<div>A regional hydrocarbon prospectivity assessment has been undertaken of the offshore Otway Basin by the Offshore Energy Systems Section. This program was designed to produce pre-competitive information to assist with the evaluation of the hydrocarbon resource potential of the offshore Otway Basin and attract exploration investment to Australia. The inboard part of the basin is an established hydrocarbon province with onshore and shallow-water offshore discoveries, whereas the outboard deep-water region, where water depths range from 500 to 6300&nbsp;m, is comparatively underexplored and considered a frontier area.</div><div><br></div><div>As part of this program, molecular and noble gas isotopic analyses were undertaken by Smart Gas Sciences, under contract to Geoscience Australia on available gas samples from the Waarre Formation in the Shipwreck Trough in the offshore eastern Otway Basin, with data from these analyses being released in this report. This report provides additional compositional information for gases in the Waarre Formation reservoirs and builds on previously established gas-gas correlations and gas-oil correlations. Noble gas isotopic data can be used in conjunction with carbon and hydrogen isotopic data to determine the origin of both inorganic and organic (hydrocarbon) gases. This information can be used in future geological programs to determine the source and distribution of hydrogen and helium in natural gases and support acreage releases by the Australian Government.</div><div><br></div><div><br></div>

<div>Exploring for the Future (EFTF) is a program dedicated to exploring Australia’s resource potential and boosting investment. This program is designed to produce pre-competitive information to assist with the evaluation of the hydrocarbon resource potential of onshore basins and attract exploration investment to Australia. This record presents geochemical analyses of natural gases sampled from Nangwarry 1, located in the onshore Otway Basin, undertaken in partnership with the Department for Energy and Mining – Energy Resources, Government of South Australia, as part of the EFTF program Natural Hydrogen module. The Nangwarry Joint Venture drilled Nangwarry 1 to investigate the potential for the development of food grade, carbon dioxide production from this well. The results of the molecular and stable carbon and hydrogen isotopic analyses undertaken by Geoscience Australia are released in this report. The molecular data show that the gas composition in this well has an average of 96 mol% CO2 with an isotopic signature indicative of a magmatic origin, being comparable with previously produced gases from onshore Otway Basin wells (e.g. Boggy Creek 1, Caroline 1) for use by the food industry. The carbon and hydrogen isotopic composition of the C1–C5 hydrocarbon gases from Nangwarry 1 are suggestive of a source from within the Crayfish Supersequence.</div>

Geoscience Australia has undertaken a regional seismic mapping study of the offshore Otway Basin extending across the explored inner basin to the frontier deep-water region. Seismic interpretation covers over 18000 line-km of new and reprocessed data acquired in the 2020 Otway Basin seismic program and over 40000 line-km of legacy 2D seismic data. We present new basin-scale isochore maps that show the distribution of the Cretaceous depocentres. Maps for the Lower Cretaceous Crayfish and Eumeralla supersequences, together with those recently published for the Upper Cretaceous Shipwreck and Sherbrook Supersequences, completes the set of isochore maps for the main tectonostratigraphic basin intervals. Mapping of basement involved faults has revealed structural fabrics that have influenced depocentre development. The tectonostratigraphic development of depocentres and maps of deep crustal units delineate crustal thinning trends related to late Cretaceous extension phases. This work highlights the need to review and update structural elements. For example, the boundary between the Otway and Sorell basins is now geologically constrained. The refinements to the tectonostratigraphic evolution of the Otway Basin presented here have important implications for the distribution and potential maturity of petroleum systems, especially with regard to heat flow associated with crustal extension. Presented at the 2024 Australian Energy Producers Conference & Exhibition (AEP) (https://energyproducersconference.au/conference/)

