In 2014, the surprise discovery of oil in the well Phoenix South 1 on Australia’s North West Shelf (NWS) heralded the Bedout Sub-basin as Australia’s new petroleum province. The well had initially targeted gas in Triassic reservoirs of the Keraudren Formation and Locker Shale but unexpectedly encountered oil instead. This has fuelled a revival of exploration in the area with subsequent wells Roc 1, Roc 2, Phoenix South 2 and Dorado 1 all successful in penetrating hydrocarbon columns. The Phoenix South 1 accumulation is significant as it demonstrates the occurrence of an effective oil-prone source rock within the Lower–Middle Triassic for the first time on the NWS. It also raised the possibility that a source rock analogue to the Hovea Member of the Lopingian–Lower Triassic Kockatea Shale, a well-established source of oil and gas in the northern Perth Basin, may also be effective on the NWS. In order to understand the origin of the Phoenix South 1 accumulation, its geochemical composition was compared to those of Triassic-sourced oils from the Perth Basin and petroleum fluids from the neighbouring Browse Basin, and Beagle and Dampier sub-basins.

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Program and Abstracts from the conference

The Roebuck Basin on Australia’s offshore north-western margin is the focus of a regional hydrocarbon prospectivity assessment being undertaken by the Offshore Energy Systems Section. This offshore program is designed to produce pre-competitive information to assist with the evaluation of the hydrocarbon resource potential of the central North West Shelf and attract exploration investment to Australia. The recent oil and gas discoveries at Phoenix South 1 (2014), Roc 1 (2015-16), Roc 2 (2016), Phoenix South 2 (2016), Phoenix South 3 (2018), Dorado 1 (2018) and Dorado 2–3 (2019) in the Bedout Sub-basin demonstrate the presence of a petroleum system in Lower Triassic strata (Grosjean et al., 2021; Rollet et al., 2019). As part of this program, a range of organic geochemical analyses were acquired on petroleum fluids from the Dorado 1 and Roc 2 wells with these data released in this report.

<div>Throughout geological history, marine organic-rich shales show variable but appreciable enrichment in uranium (U), < 5 to > 500 ppm. Here we report the results of high-energy resolution fluorescence detection (HERFD) x-ray absorption spectroscopy at U L3 and M4 edges to characterize U speciation in marine sediments.</div><div><br></div><div>We characterised U oxidation state in samples from the Cretaceous Toolebuc Formation of the Eromanga Basin, Australia. Nine samples were carbonaceous shales with high total organic carbon (TOC) content of 5.9 to 13.4 wt&nbsp;% and with low maturity organic matter. Two samples of coquinite were selected for comparison (TOC 0.3 and 2.4 wt %).</div><div><br></div><div>Our results suggest that a significant proportion of U in marine black shales (~20 to 30%) exists as U(VI) (Figures 1-2), despite the extremely reducing (anoxic to euxinic) conditions during sediment precipitation and diagenesis. Within individual samples, spot analyses indicate variation in the estimated oxidation state within a range of ~20% of U(VI). Uranium is unevenly distributed at mm to nanoscale. Nanoscale secondary ion mass spectrometry (NanoSIMS) reveals different associations that often coexist in single samples; nano-particulate uranium is associated with organic matter matrix or sulphide minerals, whereas phosphate minerals display diffuse uranium enrichment. The coquinite has a higher proportion of U(VI), consistent with the dysoxic depositional environment (Boreham and Powell, 1987).</div><div><br></div><div>The unexpectedly enhanced proportion of U(VI) relative to U(IV) within marine organic-rich shales implies that U might not be immediately fixed by reduction processes during sedimentation, but adsorbed by accumulating organic matter, at least in part as U(VI). This is consistent with the behaviour of uranium reported within the water column of the anoxic Black Sea (Anderson, 1989), experiments on U(VI) sorption by organic matter (e.g., Bhat et al., 2008), and previously documented redox state of U from continental organic-rich Eocene (56-34 Ma) sediments of paleochannel and lacustrine origin (Cumberland et al., 2018).</div><div><br></div><div>The results are significant for improving hydrocarbon exploration in known fields (covering the gap to a carbon-free economy without development of new greenfield oil provinces); economic geology (uranium, base-metal, and critical-metal deposits); and environmental management (evaluating potential mobilization of U by groundwaters).</div><div><br></div>This Abstract was submitted and presented to the 2023 Goldschmidt Conference Lyon, France (https://conf.goldschmidt.info/goldschmidt/2023/meetingapp.cgi)

