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 is considered a new and relatively untested hydrocarbon province in the central North West Shelf of Australia. Inconsistent results from drilling for hydrocarbons highlights the need to better understand the deep structures along this rifted margin that initially formed as an intra-continental, failed rift during Late Permian. Recent wells penetrated the previously unknown Lower-Middle Triassic fluvio-deltaic sedimentary package in the Bedout Sub-basin (inboard part of the Roebuck Basin), including intervals with major oil and gas discoveries. Another two wells, Anhalt 1 and Hannover South 1, only penetrated the top of this succession and they encountered volcanics in the Rowley Sub-basin (outboard part of the Roebuck Basin). Steeply dipping clinoforms observed in the seismic data in the Rowley Sub-basin have been interpreted either as a lava delta complex associated with a failed triple junction; or as a series of back-stepping, Late Permian carbonate ramps and banks, interpreted to have developed on a thermally subsiding rift flank. The implication for prospectivity between the two scenarios is significant. Geoscience Australia undertook a Triassic regional basin analyses, including potential field modelling to validate whether the two proposed models are a plausible solution. A combination of magnetic and gravity 2.5D modelling along nine key regional seismic lines, considered the distribution of potential intrabasinal volcanic rocks and the crustal structure, including Moho depth and depth to top crystalline basement. <p>New seismic interpretation correlated to recent wells, including 2D and 3D seismic reflection surveys was integrated with deep seismic reflection and refraction data resulting in an improved geometry and lithology model that was input into the potential field analyses. The results show that the combined Jurassic and Triassic successions reach up to 16 km deep in the central North West Shelf. The Lower-Middle Triassic sediment package in the Rowley Sub-basin correlates with up to 10 km of dense material (about 2.7 g/cm3 density) and contains magnetic features partially sourced from basalts at the top of the section, as intersected in Anhalt 1 and Hannover South 1. Combined with other causative sources within basement, the basalts correlate with a spatially large positive magnetic anomaly that extends north onto the Scott Plateau and into the Barcoo Sub-basin; in the offshore southwest part of the Browse Basin, where Warrabkook 1 intersected Late Jurassic volcanoclastics at its total depth. The presence of high density and high positive magnetic anomalies in the Lower-Middle Triassic and basement supports the presence of a large igneous province in this area. This interpretation in the outer Rowley Sub-basin downgrades the petroleum prospectivity in this area for this Lower-Middle Triassic interval. Petroleum prospectivity remains in the area due to the overlying sediments containing good source rocks which have been identified to have good to excellent generative potential. <p>The Lower-Middle Triassic sediment package in the adjacent northern Carnarvon Basin has been intersected only on the Lambert Shelf; encountering fluvio-deltaic sediments. In the offshore part of the northern Carnarvon Basin, the nature of this sediment package still remains enigmatic. It correlates with low density sediments (about 2.5 g/cm3 density) that include magnetic bodies on the outboard Exmouth Plateau. The basement and crust show crustal thinning with the presence of a thick layer of interpreted hyper-extended continental crust or exhumed lithospheric mantle. This crustal domain is overlain by thick onlapping Lower-Middle Triassic sediments which form a triangular shape depocentre in the inboard northern Carnarvon Basin, wrapping around the edge of the Pilbara Craton. The location of this initial thick sediment package suggests that it was controlled by the inherited thermal structure of the Late Permian-early Triassic rift architecture that is associated with some volcanics related to a large igneous province extending across the central North West Shelf. As described in the Rowley Sub-basin, the petroleum prospectivity of the northern Carnarvon Basin remains in the overlying sediments showing similar characteristics and indicating good to excellent hydrocarbon generative potential.

The upper Permian to Lower Triassic sedimentary succession in the southern Bonaparte Basin represents an extensive marginal marine depositional system that hosts several gas accumulations, including the Blacktip gas field that has been in production since 2009. Development of additional identified gas resources has been hampered by reservoir heterogeneity, as highlighted by preliminary results from a post drill analyses of wells in the study area that identify reservoir effectiveness as a key exploration risk. The sedimentary succession that extends across the Permian–Triassic stratigraphic boundary was deposited during a prolonged marine transgression and shows a transition in lithofacies from the carbonate dominated Dombey Formation to the siliciclastic dominated Tern and Penguin formations. Recent improvements in chronostratigraphic calibration of Australian biostratigraphic schemes, spanning the late Permian and Early Triassic, inform our review of available palynological data and re-interpretation and infill sampling of well data. The results provide a better resolved, consistent and up-to-date stratigraphic scheme, allowing an improved understanding of the timing, duration, and distribution of depositional environments of the upper Permian to Lower Triassic sediments across the Petrel Sub-basin and Londonderry High. <b>Citation:</b> Owens R., Kelman A., Khider K., Iwanec J., Bernecker T. (2022) Addressing exploration uncertainties in the southern Bonaparte Basin: enhanced stratigraphic control and post drill analysis for upper Permian plays. <i>The APPEA Journal</i> 62, S474-S479

<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.

<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