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ABSTRACT: Geochemical correlation of gas emissions in the Cataract River gorge, Australia

George, Simon C.1, Robert Pallasser1, Robinson A. Quezada1, Michael B. Wold2, David J. Williams3, and Jeff Wood4
(1) CSIRO Petroleum, North Ryde, Australia 
(2) CSIRO Petroleum, Melbourne, Australia (
3) CSIRO Division of Energy Technology, Sydney, Australia
(4) BHP Coal Technical Services, New South Wales, Australia

The Cataract River flows through a gorge eroded into the Hawkesbury Sandstone 60 km south-west of Sydney, Australia. During longwall mining of the underlying Bulli coal seam from Tower Colliery in 1997, gas was observed bubbling to the surface in some parts of the Cataract River. The Cataract River gas contains significant amounts of high molecular weight gas components (C2+ >2.3%), including C6-C8 n-alkanes, branched alkanes, alkylcyclopentanes and alkylcyclohexanes. In contrast the Bulli coal seam gas is very dry (C2+ <0.1%). Therefore the underlying coal seam being mined (depth below regional ground level of ~450 m) is not the direct and major source for the gas bubbling into the Cataract River. The wet gas composition and carbon isotope signature of ethane and propane in the Cataract River gas suggest that it is of thermogenic origin, although the carbon isotope signature of the methane is similar to that found in the coal seam gas. The Bulgo Sandstone (205--393 m below regional ground level) is a known gas reservoir, which at one borehole (Tower, PDH18) is also wet (C2+ 1.6--1.9%) and has a similar distribution of C6-C8 hydrocarbons to that in the Cataract River gas. This compositional data, the similarity in carbon isotopic ratios of C1-C3 hydrocarbons and the lack of significant amounts of gas in the Hawkesbury Sandstone, other than small amounts of methane, suggests that the Cataract River gas is derived from the Bulgo Sandstone, with vertical migration up structural features induced by mine subsidence.

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