This CD contains a collection of reports on samples from Arafura Basin wells (biostratigraphy, organic petrology, fluid inclusions - GOI, organic geochemistry and geohistory modelling) generated by GA staff and by external contractors and collaborators based on new analyses carried out during 2004 and 2005.

No abstract available

During ODP Leg 189, the JOIDES Resolution recovered about 4200 m of continuous core from four sites in sedimentary basins on continental crust off Tasmania. These sites, one off west Tasmania, two on the South Tasman Rise (STR), and one on the East Tasman Plateau (ETP) are 760-968 m deep and in water 2100-2700 m deep. No hydrocarbon accumulations were expected at these sites. The pre-Oligocene section is largely shallow-marine organic-rich mudstone, which seismic profiles indicate is the top of a Cretaceous-Eocene largely deltaic sequence thousands of metres thick. This siliciclastic sequence formed as Tasmania rifted from the surrounding parts of Gondwana. Sedimentation rates were relatively high until the late Eocene, when a condensed siltstone sequence formed as the Antarctic Circumpolar Current first swept the shelves of the separating land masses. From the earliest Oligocene, when Australia finally cleared Antarctica, deposition of several hundred metres of carbonate pelagic ooze and chalk predominated. Sedimentation no longer kept up with subsidence, and at most sites unconformities have removed much of the Oligocene. The cores, in conjunction with seismic profiles, provide information on tectonic and sedimentary history and petroleum potential. New data include evidence of high present-day thermal gradients; marginally mature organic matter less than 1000 m below sea bed, biogenic and probably thermogenic gas, and bitumen generation; and overall source rock potential. Tectonic histories vary, but all basins have sufficient sediment and thermal gradient to generate hydrocarbons. However, of the thick sequences interpreted as deltaic, only the upper shelf mudstones were drilled. TOC content decreased with time as the gulfs around Tasmania widened, and also eastward from the more restricted Australo-Antarctic Gulf into the less restricted early Tasman Sea. Although the thermal gradient is highest in the three western sites, the holes do not reach the petroleum window. Methane is biogenic in the younger sediments, but higher hydrocarbon gases at depth suggest a thermogenic component. In addition, Rock-Eval analyses of the oldest sediments suggest bitumen (S2 peak) and organic matter approaching maturity. Little stratigraphic section is missing, so past overburden was unlikely to exceed present. Whether there are reservoir rocks and suitable hydrocarbon traps remains unknown. On the existing evidence, west Tasmania and STR appear to be reasonably prospective for petroleum, and more prospective than ETP, and we present a speculative play concept for them.

This paper discusses the applicability of Smalll Angle X-ray Scattering (SAXS) and Small Angle Neutron Scattering (SANS) techniques for determining the porosity, pore size distribution and internal specific surface area in coals. The method is non-invasive, fast, inexpensive and does not require complex sample preparation. It uses coal grains of about 0.7mm size mounted in standard pellets as used for petrographic studies.

abstract

The Ceduna Sub-basin of the Bight Basin is a frontier region containing only one exploration well. Therefore, our assessment of the distribution of potential source rocks in the area is based on an understanding of the regional sequence stratigraphic framework and the potential petroleum systems present, along with the regionsal palaeogeography, and geochemical data from onshore and the adjacent Duntroon Basin. Studies carried out by AGSO over the past three years suggest that the thick Cretaceous succession in the Ceduna Sub-basin contains a range of fluvio-lacustrine, deltaic and marine source rocks that have the potential to generate liquid hydrocarbons.

Compelling evidence is presented for the process of lipid sulfurisation in humic coal-forming environments. The production of reduced inorganic sulfides by sulfate-reducing bacteria during early diagenetic marine transgression enabled the selective sequestration of functionalised lipids in the polar and asphaltene fractions from the Eocene, marine-influenced Heartbreak Ridge lignite deposit, southeast Western Australia. Nickel boride desulfurisation experiments conducted on these fractions released small, but significant, quantities of sulfur-bound hydrocarbons. These comprised mostly higher plant triterpanes, C29 steranes and extended 17?(H),21?(H)-hopanes, linked by one sulfur atom at, or close to, sites of oxygenation in the original natural product precursors. These sulfurised lipids mostly derive from the same carbon sources as the free hydrocarbon lipids, the exception being the sulfurised extended hopanoids, which may be partially derived from a different bacterial source compared to the free hopanoids. These results indicate that the selectivity and nature of steroid and hopanoid vulcanisation in coal-forming mires is akin to that observed in other sedimentary environments. However, the diversity of sulfurised higher plant triterpanes is greater than that typically reported in immature coals. This selective preservation mechanism explains the formation of the structurally-related biomarkers in more mature sulfur-rich humic coals.

Legacy product - no abstract available

APPEA 2000 joint paper to arrive at a better understanding of the petroleum systems active in the Northern Bonaparte Basin, geochemical data from oils and source rock-extracts were compiled and interpreted from over 20 wells in the area.

A laboratory study has been conducted to determine the best methods for the detection of C10 to C40 hydrocarbons at naturally occurring oil seeps in marine sediments. The results indicate that a commercially available method using hexane to extract sediments and gas chromatography to screen the resulting extract is effective at recognizing the presence of migrated hydrocarbons at concentrations between 50 to 5,000 ppm. When the oil charge is unbiodegraded the level of charge is effectively tracked by the sum of n-alkanes in the gas chromatogram. However, once the charge oil becomes biodegraded, with the loss of n-alkanes and isoprenoids, the level of charge is tracked by the quantification of the Unresolved Complex Mixture (UCM). The use of GC-MS was also found to be very effective for the recognition of petroleum related hydrocarbons and results indicate that GC-MS would be a very effective tool for screening samples at concentrations below 50 ppm oil charge.