COAL SEAM GAS IN THE SOUTHERN SYDNEY BASIN, NEW SOUTH WALES

Abstract

The coal seam gas content of the Late Permian Illawarra Coal Measures ranges from <1 m3/t to 20 m3/t (cubic metres per tonne). The gas mainly comprises CH4 and CO2 with subsidiary amounts of higher hydrocarbons and N2. The variations in gas composition are mainly related to geological structure and depth. Gas in the structural lows is dominantly CH4 whereas the percentage of CO2 increases towards structural highs with the highest con-centrations occurring in anticlines and near some faults. However, in some structural lows local pockets of CO2 occur adjacent to dykes. Furthermore, for most bore-holes, the volume of CO2 markedly increases at shallow depths.

Methane that occurs within the basin was mainly derived as a by-product of coalification. Most of the CO2 was derived from intermittent magmatic activity between the Triassic and the Tertiary. This gas has subsequently migrated, mainly in solution, towards structural highs and accumulated in anticlines and near sealed faults.

The total desorbable gas content of the coal seams is mainly related to depth, gas composition and geological structure. At depths <200 m the gas content is very low due to severe degassing of the coal seams. At greater depths, on average, the gas content increases by 4 m3/t per 100 m increase in depth up to 600 m before reaching a near maximum. The increase in gas content with depth is the result of increase in pressure and coal rank. Anomalously high gas contents occur in areas where the gas is high in CO2, such as in anticlines whereas depleted gas contents occur near some open faults.