Is that your methane? Greenhouse gases in aquifers and alluvium of the Great Artesian Basin near a coal seam gas reservoir and potential CO2 storage site

Abstract

The Surat Basin hosts various industries that extract groundwater including coal seam gas (CSG), feedlots, and agriculture.  With water drawdown, gas has been observed in some aquifer groundwater bores.  While methane can occur naturally in aquifers, biogenic CSG has been extracted from the Walloon Coal Measures raising questions on the sources of gas in aquifers.  Current standard monitoring uses a direct fill approach to measure dissolved methane concentrations in vials, however, this approach may lose gas present above solubility levels.  Water and gas sampling was performed on bores mainly in the Springbok, Gubberamunda, and Mooga Sandstones, the Orallo Formation, and the Condamine Alluvium.  Water bores and CSG production wells from the Walloon Coal Measures were also sampled.  We compared direct fill to a closed sampling method for dissolved gas, plus a method for sampling the total free and dissolved gas in the field.  Higher dissolved and total methane concentrations were measured using closed sampling methods, especially in gassy bores.  The majority of sampled shallow aquifer gases and waters have stable isotopic signatures distinct from CSG, where methane had likely been formed in situ in shallow aquifers by primary microbial CO2 reduction or fermentation processes.

CO2 geological storage is also being assessed in the Precipice Sandstone, where an understanding of the hydrogeochemistry and baseline conditions is important.  Many of the native groundwaters sampled from the Precipice Sandstone and Hutton Sandstone were visibly gassy.  Dissolved methane ranged from <1 to 27.7 mg/l measured from vials (open sampling method), and <1 to 2100 mg/l when measured via a closed sampling method.  Dissolved methane and carbon dioxide from the Precipice Sandstone bores mainly have a mixed origin isotopic signature, with δ13C-DIC (5.8 to -5.6 ‰) also indicating methanogenesis.  The Moonie oil field samples have a mixed thermogenic signature.  Hutton Sandstone dissolved gases, however, mainly indicate in situ primary microbial CO2 reduction producing methane.  Depth trends exist for several of the gas and DIC stable isotopes.  Water stable isotopes in these two aquifers span wide ranges and are more depleted than modern rainfall, however noble gases indicate that recharge was not during colder climates.  Recharge from major monsoonal events moving overland from northern Australia may be more likely. 

Biography

Dr Julie Pearce is a geochemist with international experience in the UK, Japan, and Australia on interdisciplinary projects.  She is currently with the University of Queensland Centre for Natural Gas, and School of Earth and Environmental Sciences.  Pearce is an expert on gas-water-rock interactions with a focus on the Surat Basin, QLD, Australia, and geochemical processes in gas and oil shales. She is currently working on field monitoring techniques for measurement of gases and waters, and understanding processes through geochemical and isotopic techniques.  She has collaborated in research projects with the gas and carbon storage industries, and provided expert opinion to the Queensland Government.  Pearce has published 40 journal articles including three in the top 3% cited in geosciences, over 35 research reports, and has gained over $ 4.5 million in industry focussed and ARC funding.  

Venue

Room: 
275, Global Change Institute (#20). Additionally, this seminar will be streamed using the video conferencing software, Zoom. If you would like the link, please email sees.seminars@uq.edu.au.

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