The Onshore Energy Systems Group has undertaken a regional study on the prospectivity of the southern Georgina Basin, and herein present a synopsis of the initial results from this multidisciplinary project.
The Georgina Basin is a Neoproterozoic to Lower Devonian sedimentary basin covering 325 000 km2 of western Queensland and the Northern Territory (Dunster et al 2007, Kruse et al 2013, Munson 2014). It is a northwest-southeast-trending extensional basin where thick marine Cambrian and Ordovician sediments are preserved in its two southern depocentres, the Dulcie and Toko synclines, with a thinner succession present in the Undilla Sub-basin to the northeast. Within these depocentres, the Thorntonia Limestone and Arthur Creek Formation (Figure 1) contain potential source rocks (Ambrose et al 2001, Boreham and Ambrose 2012). Most of the southern Georgina Basin is under license for petroleum exploration, with explorers targeting the carbonate-dominated Arthur Creek Formation for both conventional and unconventional hydrocarbons.
A data package that includes raw and reprocessed HyLogging data from 25 wells in the Georgina Basin was recently released (Smith and Huntington 2014). The HyLogging data map the mineralogical variations within formations and were used in conjunction with wireline log and biostratigraphic data to refine stratigraphic correlation. The HyLogging data were re-processed using a common set of mineral scalars (i.e., spectroscopic indices) to create an internally-consistent, basin-wide dataset. Other datasets, including total organic carbon (TOC) content, X-ray diffraction (XRD) measurements and biostratigraphy were also integrated with the Hylogging data. The 'hot shale' of the Arthur Creek Formation has a characteristic spectral response of decreasing albedo and an increased short wave infra-red (SWIR) aspectral response with increasing depth to the base of the Arthur Creek Formation (Figure 2). Both features appear to correlate with increasing core total gamma and TOC. These inter-relationships may be used to better characterise and identify potential source rock units in the basin.
Recent biostratigraphic work has highlighted an age discrepancy in the prospective organic-rich 'hot shale' in the base of the middle Cambrian Arthur Creek Formation (Figure 1). This unit is present in the two major southern depocentres, the Dulcie and Toko synclines, where it has previously been considered as correlative. Recent results, however, suggest that the basal 'hot shale' is either significantly younger in the Toko Syncline than in the Dulcie Syncline, or represents a condensed section in the former.
Middle Cambrian carbon isotope excursions have been correlated across a number of Australian basins and can be used to test correlative models across the Georgina depocentres. High resolution sampling across this middle Cambrian section has been carried out in a number of wells in the Dulcie Syncline and in the Undilla Sub-basin, where the age equivalent Inca Shale is penetrated. Carbon isotopes from organic carbon (kerogen) as well as carbon and oxygen isotope ratios of four carbonate mineral phases (calcite, ankerite, dolomite and siderite) were analysed and these data are compared with existing carbon isotope stratigraphy published from neighboring wells (Donnelly et al 1988, Lindsay et al 2005, Creveling et al 2013). Initial results corroborate the new biostratigraphic interpretation.
