Stratigraphic and depositional framework of the Walloon Subgroup, eastern Surat Basin, Queensland

Abstract

The Middle Jurassic Walloon Subgroup is a prolific coal seam gas (CSG) resource in the Surat Basin, Queensland. Sedimentary framework models constrain stochastic reservoir models of the geological heterogeneity, but there is limited basin analysis information in the public domain. Here we present a regionally consistent stratigraphic framework model for the Walloon CSG play in the eastern Surat Basin. Lithostratigraphic correlation of open-file industry and government wireline logs supports the interpretation of six subunits in the eastern Surat Basin (oldest–youngest: Durabilla Formation; Taroom Coal Measures; Tangalooma Sandstone; and Juandah Coal Measures, informally divided into three members named the lower Juandah Coal Measures, Juandah sandstone and upper Juandah Coal Measures). Important findings are that subunits within the Walloon Subgroup do not correlate along the entire CSG play area; in many places, the overlying Springbok Sandstone (Upper Jurassic) has incised to the lower Juandah Coal Measures level, removing the upper coal seam groups. The Walloon Subgroup thins to the south through a combination of depositional thinning and truncation. Lithofacies analysis and isopach maps support deposition in a southerly prograding fluvial system or clastic wedge. This stratigraphic and depositional interpretation informs models for hydrogeological studies of the Walloon Subgroup and underpins a regional assessment of controls on microbial methane distribution.

中侏罗世Walloon亚群是昆士兰州苏拉特盆地的一个多产煤层气（CSG）源地。沉积框架模型控制着地质异质性的随机油藏模型。但在公共领域，盆地分析信息有限。在这里，我们为苏拉特盆地东部Walloon CSG提出了一个区域一致的地层框架模型。文件公开的行业和政府电缆测井资料的岩石地层对比支持对苏拉特盆地东部六个亚单元的解释（从老到新为：Durabilla 组、 Taroom煤系、 Tangalooma 砂岩以及 Juandah煤系。后者非正式地分为三部分，即下 Juandah 煤系、 Juandah 砂岩和上 Juandah 煤系）。重要的发现是，Walloon亚群的亚单元沿整个CSG区并不能相对比;在很多地方，上覆Springbok砂岩（上侏罗统）已切割到下Juandah煤系，除去了上煤系群。Walloon亚群由于沉积薄化和截断的综合作用而向南变薄。岩相分析等厚图支持在向南推进的河流系统中的沉积作用或碎屑楔。这种地层和沉积的解释为Walloon亚群的水文地质模型研究提供信息，为控制微生物甲烷分布的区域评估起关键的作用。

KEYWORDS:

• Surat Basin
• Jurassic
• Walloon Subgroup
• stratigraphy
• basin analysis
• paleogeography

ACKNOWLEDGEMENTS

This research is part of a broader program investigating the controls on spatial variability of methanogenesis in the Walloon Subgroup. Many thanks to Robert Langford and Morteza Jami for their helpful comments on review. S. K. Hamilton wishes to thank John McKellar, Liam Hogan, Lesley Blight and Gina Nuttall (Geological Survey of Queensland) for assistance drafting the stratigraphic columns. Funding was obtained from the Queensland Government Healthy Headwaters Program for regional lithofacies modelling. Numerous discussions with colleagues have contributed to ideas in this paper, and it will hopefully stimulate more targeted research and publications for the Surat Basin.