The Early Cretaceous South Perth Shale has been previously identified as the regional seal in the offshore Vlaming Sub-basin. The South Perth Shale is a deltaic succession, which unfilled a large palaeotopographic low in the Early Cretaceous through a series of transgressive and regressive events. The new study undertaken at Geoscience Australia has shown that the seal quality varies greatly throughout the basin and at places has very poor sealing properties. A re-evaluation of the regional seal based on seismic mapping determined the extent of the pro-delta shale facies within the South Perth Shale succession, which are shown to provide effective sealing capacity. New sequence stratigraphic interpretation, seismic facies mapping, new and revised biostratigraphic data and well log analysis were used to produce palaeogeographic reconstructions which document the distribution of depositional facies within the South Perth Shale Formation and reveal evolution of the Early Cretaceous deltas. Our study documents spatial variations in the seal quality and re-defines the extent and thickness of the regional seal in the central Vlaming Sub-basin. It provides an explanation for the lack of exploration success at some structural closures and constraints for possible location of the valid plays.

Presentation delivered on 9 March 2012 by Marita Bradshaw.

Extended abstract version of the abstract (Geocat#73747) submitted in March 2012 and accepted for an oral presentation at the symposium.

Geoscience Australia has recently completed a marine survey in the offshore northern Perth Basin, off Western Australia (Jones et al., 2011b; Jones, 2011c, Upton and Jones, 2011). One of the principal aims of the survey was the collection of evidence for natural hydrocarbon seepage. The survey formed part of a regional reassessment of the basin's petroleum prospectivity in support of frontier exploration acreage Release Area W11-18. This reassessment was initiated under the Australian Government's Offshore Energy Security Program and formed part of Geoscience Australia's continuing efforts to identify a new offshore petroleum province. The offshore northern Perth Basin was identified as a basin with new frontier opportunities. New data demonstrated that proven onshore-nearshore petroleum system is also effective and widespread in the offshore (Jones et al., 2011a). Evidence for a Jurassic petroleum system was also demonstrated in the Release Area W11-18 (Jones et al., 2011a). The marine survey results provide additional support for the presence of an active petroleum system in the northern Perth Basin.

This study assesses the petroleum potential of the Paleo–Mesoproterozoic Birrindudu Basin in the northwestern Northern Territory, which is one of several Proterozoic basins in northern Australia with the potential to host conventional and unconventional petroleum accumulations. Historical source rock geochemistry, porosity, and permeability data from the Birrindudu Basin are collated and interpreted; in addition, new fluid geochemistry is interpreted within the context of the greater McArthur Basin. The limited data available indicate that at least four formations have good or excellent present-day organic richness (>2 wt% TOC), and several sandstone and carbonate reservoirs have good porosity data. The calculated brittleness index of a number of organic-rich shales suggests that several are likely to be favourable for fracture stimulation and therefore might constitute good unconventional hydrocarbon targets. Four continent-scale petroleum supersystems are identified, two of which are described for the first time. These supersystems are an important tool in understanding the petroleum potential in frontier basins with limited data. Additionally, a number of basin-scale petroleum systems are potentially present within the basin successions; 14 possible conventional systems and 9 possible unconventional systems are documented. Petroleum play concepts are also described to assist with assessing the potential for conventional and unconventional hydrocarbon resources. The ultimate aim is to identify areas that can be targeting for precompetitive geoscience data acquisition, so as to reduce the exploration search space. Presented at Annual Geoscience Exploration Seminar (AGES) April 2021 (p115 - p130)

The Mesozoic Beagle Sub-basin is in the Northern Carnarvon Basin, offshore Western Australia. Oil discovered at Nebo 1 in 1993 highlights an active petroleum system. The central Beagle Sub-basin, this study's focus, has a north-south trending horst-graben architecture. Detailed mapping of the 1529 km2 Beagle Multi-client 3D seismic survey gave insight into its geological history. The Rhaetian to Valanginian syn-rift succession comprises fluvio-deltaic and marine sediments deposited during low rates of crustal extension. During post-rift thermal subsidence, sediments onlapped eroded and tilted fault blocks formed during the syn-rift phase. Consequently, the Early Cretaceous regional seal is absent in the central study area. Overlying sedimentary successions are dominated by a prograding carbonate wedge. Potential source, reservoir and seal facies are present from the Triassic to earliest Cretaceous. 1D burial history modelling indicates that in Nebo 1, potential source rocks from the Middle Jurassic to Early Cretaceous became oil mature after the emplacement of the regional seal. At Manaslu 1, these sediments are immature. Potential source rocks are currently at maximum burial depth and thermal maximum. Trap integrity in the pre and syn-rift succession could be jeopardized by fault reactivation, however post-rift traps may be preserved. Potential plays include compaction folds over tilted horst blocks, anticlines, basin-floor fans and intra-formational traps. Hydrocarbons could use deep faults to migrate into Early Cretaceous plays. Younger sediments lack migration pathways so are unlikely to host significant hydrocarbons. Poor quality source rocks and reservoirs, and poor source rock distribution may also contribute to disappointing exploration results.

