Vertical geochemical profiling of the marine Toolebuc Formation, Eromanga Basin - implications for shale gas/oil potential The regionally extensive, marine, mid-Cretaceous (Albian) Toolebuc Formation, Eromanga Basin hosts one of Australia's most prolific potential source rocks. However, its general low thermal maturity precludes pervasive petroleum generation, although regions of high heat flow and/or deeper burial may make it attractive for unconventional (shale gas and shale oil) hydrocarbon exploration. Previous studies have provided a good understanding of the geographic distribution of the marine organic matter in the Toolebuc Formation where total organic carbon (TOC) contents range to over 20% with approx. half being of labile carbon and convertible to gas and oil. This study focuses on the vertical profiling, at the decimetre to metre scale, of the organic and inorganic geochemical fingerprints within the Toolebuc Formation with a view to quantify fluctuations in the depositional environment and mode of preservation of the organic matter and how these factors influence hydrocarbon generation thresholds. The Toolebuc Formation from three wells, Julia Creek-2 and Wallimbulla-2 and -3, was sampled over an interval from 172 to 360m depth. The total core length was 27m from which 60 samples were selected. Cores from the underlying Wallumbilla Formation (11 samples over 13m) and the overlying Allaru Mudstone (3 samples) completed the sample set. Bulk geochemical analyses included %TOC, %carbonate, %total S, -15N kerogen, -13C kerogen, -13C carbonate, -18O carbonate, and major, minor and tracer elements and quantitative mineralogy. More detailed organic geochemical analyses involved molecular fossils (saturated and aromatic hydrocarbons, and metalloporphyrins), compound specific carbon isotopes of n-alkanes, pyrolysis-gas chromatography and compositional kinetics. etc.

The natural gases are composed of a limited number of individual compounds, mainly of C1-C5 hydrocarbons and non-hydrocarbon compounds (CO2, N2, noble gases etc.). Their compositions and isotopes of single compounds provide critical information to decipher the origin and evolution of natural gases. Efficient analysis of these compounds is paramount for timely application of this important dataset.

The oxygen isotopic record obtained from Globigerina bulloides, Globorotalia inflata, and Neogloboquadrina pachyderma (s.) was analysed for 5 sediment traps moored in the Southern Ocean and Southwest Pacific. The traps extend from Subtropical to the Polar Frontal environments, providing the first analysis of seasonal foraminiferal d18O records from these latitudes. Comparison between the foraminiferal records and various equations for predicted d18O of calcite reveals that the predicted d18O is best captured by the equations of Epstein et al. (1953) [Epstein, S., Buchsbaum, R., Lowenstam, H.A., Urey, H.C., 1953. Revised carbonate-water isotopic temperature scale. Geological Society of America Bulletin 64, 1315-1326.] and Kim and O'Neil (1997) [Kim, S.-T., O'Neil, J.R., 1997. Equilibrium and non-equilibrium oxygen isotope effects in synthetic carbonates. Geochimica et Cosmochimica Acta 61, 3461-3475.]. The Epstein equation shows a constant offset from the -18O of G. bulloides and N. pachyderma (s.) across the full range of latitudes. The seasonal range in -18O values for these two species implies a near-surface habitat across all sites, while G. inflata most likely dwells at 50 m depth. A significant finding in this study was that offsets from predicted -18O for G. bulloides do not correlate to changes in the carbonate ion concentration. This suggests that [CO32-] in and of itself may not capture the full range of carbonate chemistry conditions in the marine system. This sediment trap deployment also reveals distinct seasonal flux patterns for each species. Comparison between flux-weighted isotopic values calculated from the sediment traps and the isotopic composition of nearby surface sediments indicates that the sedimentary records retain this seasonal imprint. At the 51°S site, G. bulloides has a spring flux peak while N. pachyderma (s.) is dominated by summer production.

