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

Abstract

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 CO₂ 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 CO₂. Mixing of carbonate-rich waters with waters of significantly different chemistries at depth causes the liberation of CO₂ gas from the solution that is adsorbed into the coal matrix in hydrodynamically closed terrains. In more open systems, excess CO₂ 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 CO₂ 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 CO₂-rich gases at depth. This deep-seated CO₂-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.

本文综述了专为悉尼盆地建立的多种煤层气（CSG）模式，并提供了气体成分分层和深层CO₂起源的另一种解释。CSG矿井资料及矿床规模信息，被用来研究从盆地边缘到中心部位的气体含量和组成的发展趋势。区域范围存在的水化学资料和流体力学的解释与传统的石油钻井孔隙度和渗透率资料结合起来。本文提出的合成气和地下水模型显示，在静水压力下的雨水水流将产甲烷流体运移到地下，在内陆补给区含钙丰富的淡水往富钠的苦咸水的流动过程中，水化学随着向下梯度和深度而产生变化。地下水化学变化导致矿物的溶解和沉淀并影响溶解气体如CO₂的行为。富碳酸盐水与深部化学物质显著不同的水的混合引起CO ₂气体从该溶液中释放出来，被吸附到流体动力闭合地体内的煤基质中。在更开放的系统中，地下水中多余的CO₂的（以碳酸氢盐为载体）可能会导致方解石沉淀在主地层中。因此，在盆地中部和东部地区不承载空间上广布的CO₂气体积累，却经历了更广泛的方解石成矿作用，气体成分以包括湿气体的烃类为主。在盆地的北部，西部和南部，盆地边界区（通常是地形和/或构造上的制高点）通常在深部包含富CO₂气体。这种深部富CO₂气体一般认为源自局部范围至大陆范围的岩浆侵入，但也可能是碳酸盐溶解或醋酸发酵的产物。

KEYWORDS:

• Sydney Basin
• coal seam gas
• gas origin
• hydrodynamics
• hydrochemistry
• CO₂
• carbonates

ACKNOWLEDGEMENTS

The authors would like to thank Australian Coal Association Research Program (ACARP) (project C21061) for the sponsorship of this research. We would also like to acknowledge and thank J. Sandford of Glencore (formerly Xstrata Coal) for his support throughout the project, as well as C. Holmes (AGL) and M. Creech (SRK) for valuable discussions that contributed to the refinement of this work. We would also like to thank C. Ward, M. Faiz and an anonymous reviewer for very helpful and constructive feedback on the manuscript.