In February-March 2004, Geoscience Australia undertook a 26 day geophysical and geological survey of the undrilled frontier depocentres of the Bremer and Denmark sub-basins. These sub-basins comprise the western part of the Bight Basin, and are located off the southern coast of Western Australia in water depths ranging from 100 to 4500 m. The survey was planned to recover rock samples and acquire high-speed seismic and swath data over the sub-basins in order to define a chronostratigraphic and depositional framework for the evolution of these depocentres. Ultimately, this information will be used to advance our understanding of the petroleum prospectivity of this frontier region.

The Bass Basin is a moderately explored Cretaceous to Cainozoic intracratonic rift basin on Australia's southeastern margin, underlying the shallow seabed between Tasmania and the Victorian mainland (Bass Strait; Figure 1). The basin contains proven commercial reserves of gas and condensate that are soon to be developed (Yolla gas field, Origin Energy Resources Ltd-operated BassGas Project). Other discoveries in the basin include the White Ibis and Pelican gas fields. To date, the 32 wells drilled have targeted Upper Cretaceous to Middle Eocene reservoirs within fault blocks and anticlinal structures. The targeted succession comprises interbedded fluvio-deltaic and lacustrine sandstones, siltstones and shales. The principal source rocks in the Bass Basin are interbedded coals (ranging from 5 to 25 m thick) and lacustrine shales of early Palaeogene age. Geochemical analyses show these source rocks have generated liquid and gaseous hydrocarbons, with the coals being the dominant source for the liquids (Boreham et al., 2003).

The Onshore Energy Security Program, funded by the Australian Government, Geoscience Australia has acquired deep seismic reflection data across several frontier sedimentary basins to stimulate interest in petroleum exploration in onshore Australia. Detailed interpretation of deep seismic reflection profiles from four onshore basins, focusing on overall basin geometry and internal sequence stratigraphy will be presented here, with the aim of assessing the petroleum potential of the basins. At the Southern end of the exposed part of the Mt Isa Province, northwest Queensland, a deep seismic line (06GA-M6) crosses the Burke River Structural Zone of the Georgina Basin. The basin here is >50 km wide, with a half graben geometry, and bound in the west by a rift border fault. The Millungera Basin in northwest Queensland is completely covered by the thin Eromanga basin and was unknown prior to being detected on two seismic lines (06GA-M4 and 06GA-M5) acquired in 2006. Following this, seismic line 07GA-IG1 imaged a 65 km wide section of the basin. The geometry of internal stratigraphic sequences and post-depositional thrust margin indicate that the original succession was much thicker than preserved today. The Yathong Trough in the southeast part of the Darling Basin in NSW has been imaged in seismic line 08GA-RS2 and interpreted in detail using sequence stratigraphic principles, with several sequences being mapped. The upper part of this basin contains Devonian sediments, with potential source rocks at depth.

Petroleum exploration in the Arafura Basin has been restricted to the Goulburn Graben, a dominant central feature within the basin. The graben is over 350 km long and up to 70 km wide, containing a highly deformed sedimentary fill of up to 10 km. To date a total of nine wells have been drilled in the region which test a variety of structural and stratigraphic play types. No commercial discoveries have been made. The most significant drilling result is an oil and gas show in Arafura-1. A review of drilling results has identified a number of exploration risks in the basin: 1) poor quality reservoirs in the Palaeozoic and/or restricted fluid movement; 2) hydrocarbon charge and timing of events; and 3) breach of structure. Most of these issues are related to a Triassic contractional event that caused uplift and erosion. Despite these risks, significant petroleum potential remains for the Arafura Basin as a whole. Firstly the identified risk factors may not apply to the undrilled northern part of the basin, which is the basin depocentre. Secondly, there is strong evidence for viable petroleum systems with source, reservoir and seal rocks present in the sedimentary succession. Evidence for active source rocks is provided by numerous hydrocarbon indications in the wells, including oil shows and bitumen in Arafura-1 and Goulburn-1. Analysis of these shows indicates a probable Cambrian source and there is evidence of significant vertical migration of fluids, with Cambrian oil signatures throughout the Palaeozoic sections. With high quality reservoirs (>20% porosity) and a regional seal, potential exists for accumulations within the overlying Money Shoal Basin. Given the limited petroleum exploration in the Arafura Basin, there remains considerable untested potential in both the Goulburn Graben and the unexplored northern region.