The 2012 Australian offshore acreage release includes exploration areas in four southern margin basins. Three large Release Areas in the frontier Ceduna Sub-basin lie adjacent to four exploration permits granted in 2011. The petroleum prospectivity of the Ceduna Sub-basin is controlled by the distribution of Upper Cretaceous marine and deltaic facies and a structural framework established by Cenomanian growth faulting. These Release Areas offer a range of plays charged by Cretaceous marine and coaly source rocks and Jurassic lacustrine sediments. In the westernmost part of the gas-producing Otway Basin, a large Release Area offers numerous opportunites to test exisiting and new play concepts in underexplored areas beyond the continental shelf. Gas and oil shows in the eastern part of the Release Area confirm the presence of at least two working petroleum systems. In the eastern Otway Basin, several Release Areas are offered in shallow water on the eastern flank of the highly prospective Shipwreck Trough and provide untested targets along the eastern basin margin southward into Tasmanian waters. To the south, a large Release Area in the frontier Sorell Basin provides the opportunity to explore a range of untested targets in depocentres that formed along the western Tasmanian transform continental margin. This year, two Release Areas offer exploration potential in the under-explored eastern deep-water part of the Gippsland Basin. Geological control is provided by several successful wells indicating the presence of both gas and liquids in the northern area, while the southern area represents the remaining frontier of the basin.

Exploration for Unconventional Hydrocarbons in Australia reached a new milestone when Beach Energy announced the first successful flow test of a shale gas target in the Cooper Basin. Significant exploration activity is being seen in the Amadeus, Pedirka and Georgina basins and Beetaloo Sub-basin, while little is known of the potential of many other Central Australian basins. The globally acknowledged large resource potential of coal seam gas, shale and tight gas on the continent in addition to low sovereign risk has put Australia firmly on the radar of many local and international exploration companies. Over the next 12 months Geoscience Australia in collaboration with its counterparts in the State and Territory resource and energy departments will undertake an initial assessment of Australia's unconventional hydrocarbon resource potential. Capitalising on decades of high quality geological data held by the Commonwealth and the States and Territories, the programme aims to compile these data using nationally consistent assessment methodologies that ultimately provide robust figures in an internationally accepted standard. The immediate goal is to provide a first-pass, high level estimate of the likely resource volumes, which will be reported in the second edition of the Australian Energy Resource Assessment (published by RET). The longer term work program aims to assess Australia's onshore basins in terms of their resource potential and provide pre-competitive data to industry. To achieve this, several geological techniques will be applied including, but not limited to, geochemical screening, mapping of source rock occurrences and their distributions as well as physical rock property studies.

Release Area W11-18 is a very large block over the offshore northern Perth Basin, covering parts of the Abrolhos, Houtman and Vlaming sub-basins and the Beagle and Turtle Dove ridges. Geoscience Australia (GA) has assessed the petroleum prospectivity of this area as part of the Australian Government's Offshore Energy Security Program. This assessment includes the first published synthesis of data from fourteen new field wildcat wells drilled in this part of the basin since the Cliff Head-1 discovery (2001), and the interpretation of new regional 2D seismic data acquired during GA survey 310 (2008-2009). A refined tectono-stratigraphic model for the offshore basin provides insights into basin evolution and prospectivity. Oil has been produced since 2006 from the Cliff Head oil field in WA-31-L, which is directly adjacent to Release Area W11-18. Three petroleum discoveries are included within the Release Area, with oil and gas in Dunsborough-1, and gas in Frankland-1 and Perseverance-1. These accumulations are reservoired in Permian sandstones and have primarily been sourced from the Hovea Member of the Kockatea Shale, which has also sourced the majority of producing oil and gas fields of the onshore Perth Basin. New seismic data show Permo-Triassic strata that are stratigraphic equivalents of the productive onshore and nearshore Perth Basin petroleum system, also occur within Permian half-graben in the outer Abrolhos and Houtman sub-basins. Source rock, oil stain and fluid inclusion sampling from this interval suggest that the proven onshore-nearshore petroleum system is also effective and widespread in the offshore. There is also evidence for an active Jurassic petroleum system within the Release Area. The Release Area offers a range of plays in a variety of water depths, predominantly less than 200 m, and is highly prospective for oil and gas.

In this study detailed mapping of seismic data from the 1529 km2 Beagle multi-client 3D seismic survey was undertaken to provide a better understanding of the geological history of the central Beagle Sub-basin. Situated in the Northern Carnarvon Basin, oil discovered at Nebo 1 in 1993 indicated the presence of at least one active petroleum system. The central part of the sub-basin has a N-trending horst-graben architecture. Two rifting events from the Hettangian to Sinemurian and the Callovian to Oxfordian were identified. A series of tilted fault blocks formed by the rifting events were locally eroded and progressively draped and buried by post-rift thermal subsidence sedimentation. Mapping indicated the Post-rift I Lower Cretaceous Muderong Shale regional seal is anomalously thin or absent in the intra-horst graben area. Burial history 1D modelling indicates that at Nebo 1, the most rospective potential source rocks within the Middle-Upper Jurassic section where in the early oil window; however, if present within the Beagle and Cossigny trough depocentres, these sediments would have entered the oil window prior to the deposition of the Muderong Shale regional seal. Upper Jurassic shales provide seal for the oil pool intersected in Nebo 1. The Tertiary section is dominated by a prograding carbonate wedge which has driven a second phase of thermal maturation observed in the Paleogene (Nebo 1) and Miocene (Manaslu 1). Potential source rocks are currently at their maximum depth of burial and maximum thermal maturity. Modest inversion on some faults prior to the Early Cretaceous has created traps and if source rocks retain generative potential, favourable traps could be now actively receiving hydrocarbon charge. Potential plays include compaction folds over tilted horst blocks, drape and small inversion induced anticlines, basin-floor fans and intra-formational traps. Deep faults may act as conduits for hydrocarbons migrating from mature potential source rocks into Jurassic to Cretaceous plays. Younger sediments appear to lack access to migration pathways provided by deeper faults.