The greater Phoenix area in the Bedout Sub-basin has experienced recent exploration success on Australia’s North West Shelf (NWS). Oil and gas discoveries in the Triassic reservoirs of the Keraudren Formation and Locker Shale have revived interest in mapping the distribution and lateral facies variation of the Triassic succession from the Bedout Sub-basin into the adjacent underexplored Beagle and Rowley sub-basins. This multi-disciplinary study integrating structural architecture, sequence stratigraphy, palaeogeography and geochemistry has mapped the spatial and temporal distributions of Triassic source rocks on the central NWS. The Lower‒Middle Triassic palaeogeography is dominated by a deltaic system building from the Bedout Sub-basin into the Beagle Sub-basin. The oil sourced and reservoired within the Lower‒Middle Triassic sequences at Phoenix South 1 is unique to the Bedout Sub-basin, compared to other oils along the NWS. Its mixed land-plant and algal biomarker signature is most likely sourced locally by fluvial-deltaic mudstones within the TR10‒TR14 or TR15 sequences and represents a new petroleum system on the NWS. A Middle Triassic marine incursion is recorded in the Bedout Sub-basin with the development of a carbonate platform while in the Rowley Sub-basin, volcanics have been penetrated at the top of the thick Lower‒Middle Triassic sediment package. The Late Triassic palaeogeographic map suggests a carbonate environment in the Rowley Sub-basin distinct to the clastic-dominated fluvial-deltaic environment in the Beagle Sub-basin. This information combined with results of well-based geochemical analyses highlights the potential for hydrocarbon generation within the Upper Triassic in these sub-basins. This extended abstract was presented at the Australasian Exploration Geoscience Conference (AECG) 2019

The central North West Shelf has been the focus of a regional mapping program by Geoscience Australia targeting the Triassic succession. Resulting updates to the regional structural understanding are presented, showcasing variations in structural style across the region. The Triassic section is affected by fault sets with two predominant orientations across the study area: N-trending and NE-trending, with localised areas of NW-trending faulting. There is typically vertical separation of faulting between the upper Triassic and lower Triassic successions, resulting in different fault fabrics mapped on the top and base Triassic surfaces. In some areas major faults penetrate through the Mesozoic section and into the Paleozoic basement, forming features with significant displacement such as the Thouin Graben, Whitetail Graben, Naranco High, and the Barcoo Sub-basin half graben. Isochore maps reveal two Triassic depocentres separated by an area of thin Triassic extending from the inboard Bedout Sub-basin out to the western Rowley Sub-basin and NE Exmouth Plateau. This new mapping shows that there is a poor correlation between Triassic depocentres and existing basin boundaries, making it difficult to clearly describe regions of interest and their evolution. Greater integration of new structural insights into a regional structural framework is necessary to improve our understanding of the tectonostratigraphic evolution of the margin, and the stratigraphic and structural aspects of exploration risk. This abstract was submitted to/presented at the 2019 Australasian Exploration Geoscience Conference (AEGC 2019) (https://www.aig.org.au/aegc-2019-data-to-discovery/)

<p>The Roebuck Basin and adjoining Beagle Sub-basin are underexplored areas on Australia’s North West Shelf and are undergoing renewed exploration interest since the discovery of oil at Phoenix South 1 and gas at Roc 1, 2 in the Bedout Sub-basin. A well folio of 24 offshore wells across the Beagle, Bedout, Rowley and Barcoo sub-basins was completed as part of Geoscience Australia’s assessment of hydrocarbon prospectivity across the region. The study consists of composite well log plots summarising lithology, stratigraphy, GA’s newly acquired biostratigraphic and geochemical data and petrophysical analysis, in conjunction with revised sequence interpretations. <p>The wells included in the well folio package are: <p>Anhalt 1, Barcoo 1 ST2, Bedout 1, Bruce 1, Cossigny 1, De Grey 1A ST1, Delambre 1, Depuch 1, East Mermaid 1B ST1, Hanover South 1, Huntsman 1, Keraudren 1. Lagrange 1, Minilya 1, Nebo 1, Omar 1, Phoenix 1, Phoenix 2, Phoenix South 1 ST1 ST2, Picard 1, Poissonnier 1, Roc 1, Steel Dragon 1 and Wigmore 1

High-precision radiometric dating using Chemical Abrasion-Isotope Dilution Thermal Ionisation Mass Spectrometry (CA-IDTIMS) has allowed the recalibration of the numerical ages of Permian and Triassic spore-pollen palynozones in Australia. These changes have been significant, with some zonal boundaries in the Permian shifting by as much as six million years, and some in the Triassic by more than twice that. Most of the samples analysed came from eastern Australian coal basins (Sydney, Gunnedah, Bowen, Galilee) where abundant volcanic ash beds occur within the coal-bearing successions. The recalibrations of these widely used palynozones have implications for the dating of geological events outside the basins from where samples were obtained. Our revised dates for the Permian palynozones can now be applied to all Permian basins across Australia, including the Perth, Carnarvon, Canning and Bonaparte basins (along the western and northern continental margins), the Cooper and Galilee basins (in central Australia), and the Bowen, Gunnedah and Sydney basins (in eastern Australia). Revised regional stratigraphic frameworks are presented here for some of these basins. The impact of an improved calibration of biostratigraphic zones to the numerical timescale is broad and far-reaching. For example, the more accurate stratigraphic ages are the more closely burial history modelling will reflect the basin history, thereby providing control on the timing of kerogen maturation, and hydrocarbon expulsion and migration. These improvements can in turn be expected to translate in to improved exploration outcomes. We have initially focused on the Permian and provide preliminary results for the Triassic, but intend to expand recalibrations to include Jurassic, Cretaceous and Paleozoic successions beyond the Permian. Preliminary data indicates that significant changes to these calibrations are also likely.

<p>A regional mapping program conducted by Geoscience Australia addressed stratigraphic and structural aspects of exploration risk within the Triassic succession of the Roebuck Basin and parts of the adjacent sub-basins (central North West Shelf, Figure 1). <p>Seismic horizons of regional significance were mapped using 2D and 3D seismic surveys. Seismic survey coverage is shown in Figure 1. 2D surveys include regional deep surveys such as AGSO s110, AGSO s120, and PGS New Dawn. 3D surveys include Admiral, Beagle, CNOOC, Curt, Lord, Naranco, Polly, Whitetail, and a 5 x 5 km extract (used with permission) from the TGS Capreolus MC3D. Synthetic seismograms (Nguyen et al., 2019) were used to tie seismic horizons to wells. <p>The mapped horizons are placed within a regional tectonostratigraphic framework by Abbott et al. (2019, their Figure 2). This data pack comprises seismic horizon grids and isochron grids generated from the TR10.0_SB (base Triassic), TR17.0_SB (Mid–Triassic), and J10.0_SB (top Triassic) seismic horizons (Figure 2). Fault maps compiled at the TR10.0 _SB and J10.0_SB are also included.