Coal seam gas distribution and hydrodynamics of the Sydney Basin, NSW, Australia

• Published in: Australian journal of earth sciences Vol. 61; no. 3; pp. 427 - 451
• Main Authors: Burra, A., Esterle, J. S., Golding, S. D.
• Format: Journal Article
• Language: English
• Published: Taylor & Francis 01.01.2014
• Subjects: Basins; Brackish; Calcite; Carbon dioxide; carbonates; Coal; coal seam gas; Computational fluid dynamics; gas origin; Groundwater; hydrochemistry; hydrodynamics; Natural gas; Precipitation; Sydney Basin
• ISSN: 0812-0099; 1440-0952
• DOI: 10.1080/08120099.2014.912991

This paper reviews various coal seam gas (CSG) models that have been developed for the Sydney Basin, and provides an alternative interpretation for gas composition layering and deep-seated CO 2 origins. Open file CSG wells, supplemented by mine-scale information, were used to examine trends in gas content and composition at locations from the margin to the centre of the basin. Regionally available hydrochemistry data and interpretations of hydrodynamics were incorporated with conventional petroleum well data on porosity and permeability. The synthesised gas and groundwater model presented in this paper suggests that meteoric water flow under hydrostatic pressure transports methanogenic consortia into the subsurface and that water chemistry evolves during migration from calcium-rich freshwaters in inland recharge areas towards sodium-rich brackish water down-gradient and with depth. Groundwater chemistry changes result in the dissolution and precipitation of minerals as well as affecting the behaviour of dissolved gases such as CO 2 . Mixing of carbonate-rich waters with waters of significantly different chemistries at depth causes the liberation of CO 2 gas from the solution that is adsorbed into the coal matrix in hydrodynamically closed terrains. In more open systems, excess CO 2 in the groundwater (carried as bicarbonate) may lead to precipitation of calcite in the host strata. As a result, areas in the central and eastern parts of the basin do not host spatially extensive CO 2 gas accumulations but experience more widespread calcite mineralisation, with gas compositions dominated by hydrocarbons, including wet gases. Basin boundary areas (commonly topographic and/or structural highs) in the northern, western and southern parts of the basin commonly contain CO 2 -rich gases at depth. This deep-seated CO 2 -rich gas is generally thought to derive from local to continental scale magmatic intrusions, but could also be the product of carbonate dissolution or acetate fermentation.