The Development of an Understanding of the Origins of the Sydney and Bowen Basin Gases

It is remarkable that the origins of the coal-associated gases in the Sydney and Bowen Basins should have been so generally and consistently misunderstood. These gases occur at depths to some 600 m, vary widely in composition and most commonly occur as mixtures of very dry methane with carbon dioxide in all proportions. The origins of these gases remain in doubt, although a thermal maturation mechanism was initially agreed. Up-lifting and erosion have resulted in the loss of the bulk of this early-formed wet thermogenic gas. It has been almost completely displaced by very dry methane and now remains as traces in seam-enclosing sandstones and in the deepest coal measures. A biogenic origin, largely via carbon dioxide reduction, is often assigned to gases of this type. However in these Basins higher temperatures associated with hydrothermal activity, and anomalies in the behaviour of carbon dioxide, appear likely to curtail biogenic activity and suggest alternative gas sources.