Regional coal seam gas distribution and burial history of the Hunter Coalfield, Sydney Basin

Abstract

Basin modelling has been used to improve understanding of the origin and temporal evolution of coal seam gas in the Hunter Coalfield of the Sydney Basin. Burial history models were produced based on data from seven boreholes located in the southern, eastern, central and western areas of the coalfield. Mean random vitrinite reflectance (R_v,r) data, derived from measurements of mean maximum reflectance (R_v,max), were used for calibration of the models. A qualitative sensitivity analysis of one model shows that varying the paleoheat flow has a greater influence on calculated R_v,r than varying the eroded overburden thickness.

The differences between the constructed models are significant enough to provide plausible explanations for regional gas distribution in the Hunter Coalfield. Coals in the south of the coalfield appear to have the greatest potential for thermogenic gas generation. Modelling has shown that areas that have low gas contents and decreased permeability have been uplifted more, and buried less, compared with areas that have high gas contents. Burial history modelling shows noticeable variations in the extent of burial and uplift, and, consequently, in thermal maturities and potential for thermogenic gas generation; together with the assessment of other coal and gas property data, it appears that present-day gas contents may partially reflect coal ranks and adsorption capacities, with late-stage biogenic gas generation replenishing CH₄ volumes that were lost following uplift during the Late Cretaceous.

盆地模拟已被用于改善对悉尼盆地的猎人煤田中的煤层天然气的起源和时间演变的理解。根据位于煤田的南部、东部、中部和西部地区的7个钻孔数据，我们制作了埋藏史模型。平均随机镜质体反射率（R_v,r）数据，得自平均最大反射率（R_v,max）的测数，被用于模型校准。对一个模型的定性敏感性分析显示，改变古热流对计算出的R_v,r的影响比改变侵蚀覆盖层厚度的影响更大。所构建的模型之间的差异足以显著到可以提供对天然气区域性分布的合理解释。在煤田南部的煤似乎有最大的热成因天然气发电的潜力。模拟已经表明，比起具有高气体含量的区域，具有低气体含量、通透性降低的地区抬升更多，埋葬更少。埋藏史模型显示出明显的埋葬和抬升程度的变化，因此也显示出热成熟度和产生热成因天然气的潜力变化；根据其它煤与天然气属性数据的评估，当今的气体含量似乎可以部分反映了煤炭等级和吸附能力；晚期生物成因天然气生成作用补充了晚白垩世隆起之后失去的CH₄量。

KEYWORDS:

• burial history
• thermal maturity
• Hunter Coalfield
• Sydney Basin
• coal seam gas
• reservoir properties

ACKNOWLEDGEMENTS

The author wishes to thank the coal mine operator for supplying the data used to conduct this study, as well as the NSW Department of Trade and Investment for assistance and consultation during public data collection and processing. The CSIRO is thanked for the continued support and resources provided for the study. Dr David Whitford and Dr Neil Sherwood from the CSIRO Division of Earth Science and Resource Engineering, and the reviewers, are also thanked for their valuable comments on the manuscript.

SUPPLEMENTARY PAPERS

Table A1 Main model inputs for Knight-1.

Table A2 Main model inputs for Pinegrove-1.

Table A3 Main model inputs for DH219.

Table A4 Main model inputs for DH225.

Table A5 Main model inputs for EHL-1.

Table A6 Main model inputs for EHRP-1.

Table A7 Main model inputs for Wybong-1.