Promotional flyer describing the GA programme in national unconventional hydrocarbon prospectivity and resource assessment commenced in 2011 by the Onshore (Unconventional) Hydrocarbons Section, Basin Resources Group, Energy Division.

The under-explored deepwater Otway and Sorell basins lie offshore of southwestern Victoria and western Tasmania in water depths of 100-4,500 m. The basins developed during rifting and continental separation between Australia and Antarctica from the Cretaceous to Cenozoic and contain up to 10 km of sediments. Significant changes in basin architecture and depositional history from west to east reflect the transition from a divergent rifted continental margin to a transform continental margin. The basins are adjacent to hydrocarbon-producing areas of the Otway Basin, but despite good 2D seismic data coverage, they remain relatively untested and their prospectivity is poorly understood. The deepwater (>500 m) section of the Otway Basin has been tested by two wells, of which Somerset 1 recorded minor gas shows within the Upper Cretaceous section. Three wells have been drilled in the Sorell Basin, where minor oil and gas indications were recorded in Maastrichtian rocks near the base of Cape Sorell 1. Building on previous GA basin studies and using an integrated approach, new aeromagnetic data, open-file potential field, seismic and exploration well data have been used to develop new interpretations of basement structure and sedimentary basin architecture. Analysis of potential field data, integrated with interpretation of 2D seismic data, has shown that reactivated north-south Paleozoic structures, particularly the Avoca-Sorell Fault System, control the transition from extension through transtension to a dominantly strike-slip tectonic regime along this part of the southern margin. Depocentres to the west of this structure are large and deep in contrast to the narrow elongate depocentres to its east. Regional-scale mapping of key sequence stratigraphic surfaces across the basins has resulted in the identification of distinct basin phases. Three periods of upper crustal extension can be identified. In the north, one phase of extension in the Early Cretaceous and two in the Late Cretaceous can be mapped. However, to the south, the Late Cretaceous extensional phase extends into the Paleocene, reflecting the diachronous break-up history. Extension was followed by thermal subsidence, and during the Eocene-Oligocene the basin was affected by several periods of compression, resulting in inversion and uplift. The new seismic interpretation shows that depositional sequences hosting active petroleum systems in the producing areas of the Otway Basin are also likely to be present in the southern Otway and Sorell basins. Petroleum systems modelling suggests that if the equivalent petroleum systems elements are present, then they are mature for oil and gas generation, with generation and expulsion occurring mainly in the Late Cretaceous in the southern Otway and northern Sorell basins and during the Paleocene in the Strahan Sub-basin (southern Sorell Basin). The integration of sequence stratigraphic interpretation of seismic data, regional structural analysis and petroleum systems modelling has resulted in a clearer understanding of the tectonostratigraphic evolution of this complex basin system. The results of this study provide new insights into the geological controls on the development of the basins and their petroleum prospectivity.

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.

Vertical geochemical profiling of the marine Toolebuc Formation, Eromanga Basin - implications for shale gas/oil potential The regionally extensive, marine, mid-Cretaceous (Albian) Toolebuc Formation, Eromanga Basin hosts one of Australia's most prolific potential source rocks. However, its general low thermal maturity precludes pervasive petroleum generation, although regions of high heat flow and/or deeper burial may make it attractive for unconventional (shale gas and shale oil) hydrocarbon exploration. Previous studies have provided a good understanding of the geographic distribution of the marine organic matter in the Toolebuc Formation where total organic carbon (TOC) contents range to over 20% with approx. half being of labile carbon and convertible to gas and oil. This study focuses on the vertical profiling, at the decimetre to metre scale, of the organic and inorganic geochemical fingerprints within the Toolebuc Formation with a view to quantify fluctuations in the depositional environment and mode of preservation of the organic matter and how these factors influence hydrocarbon generation thresholds. The Toolebuc Formation from three wells, Julia Creek-2 and Wallimbulla-2 and -3, was sampled over an interval from 172 to 360m depth. The total core length was 27m from which 60 samples were selected. Cores from the underlying Wallumbilla Formation (11 samples over 13m) and the overlying Allaru Mudstone (3 samples) completed the sample set. Bulk geochemical analyses included %TOC, %carbonate, %total S, -15N kerogen, -13C kerogen, -13C carbonate, -18O carbonate, and major, minor and tracer elements and quantitative mineralogy. More detailed organic geochemical analyses involved molecular fossils (saturated and aromatic hydrocarbons, and metalloporphyrins), compound specific carbon isotopes of n-alkanes, pyrolysis-gas chromatography and compositional kinetics. etc.

Presentation delivered on 9 March 2012 by Marita Bradshaw.

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.

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

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.

As part of the Offshore Energy Security Program (2007-2011), Geoscience Australia (GA) undertook an integrated regional study of the deepwater Otway and Sorell basins to improve the understanding of the geology and petroleum prospectivity of the region. Major outputs of this study include: - New interpretations of basement architecture and structural fabric resulting in the recognition of the Avoca-Sorell Fault System as a major control on sedimentary basin development, - Extension of the tectonostratigraphic framework of Krassay et. al. (2004) into the deepwater Otway and Sorell basins, leading to new insights into their structural and accommodation histories of both basins, and - Petroleum systems modelling indicating that these basins are mature for oil and gas generation.

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. The ever expanding coal seam gas industry on Australia's east coast in addition to the large resource potential of shale and tight gas in Australia's eastern basins 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 begin an assessment of Australia's coal seam gas, shale gas and oil and tight gas resource potential. Capitalising on decades of high quality geological data held by the Commonwealth and the States and Territories, the aim of this collaboration is to develop nationally consistent assessment methodologies and provide robust national resource estimates in an internationally accepted standard. Overall, the programme aims to answer the 'where' and 'how much' questions for government, as well as provide this new industry with pre-competitive data and tools for comparing exploration opportunities. 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.