Coal architecture, high-resolution correlation and connectivity: new insights from the Walloon Subgroup in the Surat Basin of SE Queensland, Australia

Abstract

The Middle Jurassic (Aalenian–Oxfordian) Walloon Coal Measures are the focus of major coal seam gas projects in the Surat Basin, SE Australia. The ever-growing abundance of well data linked with ongoing coal seam gas exploitation has provided an opportunity to investigate coal body lateral extents and their associated geometries. From 120 wells over a 67.4 km 2 area, 25 coal horizons were correlated establishing a local coal seam area–thickness relationship. Individual plies (coal beds) average less than 30 cm and seams reach up to 10 m thick. In this heterogeneous succession, it is evident that coal correlation does not necessarily equate to coal continuity/connectivity as coal quality locally degrades from coal- to clay-rich facies. During a QGC Pty Ltd gas storage pilot test, offset well pressure response to gas injection provided evidence for coal connectivity and local faulting. By studying the relationships of the coals and interrelated facies, fluctuations in field-scale (local) accommodation space generation can be recognized. These intuitively convey information about the scale of the coal bodies, their possible correlatability and, ultimately, their connectivity. The presented methodology – specifically, the generation of areal extent v. thickness graphs and maps of coal heterogeneity – could be carried out in other basins to better understand the variability in coal body lateral extent, geometry and connectivity (connected permeability). In the context of this coal seam gas project, our workflow can be a useful tool for static reservoir characterization and selection of optimal well spacing; suggestions that it may also be useful for dynamic characterization will require further studies to assess.