The Georgina Basin is a Neoproterozoic to Lower Devonian sedimentary basin covering 325,000 km² of western Queensland and the NT. It is a northwest-southeast-trending extensional basin, with prospective conventional and unconventional hydrocarbon targets in Cambrian and Ordovician carbonate and siliciclastic rock units. The unconventional gas and oil potential of the basin has led to recent exploration interest, although the basin has been relatively less explored in the past. At the southern end of the basin, depocentres contain up to 2.2 km of Cambrian to Devonian sedimentary rocks, overlying Neoproterozoic sedimentary rocks more than 1.5 km thick. The basin succession thins toward the north, where Cambrian sediments overlie the McArthur Basin sediments in the Beetaloo Sub-basin. Biostratigraphic interpretations of the prospective southern, central and eastern regions of the basin have been revised to reflect the 2012 Geological Time Scale (Gradstein, Ogg, Schmitz, and Ogg, 2012), resulting in an updated chronostratigraphic framework for the basin. The revised biostratigraphic interpretations have implications for important hydrocarbon source rocks. For example, the limestone unit in the southern parts of the basin, generally regarded as the Thorntonia Limestone, is of a different age to the type section for this unit, located in the Undilla Sub-basin. Additionally, the basal hot shale of the Arthur Creek Formation is diachronous across the Dulcie and Toko synclines, which may have ramifications for hydrocarbon exploration. This revised chronostratigraphic framework (by Geoscience Australia) for the Georgina Basin provides a baseline for the first basin-wide assessment of the unconventional hydrocarbon potential of the basin. Abstract prepared for the APPEA 2013 Conference & Exhibition Citation: Smith, T.E., Kelman, A., Laurie, J.R., Nicoll, R.S., Carr, L.K., Hall, L., Edwards, D. 2013. An updated stratigraphic framework for the Georgina Basin, Northern Territory and Queensland. The APPEA Journal 53(2) 487-487, Conference Proceedings, extended abstract and poster. https://doi.org/10.1071/AJ12098

Small angle neutron scattering (SANS) and ultra-small angle neutron scattering (USANS) are used to directly detect the processes of hydrocarbon generation in the 10 nm to 10 μm size pores in carbonate and siliciclastic rocks which contain no land-plant material suitable for conventional maturity determination by vitrinite reflectance. The method takes advantage of the pore-size-specific variation of neutron scattering contrast between the solid rock matrix and pore-space content with depth, which is caused by thermal maturation of organic matter through the oil and gas generation windows. SANS and USANS measurements were performed on bedding plane-orientated core slices, extracted from a series of 10 to 12 depth intervals for three wells, CKAD0001, MacIntyre 1 and Baldwin 1 in the southern Georgina Basin, central Australia. The depth intervals, intersecting the organic-rich basal ‘hot’ shales of the middle Cambrian Arthur Creek Formation, were selected based on Rock-Eval pyrolysis data. SANS and USANS results indicate that oil generation has occurred in the past in nano-sized pores in rocks that are now at depths of around 538.4 m in CKAD0001 and 799.3 m in MacIntyre 1. Furthermore, in the CKAD0001 well, the oil-wet pores extend into the larger pore-size range (at least up to 10 μm) due to the efficient expulsion of oil. At around 880 m in Baldwin 1, the influence of pyrobitumen reverts pore space from gas wet to oil wet. These hydrocarbons have remained in situ since the Devonian when the Neoproterozoic to Paleozoic section was exhumed in the Alice Springs Orogeny and subsequently eroded, preserving only remnants of the once extensive basin sediments.

The Georgina Basin is a Neoproterozoic to Lower Devonian sedimentary basin covering 325,000 km² of western Queensland and the Northern Territory. It is a northwest-southeast-trending extensional basin, with prospective conventional and unconventional hydrocarbon targets within Cambrian and Ordovician carbonate and siliciclastic rock units in the southern depocentres. 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. 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 very 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. In the current study, 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. These new data are compared with the existing carbon isotope stratigraphy from the Dulcie and Toko synclines. Initial results corroborate the new biostratigraphic interpretation. This work will provide a detailed understanding of middle Cambrian isotope signatures and correlate this prospective unit across the southern Georgina Basin. Abstract presented at AAPG/SEG International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015