In 2017, 21 new offshore petroleum exploration areas have been released. The majority of the areas are located along the North West Shelf spanning the Westralian Superbasin from the Bonaparte Basin in the north-east to the Northern Carnarvon Basin in the south-west. New areas have been released in offshore south-eastern Australia with new opportunities provided in the Otway, Bass and Gippsland basins. Two large areas in the northern Perth Basin, an offshore frontier, complete the 2017 Acreage Release. All Release Areas are supported by industry nominations and one new cash bid area has been offered in the Dampier Sub-basin. Geoscience Australia continues to support industry activities by acquiring, interpreting and integrating pre-competitive datasets that are made freely available as part of the agency’s regional petroleum geological studies. A new regional 2D seismic survey was acquired in the Houtman Sub-basin of the Perth Basin, forming the basis of the latest prospectivity study carried out by Geoscience Australia. The results of the study are presented in the technical program of the 2017 APPEA conference. A wealth of seismic and well data, submitted under the Offshore Petroleum and Greenhouse Gas Storage Act 2006 (OPGSSA) are made available through the National Offshore Petroleum Information Management System (NOPIMS). Additional datasets are accessible through Geoscience Australia’s data repository. Presented at the 2017 Australian Petroleum Production & Exploration Association (APPEA) Conference.

The Shipwreck and Sherbrook supersequences together constitute the upper Cretaceous succession in the Otway Basin that was deposited during an extensional basin phase. In the Shipwreck Trough, where the upper Cretaceous succession is well explored, gas fields are hosted by the Shipwreck Supersequence (SS). Elsewhere, the upper Cretaceous interval is lightly explored, and the deep-water area is considered an exploration frontier. We present regional gross depositional environment (RGDE) maps for the LC1.1 and LC1.2 sequences of the Shipwreck SS, and the LC2 Sherbrook SS. Fluvial Plain, Coastal-Delta Plain and Shelf RGDEs were interpreted from wireline logs, cores, and seismic facies. The Fluvial Plain and Coastal-Delta Plain RGDEs are mostly restricted to the inboard platform areas and the inner Morum Sub-basin. The mud-prone Shelf RGDE is widespread across the deep-water Morum and Nelson depocentres. The extent of the Fluvial and Coastal-Delta Plain belts progressively increases up-section, imparting a regressive aspect to the succession, and delineating a large fluvial-deltaic complex in the north-west of the basin. Thick seal development across the greater Shipwreck Trough, potentially mature source rocks in the deep-water basin, and thick reservoir development in the hanging wall of growth faults in the inner Morum Sub-basin are insights derived from this study, and will inform area selection for detailed gross depositional environment mapping, formulation of new hydrocarbon and carbon dioxide storage plays, and inputs for petroleum systems modelling. Presented at the Australian Energy Producers (AEP) Conference & Exhibition (https://energyproducersconference.au/conference/)

Geoscience Australia has undertaken a regional seismic mapping study of the offshore Otway Basin extending across the explored inner basin to the frontier deep-water region. Seismic interpretation covers over 18,000 line-km of new and reprocessed data acquired in the 2020 Otway Basin seismic program, over 40,000 line-km of legacy 2D seismic data and GA’s new 2023 Otway 3D post-stack Mega Merge seismic dataset. This work provides a new perspective on regional structural architecture and basin evolution and has important implications for hydrocarbon prospectivity of this region. This seminar was two short talks centring on the Otway Basin. <u>Post-stack 3D merging to fast-track regional interpretation - offshore Otway Basin case study, presented by Merrie-Ellen Gunning</u> This case study was to produce a regularised and seamless 3D dataset of the highest possible quality, for the offshore Otway Basin, within two-months. The input migrated volumes varied by data extent, migration methodology, angle range and grid orientation. Fourteen input volumes totalling 8,092 km2 were post-stack merged and processed to produce a continuous and consistent volume, enabling more efficient and effective interpretation of the region. The surveys were regularised onto a common grid, optimised for structural trend, prior to survey matching. A mis-tie analysis algorithm, applied over a time window optimised for interpretation of key events, was used to derive corrections for timing, phase and amplitude, using a reference. This was followed by time-variant spectral and amplitude matching to improve continuity between volumes. Additional enhancements including noise removal and lateral amplitude scaling were also applied. The final merged volume offers significant uplift over the inputs, providing better imaging of structure and events and dramatically improving the efficiency and quality of interpretation. This enables rapid reconnaissance of the area by explorers. <u>Structural architecture of the offshore Otway Basin presented by Chris Nicholson</u> We present new basin-scale isochore maps that show the distribution of the Cretaceous depocentres. Maps for the Lower Cretaceous Crayfish and Eumeralla supersequences, together with those recently published for the Upper Cretaceous Shipwreck and Sherbrook Supersequences, completes the set of isochore maps for the main tectonostratigraphic basin intervals. Mapping of basement involved faults has revealed structural fabrics that have influenced depocentre development. The tectonostratigraphic development of depocentres and maps of deep crustal units delineate crustal thinning trends related to late Cretaceous extension phases. This work highlights the need to review and update structural elements. For example, the boundary between the Otway and Sorell basins is now geologically constrained. The refinements to the tectonostratigraphic evolution of the Otway Basin presented here have important implications for the distribution and potential maturity of petroleum systems, especially with regard to heat flow associated with crustal extension.

