CO2 reduction and fermentation producing in situ CH4 in the majority of sampled GAB aquifers and alluvium overlying a coal seam gas field

Pearce, Julie1,2, Golding, Sue2, Baublys, Kim2, Hofmann, Harald2, Herbert, St.John3, Hayes, Phil1

1University Of Queensland, Brisbane, Australia, 2Arrow Energy Pty Ltd, Brisbane, Australia

Understanding the origin and source of gas in aquifers with multiple users is of increasing importance.  The Walloons coal seams are a major coal seam gas (CSG) resource in Queensland, where various shallower and deeper aquifers are part of the Great Artesian Basin (GAB).  Dissolved gases and waters were sampled from water bores in the Gubberamunda, Mooga, Orallo, Hutton, Precipice and Springbok Sandstones, the Condamine Alluvium, Walloons bores, and also CSG wells.  The majority of δ13C and δ2H of CH4 and CO2 sampled from shallow aquifer bores indicated in situ primary microbial CO2 reduction, with three water bores in the Gubberamunda, Mooga, and Walloons plotting in the fermentation pathway region.  CSG wells and a gassy Springbok bore however plotted in the secondary microbial region, typical of biogenic CSG.  The majority of the Condamine Alluvium bores sampled had very low CH4 concentrations, with δ13C-CO2 in the range -17 to -21 VPDB‰, and δ13C-DIC +2.2 to -13 ‰.  The range of δ13C-DIC from CSG wells (+16 to +19 ‰) was typical of methanogenesis.  Stable isotopes of water encompassed a wide range, with Alluvium samples and one Walloons water bore more enriched.  A gassy Springbok bore, CSG wells, and deeper bores had depleted values consistent with recharge during colder climates or greater impact from microbial-water-rock reactions.  Strontium isotopes of aquifer waters were mainly more radiogenic and distinct from CSG waters indicating dis-connectivity in the majority of cases.  Results so far suggested that CH4 was formed in situ (rather than leakage) in the majority of our samples, however in a few cases sources could not be determined.  Analysis of a subset of samples for δ14C, 36Cl, 34S, and CH4 clumped isotopes are also in progress.


Biography

Dr Julie Pearce is a geochemist with international experience in the UK, Japan, and Australia on interdisciplinary projects.  She is currently working on field monitoring techniques for measurement of methane and understanding its sources through isotopic techniques, in addition to CO2 storage, and geochemical processes in gas and oil shale. 

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