This module is part of the AGSO-APIRA Australian Petroleum Systems Project. Eight basin modules were examined which covered almost the entire North West Shelf, the Petrel Sub-basiTn, as well as the Papuan basin in PNG. Two relational databases were established containing the biostratigraphic data (STRATDAT) and reservoir, facies and hydrocarbon shows data (RESFACS). These databases were linked by application programs which allow time series searching using geologically intelligent routines. Petroleum systems analyses were conducted on each area, with key results focussing upon the comparison of source quality and timing of generation between similar systems in different areas.

A high resolution sequence stratigraphic study has been undertaken on the three wells in the Houtman Sub-basin, offshore North Perth Basin: Gun Island 1 (1968), Houtman 1 (1978) and Charon 1 (2008). The study focussed on the late Jurassic Yarragadee Formation, mid Jurassic Cadda Formation and early Jurassic Cattamarra Coal Measures. Log character (particularly gamma ray and sonic), cuttings, sidewall core and conventional core lithologies (including sedimentary structures) and palynological data were used to identify paleoenvironments. Stacking patterns of the interpreted environments were used to define systems tracts and then sequences. New palynological data have been collected by Geoscience Australia for Gun Island 1 and the palynology for all wells has been reviewed from Well Completion Reports and slides from intervals in each well. A number of transgressive systems tracts within the dominantly continental Yarragadee Formation and Cattamarra Coal Measures record small marine incursions into the Houtman Sub-basin. Within these units, the shallow marine intervals switch rapidly with non-marine intervals suggesting a more dynamic environment existed in the Houtman Sub-basin during the Jurassic than previously thought. These marine incursions are not evident in the Yarragadee Formation in Charon 1, indicating a lack of accommodation space or proximal sediment input into the north during the mid-late Jurassic. This has significant implications for reservoir and seal facies of potential Mesozoic petroleum systems in the Houtman Sub-basin.

This year, the Commonwealth Government is offering 6 large exploration areas in the frontier Bight Basin. The release areas (Figure 1) are situated in the central Great Australian Bight off southern Australia, approximately 415 to 655 km west of Port Lincoln, South Australia and 250 to 530 km southwest of Ceduna, South Australia. The areas are located within the Ceduna Sub-basin, in the eastern part of the Bight Basin, in water depths ranging from 130 to 4600 m. At present, no permits are held in this part of the basin. The release areas range in size from 85 to 90 graticular blocks (6000 to 6395 km2), and bids for all 6 areas close on 29 April 2010. Most exploration drilling in the Bight Basin has focused on the margins of the Ceduna Sub-basin and the Duntroon Sub-basin to the southeast of the current release areas. Gnarlyknots 1A, drilled by Woodside Energy and partners in 2003, is the only well to have attempted to test the thick, prospective Ceduna Sub-basin succession away from the margins of the sub-basin. Unfortunately the well was not an exploration success, as it had to be abandoned due to deteriorating weather and ocean conditions without reaching all planned target horizons. In 2007, Geoscience Australia conducted a marine sampling survey in the Bight Basin that dredged a suite of organic-rich rocks of Cenomanian-Turonian age from the northwestern exposed edge of the Ceduna Sub-basin. Geochemical analyses have characterised these samples as world-class, oil-prone, marine potential source rocks. Seismic interpretation indicates that this interval can be mapped throughout most of the basin and is mature for oil and gas generation across much of the Ceduna Sub-basin.