<div>The Sherbrook Supersequence (Campanian–Maastrichtian) is the youngest of four Cretaceous supersequences in the Otway Basin and was deposited during a phase of crustal extension. Supersequence thickness is typically less than 1000 ms TWT across the inboard platform. Beyond the platform edge up to 2 800 ms TWT of Sherbrook sediments were deposited in the deep-water Morum and Nelson sub-basins. Analysis of wireline-logs and cores from wells yielded fluvial, deltaic, coastal shelf gross depositional environments (GDEs). As the number of regionally mappable seismic facies is much less than the number of well-based GDEs, the integration of well-based environmental interpretations with seismic facies resulted in three main regional GDE (RGDE); Fluvial Plain, Coastal/Delta Plain, and Shelf. The Fluvial Plain and Coastal/Deltaic RGDEs are almost entirely restricted to the inboard platform areas of the basin. The mud-prone Shelf RGDE is widespread across the deep-water part of the basin where it forms the depocentres of the Morum and Nelson sub-basins. The Shelf RGDE is well imaged on the Otway 2020 2D seismic data that was acquired over the deep-water Otway Basin. In the Morum Sub-basin, the Shelf RGDE is strongly influenced by growth on extensional faults. In contrast, the Shelf RGDE in the Nelson Sub-bsin is a relatively unstructured progradational complex. The presence of mass-transport and incision complexes are consistent with active tectonism during Sherbrook deposition. Reservoir rocks in the deep-water basin are best developed in the Coastal/Deltaic RGDE where it encroaches into the Morum Sub-basin, and where the Austral 3 petroleum system was potentially active within the Sherbrook Supersequence.&nbsp;</div> This presentation was given at the 2023 Australasian Exploration Geoscience Conference (AEGC) 13-18 March, Brisbane (https://2023.aegc.com.au/)