Throughout New Zealand, the Torlesse Supergroup forms an extensive Permian to Cretaceous accretionary wedge of rather monotonous, sandstone-dominated turbidites. In contrast to contemporaneous rocks in neighbouring terranes within the accretionary wedge, the turbidites have less intermediate-volcaniclastic compositions, and show more quartzose, continent-derived, plutonic provenances. Petrographic, geochemical, isotopic and detrital mineral age characteristics all indicate that they did not originate at the contemporary Gondwanaland margin in New Zealand, but rather, constitute a suspect terrane (Torlesse Terrane), having sediment sources elsewhere along the margin. This latter subject has been controversial, with sediment sources suggested in Antarctica, southern South America and northeast Australia, but detailed Torlesse detrital mineral (zircon and mica) age data and bulk rock Sr-isotope patterns can be best matched for the most part with Carboniferous, Permian and Triassic sources in the New England Orogen, and the remainder with Cambrian and Ordovician sources in its hinterland.

Natural gas is Australia's third largest energy resource after coal and uranium but despite this economic importance, the gas origin is not always recognized. To address this, isotope and geochemistry data have been collated on 850 natural gases from all of Australia's major gas provinces with proposed source ages spanning the earliest Paleozoic to the Cenozoic. Unaltered natural gases have a thermogenic origin ('13C methane ranges between -49 and -27'; 'D methane ranges between -290 and -125'). Microbially altered natural gases were identified primarily on the basis of 13C and D enrichments in propane and/or 13C depletion in methane and/or 13C enrichment in CO2. The carbon isotopic composition of the gas source has been estimated using '13C iso-butane as a surrogate for '13C kerogen while for gases where biodegradation is moderate to severe, '13C neo-pentane provides an alternative measure. The '13C kerogen of gas source rocks range from -47 to -22' with the older Paleozoic sources and marine kerogen amongst the most depleted in 13C. The '13C CO2 also provides an insight into crustal- and mantle-derived components while '15N N2 (-6.0 to 2.3' for N2 up to 47 %) distinguish between organic and inorganic (groundwater) inputs. This dataset provides a better understanding on the source and preservation history of Australian gas accumulations with direct implications on improving exploration success.

In this paper, we present a high resolution study focussed mainly on the Gorgon field and associated Rankin Trend gas fields, Carnarvon Basin, Australia (Figure 1). These gas fields are characterized by numerous stacked reservoirs with varying CO2 contents and provide a relevant natural laboratory for characterizing CO2 migration, dissolution and reaction by looking at chemical characteristics of the different reservoirs (Figure 2). The data we present reveal interesting trends for CO2 mol% and -13C both spatially and with each other as observed by Edwards et al. (2007). Our interpretation of the data suggests that mineral carbonation in certain fields can be significant and relatively rapid. The Gorgon and Rankin Trend fields natural gases may therefore be a unique natural laboratory, which give further insights into the rates and extent of carbonate mineral sequestration as applied to carbon storage operations.

The Kingoonya Palaeovalley is one of the largest arid zone palaeovalley systems in South Australia. Situated in the remote central-western Gawler Craton this relict drainage network, now buried and obscured by surficial Quaternary sediments, is characterised by multiple headwater tributaries which flowed predominantly westwards towards the Eucla Basin. Fluvial and lacustrine sediments infilling the incised palaeovalley, in places forming stacked sequences >100 metres thick, were sporadically deposited from the Mid Eocene to the Early Pliocene. Previous drilling transects indicated a variety of channel shapes and heterogeneous sediment packages, with favourable aquifer sequences (sand-rich) common in deeper parts. In 2010 detailed groundwater sampling from existing bores was conducted in the Kingoonya Palaeovalley for the National Water Commission-funded Palaeovalley Groundwater Project. Analysis of these samples indicates that most Kingoonya groundwaters are moderately to highly saline and dominated by Na and Cl ions. Trace element enrichments are uncommon, although locally elevated levels of some metals (e.g., Fe and Mn) likely reflect groundwater interactions with the heterogenous sediments. The Kingoonya groundwaters also have near-neutral to slightly acidic pH, low alkalinity and are mostly oxidising. Stable water isotopes define a distinct trend away from the LMWL, interpreted as multiple stages of evaporative recycling and relative enrichment from the original isotopic signatures (precipitation derived from nearby Southern Ocean). Radiocarbon ages indicate a spectrum of groundwater residence times, ranging from modern recharge to groundwater signatures >20,000 years in deeper parts of the basal palaeovalley aquifer.

<div>The push of mineral exploration under cover requires developing new geochemical exploration approaches. Detailed hydrogeochemistry addresses these needs and is valuable as a non-invasive mineral exploration technique that can identify lithological changes and dispersion signatures associated with mineralisation. Here we integrate whole-rock geochemistry and hydrogeochemistry to evaluate suitable geochemical tracers in groundwater for detecting phosphate and/or Pb-Zn style mineralisation in the Georgina Basin. The known Georgina Basin’s phosphate deposits are within the basin’s aquifers, providing groundwater near deposits greater exposure and opportunity for water-rock interactions with mineralised geology, resulting in trace element and isotope signatures of mineralisation at detectable levels. These tracers can then be applied elsewhere in the basin as a screening tool for detecting mineralisation. To achieve this, we collected rock geochemistry from the MinEx CRC East Tennant National Drilling Initiative Campaign (ME-ET) drillcore, and integrated it with nearby hydrogeochemistry (from the Northern Australia Hydrogeochemical Survey (NAHS)). </div><div><br></div><div>The NAHS was collected by Geoscience Australia as part of EFTF, which included 170 samples from Georgina Basin aquifers. This hydrogeochemistry dataset is high quality, due to robust sampling, QA/QC procedures and a comprehensive analysis suite, making it a useful tool for mineral exploration in the Georgina Basin. The ME-ET drilled 10 stratigraphic holes east of Tennant Creek, Northern Territory, in support of Geoscience Australia’s Exploring for the Future program (EFTF). Seventy six Georgina Basin rock samples were collected for whole rock geochemistry and a subset for Pb and Sr isotopes. Samples were selected to target: 1) background unmineralised lithostratigraphy, 2) intervals with groundwater intersections, and 3) transects through zones with anomalous concentrations of P, Pb, Zn and Cu, as identified by portable XRF analysis. </div><div><br></div><div>Initial exploratory data analysis of the hydrogeochemistry is conducted at various scales using principle component analysis and clustering approaches to identify the key attributes (major and trace elements, isotopes, hydrogeology etc.) that are associated with higher P content in the groundwater. These relationships are tested by comparing groundwater samples proximal (in depth and spatially) to high P compositions in the host rock, providing insight into the water-rock interactions taking place. Additionally, vertical whole rock geochemistry transects within the drill-holes are investigated to evaluate the trace element and/or isotopic features that are diagnostic of the enriched phosphate zones. We take the robust geochemical relationships identified from both approaches and apply them as tracers across the NAHS to flag areas of potential undiscovered mineralisation. As we will demonstrate, the NAHS can detect subtle or diluted mineralisation signatures, and underpins a revised understanding of phosphate mineral prospectivity in the Georgina Basin.</div> Abstract submitted and presented at 2023 Australian Earth Science Convention (AESC), Perth WA (https://2023.aegc.com.au/)

This report contains new whole-rock and isotope geochemical data, associated sample metadata, an assessment of the data’s quality assurance, for 742 samples collected in and around the Curnamona and Delamerian provinces, across numerous drillcore sampling campaigns through 2021-23. The data can be downloaded via the Geoscience Australia EFTF portal (https://portal.ga.gov.au/persona/eftf) or in the files attached with this record (http://pid.geoscience.gov.au/dataset/ga/148651). Geochemical sampling in the Curnamona region straddles both South Australia and New South Wales. The objective of sampling was to obtain representative coverage (both stratigraphically and spatially) to support developing regional geochemical baselines (in conjunction with existing geochemistry). Thus, this sampling included both the Curnamona Province and the overlying basins (Eromanga Basin, Lake Eyre Basin). Whole-rock geochemistry is reported for 562 samples, with a subset of 13 samples analysed for Pb and Sr isotopes, and another subset of 36 samples analysed by thin section petrography (all presented herein). Geochemical sampling in the Delamerian region has focussed on available legacy drill core in South Australia, New South Wales and Victoria. The objective of sampling was to (systematically) constrain the geochemical character of magmatic rocks across the mainland extent of the Delamerian Orogen, as well as younger volcanics within the Delamerian Orogen and/or overlying cover. This geochemical sampling was conducted in conjunction with geochronology, mineral systems sampling and stratigraphic drilling (all components of the DCD project) to reinterpret the timing, character and fertility of the Delamerian Orogen. Whole-rock geochemistry is reported for 180 samples. Version 2.0 (published 28 November 2023) has added whole rock geochemistry for 22 new samples in the Delamerian region. The data products and report have been updated accordingly.

<div>Archean greenstone belts are a vital window into the tectonostratigraphic processes that operated in the early Earth and the geodynamics that drove them. However, the majority of greenstone belts worldwide are highly-deformed, complicating geodynamic interpretations. The volcano-sedimentary sequence of the 2775-2690 Ma Fortescue Group is different in that it is largely undeformed, offering a unique insight into the architecture of greenstone sequences. In the Fortescue magmatic rocks, geochemical signatures that in deformed belts in the Superior or Yilgarn Cratons might have been interpreted as arc-like, are explained by contamination of rift-related mantle and plume-derived magmas with Pilbara basement crust; understanding the wider geological and structural setting allows a more complete interpretation.</div><div> However, contamination of Fortescue magmas by an enriched sub-continental mantle lithosphere (SCLM) is an alternative hypothesis to the crustal contamination model. If demonstrated, the addition of sediments and fluids to the SCLM, required to form enriched/metasomaytised SCLM, would suggest active subduction prior to the Neoarchean. To test this hypothesis, we collected Hf-O isotopic data on zircons from felsic volcanic rocks throughout the Fortescue Group; if the contamination had a subducted sedimentary component (δ18O>20‰), then the O-isotopes should record a heavy signature.</div><div> The results show that the ca. 2775 Ma Mt Roe Formation has εHfi from 0 to -5.6, and δ18OVSMOW of +4.8- +0.3‰, with the majority of values <+3‰. The ca. 2765 Ma Hardey Formation (mostly sediments) has highly unradiogenic εHfi of -5 to -9.4, and δ18O of +7.8- +6.6‰. The ca. 2730 Ma Boongal Formation displays similar values as for Mt Roe, with εHfi +1.9 to -5.5 and δ18O +3.0 to -0.6‰. The ca. 2720 Ma Tumbiana Formation shows the greatest range in εHfi from +4.9 to -4.6, with δ18O +7.1- +0.7‰, with the majority between +4.5 and +2.5‰. Data from the 2715 Ma Maddina Formation are more restricted, with εHfi between +4.0 and -0.1, and δ18O +5.0- +3.8‰. The youngest formation, the 2680 Ma Jeerinah Formation, has εHfi +2.3 to -6.2, and δ18O +5.1 to -2.1‰.</div><div> Importantly, these data provide little evidence of a cryptic enriched SCLM source in the Fortescue magmas. Furthermore, the dataset contains some of the lightest δ18O data known for Archean zircon, highlighting a ca. 100 Myr period of high-temperature magma-water interaction, with long-term continental emergence implied by the trend to meteoric δ18O compositions. The exception to this is the Hardey Formation, which may have formed via crustal anatexis in a period of reduced heat-flow between the 2775-2665 and 2730-2680 Ma events. Data from the other formations show a broad trend of increasing δ18O and εHf from 2775 to 2680 Ma. We suggest this represents the effects of progressive cratonic rifting, allowing mantle-derived magmas to reach the surface less impeded, and also a decreasing role of meteoric water in the rift zone as the sea invades. As a result, the εHf and δ18O data from the Fortescue Group represent the evolving nature of an Archean rift zone, from an emergent volcanic centre, to a submarine environment.</div><div><br></div>This Abstract was submitted/presented to the 2023 6th International Archean Symposium (6IAS) 25 - 27 July (https://6ias.org/)