<div>Scientific studies undertaken on core from the Barnicarndy 1 well drilled in 2019 in the onshore Canning Basin in Western Australia as part of the Exploring for the Future program have shown that the well penetrated a thick section of the early Ordovician Nambeet Formation which contains abundant fossils reflective of deposition in an open marine environment. Although the calcareous shales are organically poor (average total organic carbon content 0.17 wt%) processing of 42 drill core samples recovered a plethora of acid-resistant, organic-walled microfossils. Seven core samples with the highest organic content were analysed for their molecular (biomarker) fossils and stable isotopic composition to provide insights into the type of organic matter preserved, and the redox conditions of the sediments during deposition.</div><div><br></div>This Abstract was submitted/presented to the 2022 Australian Organic Geochemistry Conference 27-29 November (https://events.csiro.au/Events/2022/October/5/Australian-Organic-Geochemistry-Conference)

This presentation describes source rock data in addition to new compositional and stable isotopic data for 17 oils and six gases from the Amadeus Basin, central Australia. All data was analysed and interpreted by Geoscience Australia. The results of this study provide new insights into the origin and preservational history of hydrocarbons in the basin and provide a platform for future research directions.

The hydrocarbon generative potential and the thermal maturity of source rocks in the offshore northern Perth Basin was reassessed based on existing and new geochemical data to get a better understanding of the basin's prospectivity. The study establishes for the first time that the main source of onshore accumulations, the Late Permian-Early Triassic Hovea Member, is well developed offshore and contains organic-rich sediments of oil-prone character. This finding shatters the long-held view that the Hovea Member was either absent or of poor quality offshore and provides a new perspective on the basin's prospectivity. The source potential of the Hovea Member varies spatially with best source rocks observed in the Beagle Ridge and Central Abrolhos Sub-basin. The Late Permian Irwin River Sequence and several Jurassic Sequences are also identified as prime potential source rocks offshore, mostly for their gas-generative potential. Oil-generative potential was identified in the Middle to Late Jurassic Yarragadee Sequence and possibly in the Middle Jurassic Cadda Sequence.

<b>Legacy service retired 29/11/2022</b> This is an Open Geospatial Consortium (OGC) web service providing access to Australian onshore and offshore borehole data conforming to the GeoSciML version 4.0 specification. The borehole data includes Mineral Drillholes, Petroleum Wells and Water Bores along with a variety of others types. The dataset has been restricted to onshore and offshore Australian boreholes, and bores that have the potential to support geological investigations and assessment of a variety of resources.

A comprehensive geochemical program was carried out on rock samples collected in the NDI Carrara 1 drill hole, the first stratigraphic test of the newly discovered Carrara Sub-basin located in the South Nicholson region of northern Australia. The drill hole recovered continuous core from 284 m to total depth at 1750 m and intersected approximately 1120 m of Proterozoic sedimentary rocks, unconformably overlain by 630 m of Cambrian Georgina Basin carbonate-rich rocks. Total organic carbon (TOC) contents from Rock-Eval pyrolysis highlight the potential for several thick black shales to be a source of petroleum for conventional and unconventional plays. Cambrian rocks contain an organic-rich section with TOC contents of up to 4.7 wt.% and excellent oil-generating potential. The Proterozoic section is overmature for oil generation but mature for gas generation, with potential for generating gas in carbonaceous mudstones showing TOC contents up to 5.5 wt.% between 680 and 725 m depth. A sustained release of methane (up to 2%) recorded during drilling from 1150 to 1500 m suggests potential for an unconventional gas system in the Proterozoic rocks from 950 to 1415 m depth, which exhibit favourable organic richness and thermal maturity. The Proterozoic rocks, which are comparable in age to the sediment-hosted deposits of the Century Mine, contain local occurrences of lead, zinc and copper sulfide minerals providing hints of mineralisation. The combined geochemical results offer the promise of a potential new resource province in northern Australia. <b>Citation:</b> E. Grosjean, A.J.M. Jarrett, C.J. Boreham, L. Wang, L. Johnson, J.M. Hope, P. Ranasinghe, J.J. Brocks, A.H.E. Bailey, G.A. Butcher, C.J. Carson, Resource potential of the Proterozoic–Paleozoic Carrara depocentre, South Nicholson region, Australia: Insights from stratigraphic drilling, <i>Organic Geochemistry</i>, Volume 186, 2023, 104688, ISSN 0146-6380, DOI: https://doi.org/10.1016/j.orggeochem.2023.104688.