The unexpected discovery of oil in Triassic sedimentary rocks of the Phoenix South 1 well on Australia’s North West Shelf (NWS) has catalysed exploration interest in pre-Jurassic plays in the region. Subsequent neighbouring wells Roc 1–2, Phoenix South 2–3 and Dorado 1–3 drilled between 2015 and 2019 penetrated gas and/or oil columns, with the Dorado field containing one of the largest oil resources found in Australia in three decades. This study aims to understand the source of the oils and gases of the greater Phoenix area, Bedout Sub-basin using a multiparameter geochemical approach. Isotopic analyses combined with biomarker data confirm that these fluids represent a new Triassic petroleum system on the NWS unrelated to the Lower Triassic Hovea Member petroleum system of the Perth Basin. The Bedout Sub-basin fluids were generated from source rocks deposited in paralic environments with mixed type II/III kerogen, with lagoonal organofacies exhibiting excellent liquids potential. The Roc 1–2 gases and the Phoenix South 1 oil are likely sourced proximally by Lower–Middle Triassic TR10–TR15 sequences. Loss of gas within the Phoenix South 1 fluid due to potential trap breach has resulted in the formation of in-place oil. These discoveries are testament to new hydrocarbon plays within the Lower–Middle Triassic succession on the NWS.

The Roebuck Basin on Australia’s offshore north-western margin is the focus of a regional hydrocarbon prospectivity assessment being undertaken by the Offshore Energy Systems Section. This offshore program is designed to produce pre-competitive information to assist with the evaluation of the hydrocarbon resource potential of the central North West Shelf and attract exploration investment to Australia. The recent oil and gas discoveries at Phoenix South 1 (2014), Roc 1 (2015-16), Roc 2 (2016), Phoenix South 2 (2016), Phoenix South 3 (2018), Dorado 1 (2018) and Dorado 2–3 (2019) in the Bedout Sub-basin demonstrate the presence of a petroleum system in Lower Triassic strata (Grosjean et al., 2021; Rollet et al., 2019). As part of this program, a range of organic geochemical analyses were acquired on petroleum fluids from the Dorado 1 and Roc 2 wells with these data released in this report.

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.

<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.

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 on Australia’s offshore north-western margin is the focus of a regional hydrocarbon prospectivity assessment being undertaken by the North West Margin Energy Studies Section (NWMES). This offshore program is designed to produce pre-competitive information to assist with the evaluation of the hydrocarbon resource potential of the central North West Shelf and attract exploration investment to Australia. <p>The recent oil and gas discoveries at Phoenix South 1 (2014), Roc 1 (2015-16), Roc 2 (2016), Phoenix South 2 (2016), Phoenix South 3 (2018) and Dorado 1 (2018) in the Bedout Sub-basin demonstrate the presence of a petroleum system in Lower Triassic strata. The current study aims to better understand this new petroleum system and establish its extent. <p>As part of this program, TOC and Rock-Eval pyrolysis analyses were undertaken by Geoscience Australia on selected rock samples from the well Roc 2 to establish their hydrocarbon-generating potential and thermal maturity.

<p>The Roebuck Basin on Australia’s offshore north-western margin is the focus of a regional hydrocarbon prospectivity assessment being undertaken by the North West Margin Energy Studies Section (NWMES). This offshore program is designed to produce pre-competitive information to assist with the evaluation of the hydrocarbon resource potential of the central North West Shelf and attract exploration investment to Australia. <p>The recent oil and gas discoveries at Phoenix South 1 (2014), Roc 1 (2015-16), Roc 2 (2016), Phoenix South 2 (2016), Phoenix South 3 (2018) and Dorado 1 (2018) in the Bedout Sub-basin demonstrate the presence of a petroleum system in Lower Triassic strata. The current study aims to better understand this new petroleum system and establish its extent. <p>As part of this program, compositional and isotopic analyses were undertaken by Geoscience Australia on gas samples from the wells Roc 1 and Roc 2 and data from these analyses are released in this dataset.

<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

In 2014, the surprise discovery of oil in the well Phoenix South 1 on Australia’s North West Shelf (NWS) heralded the Bedout Sub-basin as Australia’s new petroleum province. The well had initially targeted gas in Triassic reservoirs of the Keraudren Formation and Locker Shale but unexpectedly encountered oil instead. This has fuelled a revival of exploration in the area with subsequent wells Roc 1, Roc 2, Phoenix South 2 and Dorado 1 all successful in penetrating hydrocarbon columns. The Phoenix South 1 accumulation is significant as it demonstrates the occurrence of an effective oil-prone source rock within the Lower–Middle Triassic for the first time on the NWS. It also raised the possibility that a source rock analogue to the Hovea Member of the Lopingian–Lower Triassic Kockatea Shale, a well-established source of oil and gas in the northern Perth Basin, may also be effective on the NWS. In order to understand the origin of the Phoenix South 1 accumulation, its geochemical composition was compared to those of Triassic-sourced oils from the Perth Basin and petroleum fluids from the neighbouring Browse Basin, and Beagle and Dampier sub-basins.

<div>The Roebuck Basin on Australia’s offshore north-western margin is the focus of regional energy exploration activity. Drilling in the Roebuck Basin resulted in oil and gas discoveries at Phoenix South&nbsp;1 (2014), Roc&nbsp;1 (2015–2016) and Dorado&nbsp;1 (2018) in the Bedout Sub-basin (Figure 1‑2) and demonstrated the presence of a petroleum system in Lower Triassic strata. These discoveries have been evaluated for development and production with infill drilling at Roc&nbsp;2 (2016), Phoenix South&nbsp;2 (2016), Phoenix South&nbsp;3 (2018), Dorado&nbsp;2 (2019), and Dorado&nbsp;3 (2019). Recent drilling by Santos (2022) has resulted in the discovery of oil at Pavo&nbsp;1 (2022) and hydrocarbon shows at Apus&nbsp;1 (2022).</div><div><br></div><div>To complement this industry work, Geoscience Australia’s Offshore Energy Systems program produces pre-competitive information to assist with the evaluation of the energy and resource potential of the central North West Shelf, including both hydrogen and helium resources, and to attract exploration investment to Australia. As part of this program, determination of the molecular and noble gas isotopic composition of natural gases from selected petroleum wells in the Roebuck Basin were undertaken by Smart Gas Sciences, under contract to Geoscience Australia, with results from these analyses being released in this report. This report provides additional gas data to determine the sources of natural gases in the Roebuck Basin and build on previously established gas-gas 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>