Observations on the distribution of coal seam gas in the Sydney Basin and the development of a predictive model

Abstract

Gas content and gas composition trends with depth have been investigated for the Sydney Basin. Four distinct zones have been identified, which can be classified according to depth below ground surface. An upper low gas zone (Zone 1, 0–100 m), dominated by CO₂ and with very low gas contents (<0.7 m³/t), is underlain by a biogenic methane-rich zone (Zone 2, 100 to ∼250 m), with a rapid rate of increase in gas content with depth, followed by a mixed gas zone (Zone 3, ∼250 to ∼600 m), comprising biogenic and thermogenic methane and magmatic CO₂, and having a lower rate of gas content increase with depth relative to Zone 2, and Zone 4 (∼600 m+), which contains thermogenic methane and other ‘wet gases.’ A model is proposed that provides a rationale for the origin and timing of emplacement of the various coal seam gases in the Sydney Basin.

本文研究了悉尼盆地的天然气含量和成分与深度的关系。四个不同的天然气带根据地下深度被确定。上低气带（第1带，0-100米），以CO₂为主，气体含量很低（<0.7立方米/吨）；其下是生物源富甲烷带（第2带，100-约250 米），天然气含量随深度增加而快速增加；再下面是混合气体带（第3带，约250-约600米），包括生物成因和热成因的甲烷和岩浆成因的二氧化碳，与第2带相比，气体含量随深度的增加速率较低；第4带（约600 M +）含热成因甲烷及其它“湿气体”。我们提出的一个模型提供了对悉尼盆地各种煤层侵位的由来及时间的合理解释。

KEY WORDS:

• coal seam gas
• Sydney Basin
• biogenic gas
• methane
• coal bed methane
• thermogenic gas
• magmatic CO₂
• tectonic history
• gas layering
• fugitive emissions

ACKNOWLEDGEMENTS

The data used in this study have been gathered from a range of sites in the Sydney Basin, much of it from mining operations. The support of these companies is acknowledged. We are also grateful to the editors and reviewers for their advice and encouragement.