Major ion and isotope geochemistry of fluids and gases from coalbed methane and shallow groundwater wells in Alberta, Canada

Cheung, Katrina; Klassen, P. V.; Mayer, Bernhard; Goodarzi, Fariborz; Aravena, Ramón

doi:10.1016/j.apgeochem.2010.06.002

Cited by 86 publications

(48 citation statements)

References 46 publications

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“…Out of these, 5 samples had δ 13 C CH 4 values > −55 ‰ while 33 samples had δ 13 C CH 4 values < −55 ‰. Cheung et al (2010) reported that gases derived from the Horseshoe Canyon Formation in southeastern Alberta contained considerable amounts of ethane up to 4000 ppm in addition to methane with an average δ 13 C value of −54.0 ± 4.1 ‰. This suggests that gases from the coal-bearing Horseshoe Canyon Formation contain a minor thermogenic gas component (Cheung et al, 2010).…”

Section: Apparent or Pseudo-thermogenic Methane In Shallow Aquifers (mentioning

confidence: 99%

Redox controls on methane formation, migration and fate in shallow aquifers

Humez¹,

Mayer²,

Nightingale³

et al. 2016

Hydrol. Earth Syst. Sci.

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Abstract. Development of unconventional energy resources such as shale gas and coalbed methane has generated some public concern with regard to the protection of groundwater and surface water resources from leakage of stray gas from the deep subsurface. In terms of environmental impact to and risk assessment of shallow groundwater resources, the ultimate challenge is to distinguish (a) natural in situ production of biogenic methane, (b) biogenic or thermogenic methane migration into shallow aquifers due to natural causes, and (c) thermogenic methane migration from deep sources due to human activities associated with the exploitation of conventional or unconventional oil and gas resources. This study combines aqueous and gas (dissolved and free) geochemical and isotope data from 372 groundwater samples obtained from 186 monitoring wells of the provincial Groundwater Observation Well Network (GOWN) in Alberta (Canada), a province with a long record of conventional and unconventional hydrocarbon exploration. We investigated whether methane occurring in shallow groundwater formed in situ, or whether it migrated into the shallow aquifers from elsewhere in the stratigraphic column. It was found that methane is ubiquitous in groundwater in Alberta and is predominantly of biogenic origin. The highest concentrations of biogenic methane ( > 0.01 mM or > 0.2 mg L −1 ), characterized by δ 13 C CH 4 values < −55 ‰, occurred in anoxic Na-Cl, Na-HCO 3 , and Na-HCO 3 -Cl type groundwaters with negligible concentrations of nitrate and sulfate suggesting that methane was formed in situ under methanogenic conditions for 39.1 % of the samples. In only a few cases (3.7 %) was methane of biogenic origin found in more oxidizing shallow aquifer portions suggesting limited upward migration from deeper methanogenic aquifers. Of the samples, 14.1 % contained methane with δ 13 C CH 4 values > −54 ‰, potentially suggesting a thermogenic origin, but aqueous and isotope geochemistry data revealed that the elevated δ 13 C CH 4 values were caused by microbial oxidation of biogenic methane or postsampling degradation of low CH 4 content samples rather than migration of deep thermogenic gas. A significant number of samples (39.2 %) contained methane with predominantly biogenic C isotope ratios (δ 13 C CH 4 < −55 ‰) accompanied by elevated concentrations of ethane and sometimes trace concentrations of propane. These gases, observed in 28.1 % of the samples, bearing both biogenic (δ 13 C) and thermogenic (presence of C 3 ) characteristics, are most likely derived from shallow coal seams that are prevalent in the Cretaceous Horseshoe Canyon and neighboring formations in which some of the groundwater wells are completed. The remaining 3.7 % of samples were not assigned because of conflicting parameters in the data sets or between replicates samples. Hence, despite quite variable gas concentrations and a wide range of δ 13 C CH 4 values in baseline groundwater samples, we found no conclusive evidence for deep thermogenic gas migration into shallow aqu...

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Section: Apparent or Pseudo-thermogenic Methane In Shallow Aquifers (mentioning

confidence: 99%

Redox controls on methane formation, migration and fate in shallow aquifers

Humez¹,

Mayer²,

Nightingale³

et al. 2016

Hydrol. Earth Syst. Sci.

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“…A key pre-requisite for this monitoring approach is the establishment of a reliable baseline of methane and higher alkane concentrations and isotopic compositions in shallow groundwater against which potential subsequent impacts can be compared. Pre-drilling groundwater baseline data have been repeatedly used to assess the natural variability of methane and its sources in shallow aquifers (Cheung et al 2010;Osborn and McIntosh 2010;Hill et al 2007;Jenden et al 1993;Aravena et al 1995;Martini et al 2003;Muelhenbachs 2011, 2013;Vengosh et al 2014;Jackson et al 2013;McIntosh et al 2014;Hamilton et al 2015: McPhillips et al 2014. Assessing the sources of methane using chemical and isotopic approaches is especially powerful if the chemical and isotopic compositions of gases occurring in the production and in the intermediate zones are also known and can be compared to environmental baseline data for shallow aquifers.…”

Section: Introductionmentioning

confidence: 98%

An 8-year record of gas geochemistry and isotopic composition of methane during baseline sampling at a groundwater observation well in Alberta (Canada)

et al. 2015

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Variability in baseline groundwater methane concentrations and isotopic compositions was assessed while comparing free and dissolved gas sampling approaches for a groundwater monitoring well in Alberta (Canada) over an 8-year period. Methane concentrations in dissolved gas samples (n=12) were on average 4,380±2,452 μg/L, yielding a coefficient of variation (CV) >50 %. Methane concentrations in free gas samples (n = 12) were on average 228,756 ± 62, 498 ppm by volume, yielding a CV of 27 %. Quantification of combined sampling, sample handling and analytical uncertainties was assessed via triplicate sampling (CV of 19 % and 12 % for free gas and dissolved gas methane concentrations, respectively). Free and dissolved gas samples yielded comparable methane concentration patterns and there was evidence that sampling operations and pumping rates had a marked influence on the obtained methane concentrations in free gas. δ 13 C CH4 and δ 2 H CH4 values of methane were essentially constant (−78.6±1.3 and −300±3‰, respectively) throughout the observation period, suggesting that methane was derived from the same biogenic source irrespective of methane concentration variations. The isotopic composition of methane constitutes a robust and highly valuable baseline parameter and increasing δ 13 C CH4 and δ 2 H CH4 values during repeat sampling may indicate influx of thermogenic methane. Careful sampling and analytical procedures with identical and repeatable approaches are required in baseline-monitoring programs to generate methane concentration and isotope data for groundwater that can be reliably compared to repeat measurements once potential impact from oil and gas development, for example, may occur.

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“…Methane 62 (CH 4 ), nitrous oxide (N 2 O) and carbon dioxide (CO 2 ) are important greenhouse gases (GHGs) 63 that contribute to global warming (Ferron et al, 2007). Indirect N 2 O emissions are now 64 recognized as a quantitatively significant component of the total N 2 O emission budget from 65 agricultural activities (Reay et al, 2009 (Cheung et al, 2010). Therefore, quantification of dissolved GHGs in surface water, 82 groundwater, drainage water, pore water etc.…”

Section: Introduction 60mentioning

confidence: 99%