ABSTRACT: Rare gas and stable isotopes, and the origin of carbon dioxide-rich natural gases from the Cooper Basin, Australia

Wycherley, Helen¹, Christopher Ballentine², Andrew Fleet¹, and Harry Shaw³
(1) The Natural History Museum, Imperial College, London, England 
(2) ETH, Zurich, Switzerland 
(3) Imperial College, London, England

Rare gas isotopic (He, Ne, Ar) and stable carbon isotopic abundances have been measured on carbon dioxide-rich gases from the Cooper Basin in order to identify the source of the CO₂. The Permian age Cooper Basin has undergone a complex history of structural deformation and has experienced periods of both extension and compression. The gas reservoirs studied range in depth from 1900m to 3000m and are hosted in three fluvio-deltaic, coal-bearing sandstone sequences.

Methane is present (up to 80 vol. %) and has a d13C_CH4 signature of -42 to -28o/oo PDB. Such isotopic values indicate a thermogenic source for the methane, with the thermal cracking of coal being the most likely source.

Stable carbon isotopes suggest that large volumes of CO₂ (up to 50 vol. %) are multisourced. Isotopic signatures range from -4 to -15o/oo d¹³C_co2 PDB. CO₂ could be sourced from Permian age coal sequences, as well as from igneous sequences. Carbon isotope ratios have been interpreted with rare gas ratios to distinguish these sources.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia