<div><strong>Output type: </strong>Exploring for the Future Extended Abstract</div><div><br></div><div><strong>Short abstract: </strong>Australia's crust is blanketed by sedimentary cover. Knowledge of the distribution and thickness of sediments is crucial for assessing resource potential. Excellent constraints on sedimentary thickness can be obtained from borehole drilling or active seismic surveys, but these are expensive and often impractical in much of inland Australia. Recently, a relatively simple and low-cost method for estimating the sedimentary thickness using passive seismic data was developed and applied to seismic stations in South Australia. First-order estimates of the thickness of sediments are predicted from the delay time of the Ps converted phase generated at the sediment-basement interface via an empirical relationship constructed using borehole stratigraphy data. Here we expand the analysis to over 1500 seismic stations across Australia, covering numerous sedimentary basins that span the entire range of sedimentary basin ages from Precambrian to present-day. The new results clearly match the broad pattern of expected sedimentation based on the various Australian geological provinces. Using a new compilation of borehole data, we obtain a new, continent-wide empirical relationship from passive seismic delay to sediment cover thickness, offering a fast, simple and cheap way to characterise sediment thickness in under-explored areas. We apply the new relationship to the Exploring for the Future AusArray 2ºx2º data. </div><div><br></div><div><strong>Citation: </strong>Marignier, A., Eakin, C.M., Hejrani, B., Agrawal, S. & Hassan, R., 2024. Australian sedimentary thickness from passive seismic methods. In: Czarnota, K. (ed.) Exploring for the Future: Extended Abstracts. Geoscience Australia, Canberra. https://doi.org/10.26186/149319</div>

The ⁶⁰Co gamma ray radiolysis of methane was undertaken at room temperature and at a low proportion of methane conversion to observe initial polymerisation reactions in the formation of C₂─C₅ saturated hydrocarbon gases. The gamma ray radiolysis product follows a Schulz-Flory distribution model for the straight-chained C2+ alkane gases consistent with chain propagation by addition of C₁ intermediates. The high relative concentration of branched isomers suggests the total product distribution is likely controlled by propagation reactions involving mainly radical intermediates. In particular, the pentane isomers show a relative dominance of the branched isomers with sub-equal amounts of iso-pentane and neo-pentane. Carbon isotopes of the C₂─C₅ wet gases are all depleted in 13C relative to δ13C methane reactant with δ¹³C C₂ < δ¹³C C₃ and carbon isotope reversals in the C₃─C₅ gases while only ethane and neo-pentane are depleted in ²H compared to δ²H of the starting methane. As such, these molecular and isotopic features show similar characteristics to abiogenic wet gases found in terrestrial settings. Bibliographic Citation: Boreham, C. J., & Davies, J. B. (2020). Carbon and hydrogen isotopes of the wet gases produced by gamma-ray-induced polymerisation of methane: Insights into radiogenic mechanism and natural gas formation. Radiation Physics and Chemistry, 168, 108546. https://doi.org/10.1016/j.radphyschem.2019.108546

<div><strong>Output Type: </strong>Exploring for the Future Extended Abstract</div><div><br></div><div><strong>Short Abstract: </strong>Airborne electromagnetic surveys are widely used in Australia for mineral exploration, groundwater assessment (i.e. hydro-stratigraphy and water quality) and natural resource management (i.e. salinity assessment). In the last decade, regional surveys have been acquired covering approximately two thirds of the continent and resulting in a large volume of data to interpret. To address this challenge, we have developed a machine learning workflow to assist with the interpretation of AEM conductivity depth sections.</div><div>‘AEM assist’ is an open-source machine learning algorithm that allows the user to interpret AEM sections from drillhole observations and/or interpreted segments along the conductivity depth section. AEM assist finds predictive relationships between the training observations (drillhole and/or interpreted sections) and the conductivity value which also includes the first vertical derivative of the conductivity. Due to the non-uniqueness of the conductivity response, we have also built in a suite of supplementary covariates or features to help improve the model prediction. These features include terrain indices, gamma radiometric, surface weathering intensity, distance proxies (e.g., distance from rocks of a known age), climate indices, gravity, and magnetic derivatives. We have built the AEM assist into a national mapping framework to facilitate model interpretation and training anywhere in Australia. Although local training of sections is recommended the national framework provides an opportunity to train a model in one region and predict into another area given similar geological and landscape histories. The AEM assist has the potential to speed up the interpretation of AEM flightline sections with statistical models of interpretation uncertainty. AEM assist can be used to provide a first pass interpretation of a survey area that can later be revised by the domain expert. A feature of AEM assist is that it systematically integrates many datasets that would otherwise be difficult to do from traditional methods.</div><div><br></div><div><strong>Citation:</strong> Basak S., Wilford J., Wong S.C.T., Ley-Cooper Y. & Ray A., 2024. AEM assist - a national predictive machine learning framework for airborne electromagnetic interpretation and extrapolation. In: Czarnota, K. (ed.) Exploring for the Future: Extended Abstracts. Geoscience Australia, Canberra, https://doi.org/10.26186/149495</div>

<div>The Delamerian Orogen is a spatially (~1500&nbsp;km long by ~300 km wide) and temporally (ca. 830 to 495 Ma) extensive orogenic system spanning five states in central and eastern Australia. The spatial extent of the orogen was defined by Glen (2005) as the distribution of rocks that underwent their first deformation in the mid-to-late Cambrian Delamerian Orogeny, with the orogen impacted by younger geodynamic events in the Phanerozoic. The orogen is poorly exposed overall and is mostly covered by a series of younger sedimentary basins, volcanic rocks and regolith.</div><div>In the last few years, a multi-disciplinary program of geological and geophysical data collection and integration, focused on the Delamerian Orogen, was undertaken as part of Geoscience Australia’s (GA) Exploring for the Future (EFTF) program and the Mineral Exploration Cooperative Research Centres (MinEx CRC) National Drilling Initiative (NDI). This work program, outlined by Gilmore et al. (2023), compliments the Geological Survey of South Australia’s recent MinEx CRC NDI drilling campaign in the Loch Lilly-Kars Belt in South Australia (Wise, 2023). These efforts have resulted in a greatly increased volume of lithological, geochronological, geochemical, and geophysical datasets, especially in the vicinity of the Loch Lilly-Kars Belt (e.g. Brotodewo et al., 2021; Cheng et al., 2024a; Cheng et al., 2024b; Clark et al., 2024; Curtis, 2022; Doublier et al., 2024; Mole et al., 2024; Pitt et al., 2023; Taylor et al., in prep.; Wise, 2023). Two time-space plots and an accompanying map (appendices A, B and C)&nbsp;have been prepared to convey the geological history of the Delamerian Orogen within the DCD project area on mainland Australia:</div><div><br></div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;An overview time-space plot designed to provide a simplified geological history through the orogen from the Archaean to the present.</div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;A detailed time-space plot with three (northern, central and southern) traverses to reflect changes in geological history and stratigraphy along the strike and width of the Delamerian Orogen, focussed on the start of the Ediacaran to the end of the Devonian.</div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Location map showing the traverses used in the detailed time-space plot.</div><div>The time-space plots are the first holistic time-space plots for the Delamerian Orogen on mainland Australia. They allow a comparison of the similarities and differences in the age and location of magmatism, sedimentation, deformation and mineralisation across the Delamerian Orogen in New South Wales (NSW), South Australia (SA) and Victoria, at both a high level (overview) and stratigraphic unit level (detailed).</div><div>This record outlines the methodology and data used (Appendix D)&nbsp;to compile the time-space plots.</div><div><br></div>

Thirteen Australian oils and one condensate, covering oil reservoir ages from Mesoproterozoic to Early Cretaceous, show monoalkene contents varying from 0.01 to 22.3 wt% of the whole liquid. Radiolysis of saturated hydrocarbons is the most likely process leading to oils with high alkene contents. The major radiolytic component is an unresolved complex mixture (UCM). The bulk of the resolved alkene compounds are positional isomers of n-alkenes. Methyl branched and cyclohexyl alkenes are minor components. Internal n-alkene isomers have a trans configuration dominant over the cis isomer. The oil with the longest reservoir residence time shows the highest content of internal n-alkenes relative to terminal 1-alkenes as well as the highest trans/cis ratio, suggesting the extended time has resulted in rearrangement to near thermodynamic equilibrium of the congruent monoalkenes. The radiolytic monoalkenes in the Ordovician-reservoired oil with the highest alkene content is likely influenced by a higher probability of intermolecular interactions and different product pathways in a complex mixture. Here, the relative proportion of alkene mimics the relative abundance of n-alkanes, suggesting that radiolytic C–C bond cleavage is suppressed when the alkene/alkane ratio is elevated and that the preferred pathway of n-alkane radiolysis favours the production of terminal monoalkenes. Radiolysis of the alkane UCM together with crosslinking and branching of n-alkane-derived radiolysis products contribute to the higher relative proportion of the alkene UCM. The similar carbon and hydrogen isotopic ratios of the n-alkanes and n-alkenes supports a parent–daughter relationship. <b>Citation:</b> Christopher J. Boreham, Neel Jinadasa, Jacob Sohn, Ziqing Hong, Christopher Blake, Characterisation of radiogenic monoalkenes in Australian oils and condensate, <i>Organic Geochemistry</i>, Volume 163, 2022, 104332, ISSN 0146-6380, https://doi.org/10.1016/j.orggeochem.2021.104332.

<div>AGRF25 is a regional magnetic field model for Australia covering the interval 2020-2030.&nbsp;</div><div>The Australian Geomagnetic Reference Field (AGRF) is a set of numerical models of the geomagnetic field for Australia and near-neighbouring regions. It represents a combination of the Earth's main field originating from the core and the broad-scale crustal field. AGRF describes the geomagnetic field on a regional scale, intermediate between the global scale of the International Geomagnetic Reference Field (IGRF) and the local scale provided by detailed ground and aeromagnetic surveys.</div><div><br></div><div>Included with the dataset is evaluation software:</div><div>README.TXT:&nbsp;This text file of information</div><div><br></div><div>AGRF25S.FOR: Source code of a main program for generating AGRF25 values</div><div>&nbsp;&nbsp;&nbsp;&nbsp;at a sequence of single sites. The program may also be used to</div><div>&nbsp;&nbsp;&nbsp;&nbsp;generate values of the International Geomagnetic Reference Field,&nbsp;</div><div>&nbsp;&nbsp;&nbsp;&nbsp;revision 14 for epochs after 2025.0</div><div><br></div><div>AGRF25S.EXE: A Windows console application of program AGRF25S</div><div><br></div><div>AGRF25G.FOR: Source code of a main program for generating AGRF25 values</div><div>&nbsp;&nbsp;&nbsp;&nbsp;for a single field element on a latitude-longitude grid at&nbsp;</div><div>&nbsp;&nbsp;&nbsp;&nbsp;a constant altitude</div><div><br></div><div>AGRF25G.EXE: A Windows console application of program AGRF25G</div><div><br></div><div>AGRF25LB.FOR: Source code for subroutines called by the main programs.</div><div><br></div><div>COEF25:&nbsp;&nbsp;A data file of model coefficients and parameters.</div><div><br></div><div>GRID2GEO.FOR: Source code of a program to convert Universal Transverse</div><div>&nbsp;&nbsp;&nbsp;&nbsp;Mercator Easting and Northings to geodetic latitude and longitude</div><div><br></div><div>GRID2GEO.EXE: A Windows console application of GRID2GEO</div><div><br></div><div>GEO2GRID.FOR: Source code of a program to convert geodetic latitude and&nbsp;</div><div>&nbsp;&nbsp;&nbsp;&nbsp;longitude to UTM Easting and Northings</div><div><br></div><div>GEO2GRID.EXE: A Windows console application of GEO2GRID</div><div><br></div><div>SPHEROID.TXT: A text data file of geodetic spheroid parameters,&nbsp;</div><div>&nbsp;&nbsp;&nbsp;&nbsp;used by GRID2GEO and GEO2GRID</div>

High concentrations of hydrogen (H₂) and methane have been detected in shallow open-hole exploration wells surrounding the Neoarchean Frog’s Leg gold camp in the Eastern Goldfields, Yilgarn Craton, Western Australia. After corrections for modern air contaminants and excess nitrogen (N₂) the Boomer deposit gases contain: 19.9‒68.7 mol% H₂; 28.7‒76.9 mol% CH₄; 0.47‒1.6 mol% heavier hydrocarbons (C₂‒C₅), which follow an Anderson-Schulz-Flory distribution; 0.11‒3.3 mol% carbon dioxide; and 0.69‒1.87 mol% helium (He). The isotopic composition of the gas components was further investigated: helium has ³He/⁴He ratios of 1.82‒3.33×10-8 (or 0.01-0.02 Ra, where Ra is the atmospheric value) indicating a purely crustal origin; hydrogen has δ²H between -781.3 ‰ and -759.5 ‰; and methane hasδ¹³C between -20.3 and -2.42 ‰ and δ²H between -382.5 ‰ and -342.2 ‰. The C₂-C₅ gaseous hydrocarbons are commonly depleted in ¹³C (up to 22.75 ‰ for ethane) and enriched in ²H (up to 117.3 ‰ for iso-butane) compared to methane while carbon isotope reversal is observed between methane and ethane. These molecular and isotopic characteristics of the gas are consistent with 1) H₂ generation controlled by redox and/or radiolytic reactions within basic and felsic igneous rocks, and 2) methane and C₂₊ gaseous hydrocarbons produced during serpentinization of mafic‒ultramafic rocks. Serpentinization due to ingress of groundwater can produce voluminous free H₂. Subsequent, gas phase Fischer-Tropsch type reactions with limiting CO₂ (from carbonate dissolution) lead to abiogenic methane enriched in ¹³C and to the generation of C₂‒C₅ wet gases. Radiolytic-controlled processes also occur in parallel, where mafics‒ultramafics and granites and their related eroded sediments promote abiogenic radiolysis of water and polymerisation of methane. Using the H₂‒CH₄ hydrogen isotopic fractionations as a geothermometer, isotopic equilibrium temperatures are calculated between 42.2 °C and 57.0 °C, representing minimum 1.4‒2.7 km sub-surface depths for the gas sources, corresponding to the Gleesons Basalt as a major gas contributor. The processes of gold mineralisation billions of years ago laid the foundation for the supply of catalytic metal/metal ions and carbonate (CO₂) source for later independent, abiogenic gas formation. The free gases sampled from exploration wells in the Boomer deposit, Frog’s Leg Gold Camp represent the first positive identification of abiogenic natural gas in Australia, with gas compositions similar to abiogenic gases previously reported from overseas Archean cratons. Interestingly, gases preserved in fluid inclusions associated with some of the region’s other lode gold deposits demonstrate CH₄ production has likely occurred over a period from the Neoarchean to present in the Yilgarn Craton. <b>Citation:</b> Christopher J. Boreham, Jacob H. Sohn, Nicholas Cox, Jodi Williams, Ziqing Hong, Mark A. Kendrick, Hydrogen and hydrocarbons associated with the Neoarchean Frog's Leg Gold Camp, Yilgarn Craton, Western Australia, <i>Chemical Geology</i>, Volume 575, 2021, 120098, ISSN 0009-2541, https://doi.org/10.1016/j.chemgeo.2021.120098.

We have recently shown that molecular hydrogen generation from organic matter occurs at high maturity levels (vitrinite reflectance 3–5%) in Lower Cretaceous shales of the Songliao Basin. To evaluate and extend these implications to a wider range of source rock types and organofacies, we report on two Paleozoic maturity suites from Australia, namely the Permian Patchawarra Formation (fluviodeltaic; Type-III; Cooper Basin) and the middle Cambrian Arthur Creek Formation (marine; Type-II; Georgina Basin), and additional mature marine source rocks from Europe and the USA. It can be inferred from high resolution mass spectrometry that rapid growth of aromatic ring systems is the major pathway for the formation of thermogenic molecular hydrogen from all organic matter types. Extensive open system pyrolysis experiments indicate that the main generation pulse occurs in the vitrinite reflectance range 3.5–5.0%. Kinetic parameters were constructed by subtracting the hydrogen associated with hydrocarbon formation from total hydrogen in the open-system experiments via adjustment factors defined by the relative yields of CH₄ and H₂. A cumulative H₂ potential of 20 mg/g TOC is found with maximum rates of generation that are sufficient for feeding the deep biosphere. Back of the envelope calculations indicate ∼3.5E+10 tonnes of in-place accessible H₂ globally, which is an order of magnitude lower than in-place shale gas resource estimates. Regionally, inferred here for the Patchawarra Formation in the Nappamerri Trough (Cooper Basin), yields per unit rock volume resemble those of economic shale gas in the Barnett Shale, Fort Worth Basin, USA. Organic particles are, at the SEM-scale (>30 nm), barren of secondary porosity in the case of terrigenous samples at all maturity stages, but show sponge-like porosity in the investigated marine source rocks exhibiting vitrinite reflectance >∼2.0%. Presence of such meso- and macropores is crucial for H₂ storage in marine shales, as microporosity (<2 nm) yielding sorptive storage space for H₂, is usually much higher in mature terrigenous kerogens. <b>Citation:</b> Nicolaj Mahlstedt, Brian Horsfield, Philipp Weniger, David Misch, Xiangyun Shi, Mareike Noah, Christopher Boreham, Molecular hydrogen from organic sources in geological systems, <i>Journal of Natural Gas Science and Engineering</i>, Volume 105, 2022, 104704, ISSN 1875-5100, https://doi.org/10.1016/j.jngse.2022.104704.

Over 800 natural gases representative of Australia's hydrocarbon-producing sedimentary basins have been analyzed for their helium abundance and around 150 gases for their helium isotopic composition (supplemented by isotopic compositions of the higher noble gases Ne, Ar, Kr and Xe). Helium shows abundance up to over 10% with the highest values in the Amadeus Basin (central Australia), while ³He/⁴He ratios range from around 0.01 to 4.2 Ra (Figure 1). The Gunnedah Basin of south-east Australia and the Bass and Otway basins in southern Australia show the highest ³He/⁴He ratios, indicating a significant mantle contribution. Interestingly the adjacent Gippsland Basin has slightly lower ³He/⁴He ratios. The associated CO₂ has a relatively low abundance in the Gunnedah Basin (highest ³He/⁴He ratio) compared to some extreme concentrations of CO₂ in the Otway Basin, which are associate with recent volcanism. The onshore Bowen and Cooper basins of eastern Australia, where natural gases are predominately sourced from Permian coals, show intermediate ³He/⁴He ratios with the former having a higher mantle contribution. At the other end of the spectrum, low ³He/⁴He ratios characterize natural gases of the Bonaparte, Browse, Carnarvon and Perth basins in northern and south-western Australia where radiogenic helium predominates. The minor mantle contribution that is inferred from the He isotopes in these regions has resulted from the limited volcanic activity and igneous intrusions throughout the basins' evolution. The accompanying high CO₂ contents of some of these gases, together with their carbon isotopic composition, infer an inorganic source most likely from thermal decomposition of carbonates. The geochemical data suggest that the origin of helium in Australian natural gas accumulations is region specific and complex with the component gases originating from multiple sources. The relative low CO₂/³He ratio for many natural gases indicates a systematic loss of CO₂ from most basins. The process by which CO₂ has been lost from the system is most likely associated with precipitation of carbonates (Prinzhofer, 2013). The age of the source (or reservoir) rock has a primary control on the helium content with radiogenic ⁴He input increasing with residence time (Figure 1). With the recent acceleration in the exploitation of Australia's enormous reserves of natural gas, the LNG processing plants in western, northern and eastern Australia offer the opportunity to commercialize helium in gases with as low as 0.1% He. However, a key factor to the gases monetary value is its inherent N₂ content. From the ¹⁵N/¹⁴N ratios, N₂ associated with mineral decomposition severely impacts the economics of helium extraction. Presented at the 2017 International Meeting in Organic Geochemistry (IMOG)

<div>&nbsp;Seismic site classification is essential for seismic hazard analysis as it helps constrain the impact of local geological conditions on the near-surface seismic-wave propagation and observed ground motion. The Southwest Australia Seismic Network (SWAN) temporary array was established to record local earthquakes for seismic hazard applications and to improve rendering of the 3D seismic structure of the crust and mantle lithosphere in southwestern Australia. Notably, the SWAN project has recorded significant seismic events, including the 2022 Arthur River earthquake sequence and the 2023 MW 5.0 Gnowangerup earthquake. These earthquakes, together with other well-recorded events across the SWAN network, offer a rare opportunity to assess the utility of published ground-motion models (GMMs) for large-magnitude earthquakes, thereby significantly improving seismic hazard assessment in the region. Moreover, the importance of site classification is underscored as it is a critical component of GMMs, and can substantially enhance the accuracy and reliability of these models. This study uses microtremor survey methods to estimate the shallow shear-wave velocity profiles and VS30 values, which are the primary factors for site classification at seismic stations. Microtremor array measurements, such as high-resolution frequency-wavenumber and modified spatial autocorrelation methods, were utilized to analyse ambient vibrations, producing detailed dispersion curves for each station. To enhance the depth accuracy of velocity profiles, ellipticity curves were extracted using the RayDec method and jointly inverted with the dispersion curves. Additionally, OpenHVSR software was employed for the inversion of single-station ellipticity curves.</div><div><br></div><b>Citation</b>: Ebrahimi, R and Allen, TI 2024, Site classification and VS30 determination for seismic hazard evaluation in the SWAN seismic network, Western Australia, in South West Australia Network (SWAN): passive seismic imaging and hazard analysis compiled by RE Murdie and MS Miller: Geological Survey of Western Australia, Report 255, p. 58–67