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

Abstract

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...

Origin of Methane and Sources of High Concentrations in Los Angeles Groundwater

“…Figure shows a plot of SO 4 versus DOC and includes the concentrations of methane in each sample depicted with symbols in three colors. Methane concentrations increase with increasing DOC and decreasing sulfate concentrations that typify reducing conditions and are consistent with the methane originating as microbial gas from shallow in situ production in the aquifer (Humez, Mayer, Nightingale, et al, ).…”

“…These elevated gas concentrations may pose a threat to public safety from explosions, groundwater contamination, and impaired air quality (Chilingar & Endres, ; Eltschlager et al, ; Vidic et al, ). Sources of methane in groundwater commonly include the accumulation of methane from in situ microbial production, for example, (Beeman & Suflita, ; Humez, Mayer, Nightingale, et al, ) or the migration of thermogenic oil field gases toward the surface along faults or as a result of well‐integrity failure in poorly completed or abandoned oil wells, for example (Breen et al, ). Incidents related to gas seeps and gas migration from oil field operations or storage facilities have resulted in methane gas release, explosions, loss of life, and extensive damage to property, including the 24 March 1985 Ross Department Store explosion in the Fairfax district of Los Angeles California (Chilingar & Endres, ; Jenden, ; Schoell et al, ) and the 17 January 2001 explosion and fires that destroyed most of downtown Hutchinson Kansas (Allison, ), and in 2013, a water well in Hawthorne, CA, began leaking water and methane, requiring the evacuation of 37 nearby homes, and the shutdown of the surrounding freeways, before it was plugged (Jennings, ).…”

Microbial and Isotopic Evidence for Methane Cycling in Hydrocarbon-Containing Groundwater from the Pennsylvania Region

“…However, the absence of higher molecular weight gaseous alkanes in the majority of the water samples ( n = 11) did not support this hypothesis [C 1 /(C 2 + C 3 ) > 1,000; Figure 1B]. The heavy isotopic signature of methane, sampled at the same time as samples for microbial community analysis, coupled with the absence of C 2 and C 3 gases indicated that biogenic methane has likely undergone oxidation in most of the aquifers sampled (Humez et al, 2016). The isotope signature of methane therefore, probably has been influenced by the microbial activity and the proportion of biogenic methane may be underestimated owing to the oxidation of methane (Barker and Fritz, 1981).…”

Analysis of gas leakage occurrence along wells in Alberta, Canada, from a GHG perspective – Gas migration outside well casing

“…It is worth noting that heating of soil and underlying strata by thermal operations in this area likely accelerate bacterial activity (Madigan et al, 2003), hence the occurrence of biogenic methane. In central and southern Alberta, besides gas of biogenic origin, coal gas originating in the many coal seams above the Colorado Group has been identified in a significant number of water samples (Humez et al, 2016). To conclude, in the great majority of cases the gas migrating outside well casing does not originate in the production reservoir, but in overlying strata, particularly organic rich shales and coal beds.…”

“…Because gas migrating to the surface outside well casing is not easy to capture, several isotopic studies on GM in Alberta and western Saskatchewan (20 km east of Lloydminster) have focused on gas found in shallow groundwater aquifers, gas captured at surface casing vents and gas found in mud samples from newly drilled wells (Rowe and Muehlenbachs, 1999a;Szatkowski et al, 2002;Muehlenbachs, 2006, 2011;Humez et al, 2016). Methane mostly of biogenic origin is ubiquitous in shallow groundwater aquifers, hence also in the overlying vadose zone and soils.…”