Understanding the geometry and distribution of fluvial channel sandstones and coal in the Walloon Coal Measures,Surat Basin,Australia

Understanding the controls on coal seam distribution and geometry is fundamental for planning coal seam gas production. In the Jurassic Surat Basin of South East Queensland, Australia, the spatial continuity of coal seams in the Walloon Coal Measures is highly variable and often difficult to map and predict, even with closely spaced (<1000 m) drillings. This paper investigates the frequency and location of thick sandstone in relation to thick coal seams or plies across three broad stratigraphic divisions, Upper Juandah (UJ), Combined Lower Juandah-Taroom (CLJT) and Condamine Coal Measures (CCM), within the Walloon Coal Measures. Basic depositional facies, e.g. channel, floodplain, marginal mire, and coal mire, were interpreted from geophysical logs. An in-house code was used to count the number of coal plies thicker than 2 m, and channel sandstones thicker than 5 m for the UJ and CLJT and 3 m for the CCM at each borehole. Isopleth maps of the numbers of both coal plies and channel sandstones were generated across the basin for the three subdivisions. Results show that there is an upward stratigraphic trend from thick to thin, and then to thick stacked coal plies. This corresponds to a similar vertical thickness change in channel sandstones. The incidence of thick coal and thick sandstone is associated with rising base level within an early transgressive systems tract which was followed by a high stand prior to a regional erosive event above the UJ. Thick and stacked coal plies have a marked tendency to occur in belts adjacent to the thick channel thoroughfares in the basin.