Sedimentological analysis of the subsurface Mulga Downs Group in the central part of the Darling Basin, western New South Wales

Abstract

Detailed sedimentological analysis, using lithological samples (cores and cuttings) and wireline-log information, has been carried out on the Devonian subsurface strata of the Darling Basin, to aid in the development of a geological model for the Mulga Downs Group in the Blantyre, southern Pondie Range and western Neckarboo Sub-basins. The Mulga Downs Group displays five different lithofacies successions, which are visible in both vertical and horizontal sections. The lower part of the Mulga Downs Group (Snake Cave Interval) is a succession composed mainly of braided fluvial and meandering fluvial lithofacies, with minor fluvial–shallow lacustrine complex lithofacies. The upper part of the Mulga Downs Group (Ravendale Interval) contains braided fluvial and meandering fluvial lithofacies passing upwards into estuarine tidal channel deposits and a nearshore lithofacies complex contemporaneous with a nearshore braided-deltaplain complex lithofacies. The results of this study, which included core and cutting descriptions and geophysical log (mainly gamma-ray logs) correlations, and comparisons with other large sandstone- and siltstone-rich sediment-supply fluvial systems in the subsurface data in the central part of the Darling Basin, suggest that the Mulga Downs Group has excellent reservoir potential and stratigraphic-trap configuration.

Key Words:

• Blantyre Sub-basin
• Darling Basin
• Devonian
• fluvial sediments
• geophysical log data
• lithofacies
• Mulga Downs Group
• Ravendale Interval
• Snake Cave Interval

Acknowledgements

This paper represents part of a MKK's PhD thesis completed at the University of New South Wales. Financial support was provided by grant number 614/2000 from the Libyan Government, through the Department of Geology, 7th April University in Zawia, western Libya. Thanks are also expressed to the New South Wales Department of Primary Industries Mineral Resources Division (Coal & Petroleum Development) for suggesting the research, and for provision of the core and cuttings samples, wireline logs, seismic data and well completion reports. Thanks are also due to our colleagues Fethi Ahmed and Phillip Cooney for their help, and Gerrit Neef for providing advance copies of papers on other geological aspects of the study area. Thanks also to Grant-Mackie for raising some useful issues in his review of this manuscript. Finally, we thank journal reviewers Brian Stevens and Kingsley Mills for important suggestions that helped to improve the manuscript.