Regional insights into the sedimentary organisation of the Walloon Subgroup, Surat Basin, Queensland

Abstract

Despite more than a decade of intensive coal seam gas exploration and development in the Surat Basin, fundamental aspects surrounding the sedimentary organisation of the Walloon Subgroup remain unresolved. While generally agreed to have been deposited on a waterlogged alluvial plain, contradictory paleogeographic models describe the Walloon Subgroup as an internally draining fluvio-lacustrine system or, alternatively, as a southeast-prograding distributive trunk drainage system. The former invokes a radially organised fluvial catchment feeding a basin depocentre, suggesting that rivers within the study area flowed from east to west. In contrast, a prograding trunk drainage system assumes an axial system flowing southeastward towards a distant paleocoastline. Without a unifying conceptual model describing the organisation of the Jurassic depositional system, predictions about the distribution of alluvial subenvironments within the Surat Basin are difficult. This study tests previous hypotheses surrounding the Walloon Subgroup's sedimentary organisation by reconstructing regional variations in alluvial architecture, fluvial styles and sediment dispersal patterns. The workflow integrates borehole image log interpretation, wireline log analysis, core description and numerical modelling to explore the character and occurrence of subenvironments throughout the basin. Synthesis of interpretations suggests a composite meandering to anastomosing fluvial system containing both a south-oriented trunk drainage component and tributary rivers feeding the axial system from the east.