternative electron acceptors appears to be the dominant control on the occurrence of absence of alternative electron acceptors appears to be the dominant control on the occurrence of detectable dissolved oxygen, and had total Fe, NO3, and SO4 concentrations less than 3.13, 0.23, and 41.80 mg/L, respectively. Microbial methane has likely been accumulating in the study area, since at least the Late Pleistocene to the present, as indicated by the dD values of CH4 and associated groundwater (-19 to '9').
Summary(Plain Language Summary, not published)
A collaborative study was conducted to examine the occurrence of methane in shallow groundwater of southern Ontario. This region has equivalent organic-rich shales that are currently being developed through hydraulic fracturing in the United States. As there is no commercial shale-gas development occurring as yet in southern Ontario, it offers the opportunity to examine the natural occurrence of methane in a shale-gas prone region in a predevelopment state. Methane was commonly found in potable groundwater wells completed in or above these organic rich shales. Nearly 10% of domestic wells had methane in concentrations that exceed the explosion hazard limit. The methane is largely formed through microbial degradation of organic matter in the shales. Results show that methane occurs commonly, and in natural high abundance, in groundwater wells. Baseline studies are critical to characterise the natural occurrence of methane in domestic water wells prior to development.