The well folio package contains the combined data of 23 offshore wells in the northern Perth Basin, including 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). Completed as part of the Australian Government's Offshore Energy Security Program, the well folio package will improve the assessment of petroleum prospectivity in the offshore Perth Basin. The well folio package consists of 23 composite well log plots, six well to well correlations and a new chronostratigraphic sequence framework and includes Geoscience Australia's revised Offshore Perth Basin Biozonation and Stratigraphy 2011 Chart 38 (Jones et al., 2011a). The well folio package summarises key stratigraphic, biostratigraphic and geochemical data and sequence interpretations for wells in the offshore northern Perth Basin from the Beagle and Turtle Dove ridges, Houtman and central Abrolhos sub-basins and the Wittecarra Terrace. Composite well log plots at 1:5000 scale summarise, in graphic form, the main stratigraphic features and hydrocarbon occurrences. The composite well logs also graphically summarise results from 120 new palynological samples and 244 new Rock-Eval pyrolysis/Total Organic Carbon (TOC), 85 new Vitrinite Reflectance (VR), 14 new FAMM maturity and compiled Grains with Oil Inclusions (GOITM) measurements. Compilations of new and open-file biostratigraphic and geochemical data are also included in digital spreadsheet formats. Data from the 23 offshore wells show that the main sedimentary succession in the offshore northern Perth Basin is Permian to Late Jurassic in age: two wells intersected a sandstone section of Late Cambrian to Early Ordovician age. Reviewed open-file biostratigraphic data, new biostratigraphic data and interpretations from these samples have been used in conjunction with well logs and lithological interpretations of cuttings, cores and side wall cores to define a new chronostratigraphic sequence framework for this part of the basin. Sequences and major maximum flooding surfaces are correlated between wells to show the spatial and temporal distribution of these sequences through the offshore northern Perth Basin.

The rifted margins of eastern and southern Australia formed during multiple periods of extension associated with the fragmentation and dispersal of Gondwana in the Late Jurassic to Early Eocene (Veevers & Ettreim 1988; Veevers et al. 1991). The sedimentary basins of the Southern Rift System (Stagg et al. 1990) extend from Broken Ridge in the west, to the South Tasman Rise (STR) in the east. Collectively, these depocentres cover an area in excess of 1 million square kilometres (excluding the STR), with the thickest sediments (up to 15 km) occurring in the Ceduna Sub-basin of the Bight Basin. Early phases of the extension during the late Middle Jurassic to Early Cretaceous resulted in the formation of a series of west-northwesterly trending continental rift basins along the southern margin of Australia and a series of north-northwest trending transtensional basins along the western margin of Tasmania. The amount of upper crustal extension varied between basins of the rift system. This phase of upper crustal extension preceded eventual breakup between the Australian and Antarctic plates off the Bight Basin in the latest Santonian to earliest Campanian (Sayers et al. 2001). The nature of source rocks within the rift basins reflects the eastward propagation of the rift system through time, with largely terrestrial systems dominating in the early rift stages, followed by marine inundation from the Aptian onwards (west of the Otway Basin). In the Otway Basin, the first marine influence is recorded during the early Turonian, while in the Sorell and Bass basins marine conditions prevailed from ?Maastrichtian and middle Eocene time, respectively. Terrestrial progradational systems in the Late Cretaceous are important in the maturation of potential source rocks in the Bight and Otway basins, while Neogene carbonate-dominated systems are important in the Sorell, Bass and Gippsland basins. Outside of the Gippsland Basin where exploration has reached a mature status, the southern margin basins remain frontier to moderately exploration areas, with an overall drilling density (excluding the Gippsland Basin) of approximately 1 well per 6,000 square kilometres. Key Words: Australian Southern Margin, Southern Rift System, petroleum systems References SAYERS, J., SYMONDS, P.A., DIREEN, N.G. and BERNARDEL, G., 2001. Nature of the continent-ocean transition on the non-volcanic rifted margin of the central Great Australian Bight. In, Wilson, R.C.L., Whitmarsh, R.B., Taylor, B., and Froitzheim, N., (Eds), Non-Volcanic Rifting of Continental Margins; A Comparison of Evidence from Land and Sea. Geological Society, London, Special Publications, 187, 51?77. STAGG, H.M.J., COCKSHELL, C.D., WILLCOX, J.B., HILL, A., NEEDHAM, D.J.L., THOMAS, B., O?BRIEN, G.W. and HOUGH, P., 1990. Basins of the Great Australian Bight region, geology and petroleum potential. Bureau of Mineral Resources, Australia, Continental Margins Program Folio 5. VEEVERS, J.J. and ETTREIM, S.L., 1988. Reconstruction of Australia and Antarctica at breakup (95 ? 5 Ma) from magnetic and seismic data at the continental margin. Australian Journal of Earth Sciences, 35, 355?362. VEEVERS, J.J., POWELL, C.MCA. and ROOTS, S.R., 1991. Review of seafloor spreading around Australia, I. Synthesis of the patterns of spreading. Australian Journal of Earth Sciences, 38, 373?389. WILLCOX, J.B. and STAGG, H.M.J., 1990. Australia?s southern margin, a product of oblique extension. Tectonophysics, 173, 269?281.

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