<div>Ideally when combining different 3D seismic surveys differences in acquisition parameters warrant full pre-stack reprocessing from field data. However, there are occasions where this is not possible due to time, financial or data access constraints; a valuable alternative is post-stack merging and enhancement of existing migrations. The offshore Otway Basin was the subject of such a project, the objective of which was to produce a regularised and seamless 3D dataset of the highest possible quality, within a two-month turnaround time. The input migrated volumes varied by data extent, migration methodology, angle range and grid orientation. 14 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. DUG’s mis-tie analysis algorithm, applied over a time window optimised for interpretation of key</div><div>events, was used to derive corrections for timing, phase and amplitude, using the Investigator North survey as a reference. This was followed by time-variant spectral and amplitude matching, with gain corrections applied, 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 event and dramatically improving the efficiency and quality of interpretation. This enables rapid reconnaissance of the area by explorers. Presented at the Australian Energy Producers (AEP) Conference & Exhibition

Reports of bitumen stranding on the ocean beaches of southern Australia date back to the early days of European settlement. Previous investigations have shown that this ‘coastal bitumen’ comprises three categories of stranded petroleum: waxy bitumen, asphaltite and oil slicks. All three varieties are physically and chemically distinct from each other, and bear no geochemical resemblance to any indigenous Australian crude oil. This study focuses on the most common variety, waxy bitumen, which accounted for 90% of the strandings on six South Australian beaches repeatedly surveyed during 1991–1992. Geochemical analysis of 96 individual specimens collected from these survey sites and other beaches in South Australia and western Victoria has shown them to be variously weathered high-wax crude oils of paraffinic to aromatic-intermediate bulk composition. Elemental, isotopic and biomarker differences allow their assignment to at least five oil families with inferred source facies that range from deep freshwater lacustrine through paludal and deltaic to euxinic marine, possibly deposited within different sedimentary basins. Family 1, 2 and 3 waxy bitumens all contain biomarkers derived from the freshwater alga Botryococcus sp. and tropical angiosperms (notably dipterocarps). Similar biomarker assemblages are unknown in Australian sedimentary basins but are common in Cenozoic crude oils and source rocks throughout western Indonesia. Family 4 waxy bitumens lack these biomarkers, but do contain dinosterane and 24-n-propylcholestane, indicative of a marine source affinity, while the carbon isotopic signatures and high pristane/phytane (Pr/Ph) ratios of Family 5 waxy bitumens are consistent with their origin from coal-rich source rocks deposited in fluvial to deltaic sedimentary successions. The majority of these waxy bitumens represent an oceanic influx of non-indigenous, Southeast Asian crude oils carried into the waters of southern Australia by the Leeuwin Current. Although they are likely to originate from natural seepage within the Indonesian Archipelago, it is unknown whether the parent oils emanate from submarine seeps or from inland seepages which are then carried to the sea by rivers. The common practice of tanker cleaning operations in the Java and Banda seas may augment the supply of natural bitumen to the beaches of Australia.

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]

During 2021–2024 Geoscience Australia conducted regional seismic mapping across the offshore Otway Basin that extended into the frontier deep-water region. This work was part of a broader pre-competitive study undertaken in support of petroleum exploration. Seismic horizons and faults were interpreted on three regional data sets, including: over 18 000 line-km of new and reprocessed data compiled for the 2020 offshore Otway Basin seismic program; over 40 000 line-km of legacy 2D seismic data; and the Otway 3D Megamerge dataset. This digital dataset (publication date 9 September 2024) updates and replaces a previously released dataset (publication date 16 May 2022). This updated dataset includes 8 surface grids and 11 isochron grids generated from the following seismic horizons (in ascending stratigraphic order); MOHO (Mohorovičić discontinuity), TLLCC (top laminated lower continental crust), Base (base Crayfish Supersequence), EC2 (base Eumeralla Supersequence), LC1 (base Shipwreck Supersequence), LC1.2 (base LC1.2 Sequence), LC2 (base Sherbrook Supersequence), and T1 (base Wangerrip Supersequence). Fault polygons created for all surfaces (except for MOHO, TLLCC, and LC1.2) are also included in the dataset. Maps generated from the dataset depict deep-water Cretaceous depocentres, and trends in crustal thinning and rifting during the Cretaceous. This revised dataset has underpinned updates to regional structural elements, including a revision of the boundary between the Otway and Sorell basins.

<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>

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).

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.

Over 8,200 line kilometres of gravity and magnetic data, acquired during the 2020 Otway Basin Seismic Program (OBSP), were combined with public domain survey and satellite data to produce seamless maps of the NW-SE trending deep-water Otway Basin. These data provide valuable information on the geometry and spatial extent of igneous rocks in the deep-water basin. While the top of basement can effectively be imaged from seismic reflection datasets onshore in the Otway Basin, it remains problematic in parts of the deep-water offshore region due to variable seismic data quality. Modelling of the magnetic line data provides an estimate of the depth to the top of basement, an important interface for understanding hydrocarbon prospectivity because it plays a key role in characterising the tectonic evolution of the basin, and thus the thermal maturation history of hydrocarbons. Magnetic modelling was performed using a profile-based curve matching technique producing a depth estimate to the top of the magnetic body that is assumed to be the top of the basement. However, this assumption is flawed where there are volcanic or igneous intra-sedimentary rocks in the basin, as is the case for the Otway Basin where the interpretation of seismic reflection data shows highly reflective events corresponding to igneous features. In most parts of the basin, the modelling results show two layers: a shallow layer (depths < 1000m) corresponding to near surface volcanics, and a deeper layer (depths > 1000m) attributed to the top of the magnetic basement. Magnetic basement shows some similarities with basement picked on seismic reflection data, though in some areas the magnetic basement is shallower. The results also show that the depth to basement is not well resolved in areas with abundant intra-sedimentary igneous rocks. Further investigation is needed in such areas. Presented at the 2024 Australian Society of Exploration Geophysicists (ASEG) Discover Symposium

<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/)