Stable isotopic and molecular composition of desorbed coal seam gases from the Walloon Subgroup, eastern Surat Basin, Australia

Hamilton, S. K., Golding, S. D., Baublys, K. A. and Esterle, J. S. (2014) Stable isotopic and molecular composition of desorbed coal seam gases from the Walloon Subgroup, eastern Surat Basin, Australia. International Journal of Coal Geology, 122 21-36. doi:10.1016/j.coal.2013.12.003


Author Hamilton, S. K.
Golding, S. D.
Baublys, K. A.
Esterle, J. S.
Title Stable isotopic and molecular composition of desorbed coal seam gases from the Walloon Subgroup, eastern Surat Basin, Australia
Journal name International Journal of Coal Geology   Check publisher's open access policy
ISSN 0166-5162
1872-7840
Publication date 2014-02-01
Year available 2013
Sub-type Article (original research)
DOI 10.1016/j.coal.2013.12.003
Open Access Status
Volume 122
Start page 21
End page 36
Total pages 16
Place of publication Amsterdam, Netherlands
Publisher Elsevier
Collection year 2014
Language eng
Abstract This study used compositional and stable isotopic analysis to test hypotheses on the distribution and origins of Walloon Subgroup coal seam gas (CSG) in the eastern Surat Basin, Queensland, Australia. The Middle Jurassic Walloon Subgroup play differs from many other low-rank CSG plays-particularly in methane carbon isotopic signature, i.e., the CSG is not as 'microbial' as could be expected. The carbon isotope compositions of desorbed methane from three cored appraisal wells fall within the generally accepted range for thermogenic or mixed gas (δ13C -58.5‰ to -45.3‰). The δ13C-CH4 values from stratigraphically placed coal core samples increased (became more 'thermogenic') from the top of the upper (Juandah) coal measures to the base of the Tangalooma Sandstone. Below the Tangalooma Sandstone, in the lower (Taroom) coal measures, the δ13C-CH4 values decreased with increasing depth. These positively parabolic δ13C profiles tracked total measured gas content in two out of the three wells studied. The third well displayed lower variance of δ13C-CH4 and gas content increased uniformly with depth.A genetic classification based on methane stable carbon isotopes alone might interpret this pattern as a transition from microbially- to thermogenically-sourced methane in the central coal seams. However, a δ13C-CO2 profile for one well tracks total gas content and δ13C-CH4, and exhibits an inverse relationship with δD-CH4. These results, together with the mostly dry nature of the gas samples [(C1/(C2+C3)) ratios up to ~10,000] and relatively uniform δD-CH4 values (δD -238‰ to -202‰), suggest that microbial CO2 reduction is the primary source of Walloon Subgroup methane. As such, stratigraphic variations in gas content mainly reflect the extent of microbial methanogenesis. We suggest that peak microbial utilisation of H2-CO2 occurred at the Tangalooma Sandstone level, enriching the residual CO2 pool and derived methane in 13C. Carbon [δ13C(CO2-CH4)] and deuterium isotopic differences [δD(H2O-CH4)], and cross-plots of δD-H2O and δ18O-H2O are also consistent with kinetic isotope fractionation during microbial-mediated carbonate reduction. The results are relevant for applying microbially enhanced coal bed methane (MECoM) in the Surat Basin.
Keyword Carbon dioxide
Coal bed methane (CBM)
Coal seam gas
Methane
Methanogenesis
Stable isotope
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: School of Earth Sciences Publications
Official 2014 Collection
 
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