Paleozoic-aged microbial methane in an Ordovician shale and carbonate aquiclude of the Michigan Basin, southwestern Ontario. (June 2015)

• Record Type: Journal Article
• Authors: Clark, I.D.; Ilin, D.; Jackson, R.E.; Jensen, M.; Kennell, L.; Mohammadzadeh, H.; Poulain, A.; Xing, Y.P.; Raven, K.G.
• Is Part Of: Organic geochemistry. Volume 83/84(2015)
• Page Start: 118
• Page End: 126
• Publication Date: 2015-06
• Subjects: Paleozoic methane -- Archaea -- Helium -- Aquiclude -- Shale -- Ordovician -- Nuclear waste repository -- Basin brine; Organic geochemistry -- Periodicals; Biogeochemistry -- Periodicals; Géochimie organique -- Périodiques; 553.205
• DOI: 10.1016/j.orggeochem.2015.03.006
• Languages: English
• ISSNs: 0146-6380

Abstract

Highlights: CH4 and He have been extracted for the first time from Ordovician shale porewaters. We identify the earliest (Paleozoic) example of authigenic archaeal methane. The Ordovician shales constitute a closed system aquiclude since Paleozoic time. Bacterial DNA has been extracted and cloned from Ordovician shales. The shale aquiclude is proposed as the barrier for a nuclear waste repository. Abstract: Interest in the origin and migrational history of methane in low permeability shales reaches beyond that of resource potential to include the evaluation of such formations as geological barriers for the purposes of isolating surface environments from the impacts of deep exploitation and the deep disposal of waste. Here, we present detailed isotope and geochemical profiles of porewaters and methane from an Ordovician shale and carbonate aquiclude on the eastern flank of the Michigan Basin, where a deep geological repository for low and intermediate level nuclear waste is proposed. The solute concentrations and stable isotopes of water (δ 18 O and δD) indicate that this aquiclude hosts saline brine (39% TDS), originating as evaporated seawater. Methane concentrations are < 4 mmol/g in the shale section, with negative δ 13 C and δD signatures that are consistent with archaeal methanogenesis and are accompanied by a well defined positive excursion in δ 13 C of CO2 . Below the aquiclude, porewaters contain lower concentrations of methane with a thermocatalytic signature, consistent with regional methane elsewhere in the Michigan Basin. No evidence for archaea was identified by microcosm experiments or through PCR rRNA analysis, suggesting that the microbial methane present in these sediments is not the result of recent microbial activity. Based on these results and considering that archaeal activity has not been observed in such hypersaline brines with low water activities (< 0.75), it is concluded that the methane likely formed prior to the infiltration of hypersaline Silurian seawater. The methane has since remained in place, making it, perhaps, the oldest documented occurrence of biogenic methane. This is consistent with helium isotope data, which also suggests authigenic production and accumulation in this aquiclude since the Paleozoic.