The offshore Gippsland Basin is Australia's major producing hydrocarbon province. Acreage has historically been tightly held, and opportunities for new players in this highly prospective basin have been limited. However, recent relinquishments have allowed the Australian Federal and Victorian State Governments to offer three potential permits to petroleum exploration companies and consortia. The Bureau of Mineral Resources Petroleum Group, in collaboration with the Victorian Department of Manufacturing and Industry Development's Petroleum Branch, has produced a hydrocarbon prospectivity package for the Southeast Gippsland Basin, with particular emphasis on the three areas to be released. The package takes the form of this BMR Record 1991/9. The Package covers regional geology, geophysics, palaeogeography, and hydrocarbon play concepts, together with a new structural interpretation for the Gippsland Basin developed at BMR. In addition, for each release area the package covers previous exploration, local geology and play concepts, reservoir geology and engineering, and geohistory. Prospects and leads are described in detail, and the text is complemented by some 80 Plates and Figures.

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The extreme variation in the natural endowment of petroleum resources between regions has been a key geo-political driver in the last century and may well remain so in the decades ahead. Most of the world?s oil is located in a latitudinal belt lying predominantly north of the equator, running from the Gulf of Mexico and Venezuela, to North Africa, through the Middle East, the Caspian and Central Asia and down to Indonesia. Klemme and Ulmishek (1991) calculated that this Tethyan Petroleum Province contained 68% of global original petroleum reserves. Its vast petroleum resources were derived largely from the organic rich marine rocks deposited in low latitude in restricted basins and on shallow carbonate shelves flanking the various Palaeozoic, Mesozoic and Cainozoic incarnations of the east-west orientated Tethys Ocean.

A wide variety of studies have been carried out around the Australian margin to infer or detect natural hydrocarbon seepage. Hydrocarbon seepage can, in selected geological settings, delineate subsurface petroleum accumulations and provide information on hydrocarbon charge type. However, the relationship between near-surface hydrocarbon seepage and subsurface petroleum generation and entrapment is often complex. Rates and volume of hydrocarbon seepage to the surface produce a variety of near-surface geological and biological responses, which require a range of sampling techniques to detect the seepage effectively. Interpreters must firmly grasp these issues to understand the significance of migrated hydrocarbons within near-surface sediments. Thus, it is important to understand the data types that have been used to infer seepage in Australia and the results of these studies, if natural hydrocarbon seepage is to be assumed in this region. Furthermore, the strengths and weaknesses of different approaches need to be understood and the data often need to be set in a global context to appreciate the significance of results obtained. This report is aimed at providing an overview of natural hydrocarbon seepage studies that have been carried out around Australia and to provide information on techniques and approaches that have proved to be successful during studies carried out by Geoscience Australian between 2004 and 2007. ... This investigation provides an increased understanding of seepage detection technologies and techniques, particularly in relation to the Australian environment, and appropriate interpretation of potential seepage indicators in a global context. Consequently, seepage studies can be undertaken with greater confidence in Australia's offshore jurisdiction, in locations and at times that are optimal for effective seepage detection.

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During April/May 1988, the BMR research vessel 'Rig Seismic' carried out a 21 day geochemical and sedimentological research program in the Otway (17 days) and Gippsland (4 days) Basins. Light hydrocarbon gases (C1-C6) were measured in sediments at 342 locations on thecontinental shelf and upper continental slope. Thermogenic hydrocarbons were identified in near-surface sediments at forty-two locations in the Otway (32) and Gippsland (10) Basins. The major results from the Otway Basin include: 1. Evidence of thermogenic hydrocarbon sediments was found at seven locations on the Crayfish Platform, seven locations on the Mussel Platform and eighteen locations in the VolutaTrough. 2. Wet gas contents ([C2-C4/C1-C4] x 100), which provide some indication of both hydrocarbon source type and maturity,are highest on the basin margins, i.e. the Crayfish and Mussel Platforms. Wet gas contents were consistently lower in the Voluta Trough. 3. Total C1-C4 gas concentrations were higher in the Voluta Trough than on the basin margins, probably because of more intense near-surface faulting in the trough. 4. The geochemical data, when integrated with thermal maturation modelling and well data, suggest that the principal liquidhydrocarbon source rocks are located at the base of the Early Cretaceous Otway Group (i.e. basal Pretty Hill Sandstone). The Late Cretaceous Sherbrook Group appears to be gas-prone. Preliminary data from the Gippsland Basin identify ten locations which show evidence of thermogenic hydrocarbons in near-surface sediments.