<div>This data package contains interpretations of airborne electromagnetic (AEM) conductivity sections in the Exploring for the Future (EFTF) program’s Eastern Resources Corridor (ERC) study area, in south eastern Australia. Conductivity sections from 3 AEM surveys were interpreted to provide a continuous interpretation across the study area – the EFTF AusAEM ERC (Ley-Cooper, 2021), the Frome Embayment TEMPEST (Costelloe et al., 2012) and the MinEx CRC Mundi (Brodie, 2021) AEM surveys. Selected lines from the Frome Embayment TEMPEST and MinEx CRC Mundi surveys were chosen for interpretation to align with the 20&nbsp;km line-spaced EFTF AusAEM ERC survey (Figure 1).</div><div>The aim of this study was to interpret the AEM conductivity sections to develop a regional understanding of the near-surface stratigraphy and structural architecture. To ensure that the interpretations took into account the local geological features, the AEM conductivity sections were integrated and interpreted with other geological and geophysical datasets, such as boreholes, potential fields, surface and basement geology maps, and seismic interpretations. This approach provides a near-surface fundamental regional geological framework to support more detailed investigations. </div><div>This study interpreted between the ground surface and 500&nbsp;m depth along almost 30,000 line kilometres of nominally 20&nbsp;km line-spaced AEM conductivity sections, across an area of approximately 550,000&nbsp;km2. These interpretations delineate the geo-electrical features that correspond to major chronostratigraphic boundaries, and capture detailed stratigraphic information associated with these boundaries. These interpretations produced approximately 170,000 depth estimate points or approximately 9,100 3D line segments, each attributed with high-quality geometric, stratigraphic, and ancillary data. The depth estimate points are formatted for compliance with Geoscience Australia’s (GA) Estimates of Geological and Geophysical Surfaces (EGGS) database, the national repository for standardised depth estimate points. </div><div>Results from these interpretations provided support to stratigraphic drillhole targeting, as part of the Delamerian Margins NSW National Drilling Initiative campaign, a collaboration between GA’s EFTF program, the MinEx CRC National Drilling Initiative and the Geological Survey of New South Wales. The interpretations have applications in a wide range of disciplines, such as mineral, energy and groundwater resource exploration, environmental management, subsurface mapping, tectonic evolution studies, and cover thickness, prospectivity, and economic modelling. It is anticipated that these interpretations will benefit government, industry and academia with interest in the geology of the ERC region.</div>

Geoscience Australia currently uses two commercial petroleum system modelling software packages, PetroMod https://www.software.slb.com/products/petromod and Zetaware http://www.zetaware.com, to undertake burial and thermal history modelling on wells in Australian sedimentary basins. From the integration of geological (age-based sedimentary packages, uplift and erosional events), petrophysical (porosity, permeability, and thermal conductivity) and thermal (downhole temperature, heat flow, vitrinite reflectance, and Tmax) input data, to name the most significant, a best-fit model of the time-temperature history is generated. Since the transformation of sedimentary organic matter (kerogen) into petroleum (oil and gas) is a chemical reaction, it is governed by chemical kinetics i.e. time and temperature (in the geological setting, pressure is of secondary importance). Thus, the use of chemical kinetics associated with a formation-specific, immature potential source rock (where available) from the basin of interest is considered a better practical approach rather than relying on software kinetic defaults, which are generally based on the chemical kinetics determined experimentally on Northern Hemisphere organic matter types. As part of the Offshore Energy Systems program hydrocarbons from the Lower Cretaceous Eumeralla Formation were selected where available from onshore wells; compositional kinetics (1-, 2-, 4- and 14-component (phase) kinetics) were undertaken by GeoS4, Germany. The phase kinetics approach is outlined in Appendix 1. This report provides the compositional kinetics for potential source rocks from the Lower Cretaceous Otway Group, Otway Basin, Australia. The kinetic data were used in the offshore petroleum system modelling reported in Schenk et al. (2021).

Exploring for the Future (EFTF) is a multiyear (2016–2024) initiative of the Australian Government, conducted by Geoscience Australia. This program aims to improve Australia’s desirability for industry investment in resource exploration of frontier regions across Australia. This paper will focus on the science impacts from the EFTF program in northern Australia derived from the acquisition and interpretation of seismic surveys, the drilling of the NDI Carrara 1 and also complementary scientific analysis and interpretation to determine the resource potential of the region. This work was undertaken in collaboration with the Northern Territory Geological Survey, the Queensland Geological Survey, AuScope and the MinEx CRC. These new data link the highly prospective resource rich areas of the McArthur Basin and Mt Isa Province via a continuous seismic traverse across central northern Australia. The Exploring for the Future program aims to further de-risk exploration within greenfield regions and position northern Australia for future exploration investment. [Carr] The Sherbrook Supersequence is the youngest of four Cretaceous supersequences in the Otway Basin and was deposited during a phase of crustal extension. This presentation shows how a basin-scale gross depositional environment (GDE) map for the Sherbrook SS was constructed, the significance of the map for the Austral 3 petroleum system, and why GDE mapping is important for pre-competitive basin studies at Geoscience Australia. [Abbott]