Surprisingly few natural hydrocarbon seeps have been identified in Australia's offshore basins despite studies spanning thirty years. Initial studies of natural hydrocarbon seepage around the Australian margin were generally based around the geochemical analysis of stranded bitumens, water column geochemical `sniffer' sampling, synthetic aperture radar or airborne laser fluorsensor. Later studies involved the integration of these remote sensing and geochemical techniques with mutli-channel and shallow seismic. A review of these earlier studies indicates that many seepage interpretations need to be re-evaluated and that previous data sets, when set in a global context, often represent normal background hydrocarbon levels. Relatively few sites of proven natural hydrocarbon seepage in Australia's offshore sedimentary basins can be reconciled with the dominantly passive margin setting and low recent sedimentation rates, which are not favourable for high rates of seepage, and difficulties in proving seepage on high energy, shallow carbonate shelves, where seabed features may be rapidly reworked and modern marine signatures are overprinted on authigenic seep carbonates. Active thermogenic methane seepage on the Yampi Shelf, the only proven documented occurrence in Australia, is driven by deposition of a thick Late Tertiary carbonate succession and Late Miocene tectonic reactivation. Therefore, to increase the success of detecting and correctly interpreting natural hydrocarbon seepage, data need to be analysed and integrated within the context of the local geological setting, and with an understanding of what is observed globally.

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The North West Margin Transects Cruise is part of a program being undertaken by AGSO to determine the structural architecture of the north-western margin of Australia and the influence of structuring on the location, migration and trapping ofhydrocarbons in the region. An important aspect of this program is the acquisition ofa series of full margin 'dip' transects extending from unrifted cratonic basement tobeyond the continent-ocean boundary, and at least two 'strike' transects that willextend along the full length of the shelf from the eastern Timor Sea to North WestCape. The major objective of the proposed cruise is to extend previous AGSO deepseismic surveys across the Browse (Survey 119), offshore Canning (SNOWS-3/Survey 120) and northern Carnarvon Basins (SNOWS-/Survey 101, SNOWS-2/Survey 110) to complete these margin transects. Specific objectives of theproposed survey are: *Determine the regional structural framework of the Scott Plateau area and itsrelationship to adjacent features such as the Argo Abyssal Plain, the BrowseBasin and the Rowley Sub-Basin of the offshore Canning Basin; *Determine the regional structural framework of the Exmouth Plateau and itsrelationship to adjacent features such as the Barrow-Dampier Sub-basin, and theArgo and Gascoyne Abyssal Plains. *Define the broad deep-crustal structure of the region and develop a modelexplaining the tectonic, subsidence and thermal history of the Scott Plateau andthe Exmouth Plateau in relation to the development of the continental margin andadjacent ocean basins. *Assess the effects of the deep crustal structures and their reactivation phases onthe development of known petroleum accumulations. To address these objectives it is proposed that RV Rig Seismic be used to acquireabout 3276 km of deep crustal (16 second record length) multichannel seismic andother geophysical data along 9 transects across the outer margins of the Browse,offshore Canning and northern Carnarvon Basins. The survey will tie into the 1991SNOWS-1 (101), 1992 SNOWS-2 (110), 1993 Browse Basin (119) and SNOWS-3(120) surveys.

The natural gases are composed of a limited number of individual compounds, mainly of C1-C5 hydrocarbons and non-hydrocarbon compounds (CO2, N2, noble gases etc.). Their compositions and isotopes of single compounds provide critical information to decipher the origin and evolution of natural gases. Efficient analysis of these compounds is paramount for timely application of this important dataset.