This article focuses on the re-evaluation of the source rock potential of the basal Kockatea Shale in the offshore portion of the northern Perth Basin.

Organic geochemists are increasingly involved in multi-disciplinary collaborative studies but not often in the initial sample collection phase, so understanding the origin and source of contaminants derived from sample handling and containers is of vital importance as standard laboratory blanks cannot assess this contamination. A variety of organic contaminants was detected in different sediments collected during Geoscience Australia marine survey S282. These include fatty acid amides, chemical antioxidants such as butylated hydroxytoluene and octadecyl-3, 5-di-tert-butyl-4-hydroxyhydrocinnamate (Irganox 1076), plus the UV absorbers octabenzone and octyl methoxycinnamate. These compounds were introduced during sampling on board the research vessel or during subsequent handling. Solvent extraction of potential contamination sources identified two brands of plastic sampling bags as the main source for the fatty acid amides, butylated hydroxytoluene and Irganox 1076. Direct contact of samples with hands covered with sunscreen appears to have caused contamination by octabenzone and octyl methoxycinnamate. As the primary aim of the survey was to detect evidence for hydrocarbon seepage in the Arafura Sea, care was also taken to identify potential sources of hydrocarbons that might have been introduced during sampling and storage. Detailed examination of solvent extracts from plastic bags revealed the occurrence of several homologous series of branched alkanes with quaternary carbon atoms (BAQCs), as well as distributions of alkyl cyclohexanes and alkyl cyclopentanes with strong even over odd carbon predominance. These compounds were also found in sediment samples collected during the survey. Other potential sources of contamination used on board the ship, such as PVC core liners and lubricants, yielded hydrocarbons that could easily be mistaken for evidence of naturally occurring petroleum if care is not taken during interpretation.

This study undertook geochemical and isotopic analyses on a wide selection of oil stains from the Thorntonia Limestone, Arthur Creek Formation and the Arrinthrunga Formation and its lower Hagen Member in order to define geochemical inter-relationships between the oils, characterize their source facies and to determine the extent of post-emplacement alteration. Oil stains were collected from BHD-4 and -9, Elkedra-2 and -7A, Hacking-1, MacIntyre-1, M13 PD, NTGS99/1, Owen-2, Randall-1 and Ross-1 over a depth range from 91 to 1065 m and were analysed for bulk, molecular (biomarkers) and carbon isotopic compositions. Gas chromatograph of the saturated hydrocarbon fraction clearly showed biodegradation as the main alteration process in the shallow reservoirs. Unaltered oil stains show a dominance of medium weight n-alkanes with a maximum at n-C15. Biodegradation results in a progressive loss of the lighter hydrocarbons and an accompanying shift in n-alkane maximum to C27, to finally a complete loss of n-alkanes and a large unresolved complex mixture (UCM). The absence of 25-norhopanes suggests a mild level of biodegradation. The low ratio of saturated hydrocarbons/aromatic hydrocarbons (<1, down to 0.42) compared to high ratios (up to 4.35) for oils with abundant lower molecular weight n-alkanes is consistent with biodegradation. However, low ratios are also seen for otherwise pristine oils, suggesting a complex charge history of initial biodegraded and subsequent re-charge with n-alkane-laden oil. The level of biodegradation is not too severe as to overtly affect the distribution of the biomarkers C19 - C26 tricyclic terpanes, C24 tetracyclic terpane, C27 - C35 hopanes, C30 triterpane (gammacerane) and C27- C29 desmethylsteranes, enabling their use in oil-oil correlation and definition of oil populations. To clarify the inter-relationships among the Georgina Basin oil stains multivariate statistical analysis was used involving a wide range of biomarker ratios that are source-specific and environmental indicators. Resulting oil populations showed a strong correlation with their reservoir unit across the basin, suggesting juxtaposition of source and reservoir within the same stratigraphic unit. Oil-source correlation based on biomarker, bulk carbon isotopes of saturated and aromatic hydrocarbons and n-alkane-specific carbon isotopes identified Thorntonia(!), Arthur Creek(!) and Hagen(.) Petroleum Systems. The latter petroleum system is characterised by relatively high gammacerane, indicating an evaporitic depositional environment. Alternatively, an evaporatic organic facies from an Arthur Creek Formation source may have sourced the Hagen Member oil stains, considering that other oil stains reservoired within the Arrinthrunga Formation show a close affinity with oil stains from the Arthur Creek(!) Petroleum System, suggesting an inter-formational Arthur Creek-Hagen Petroleum System at Elkedra-2. An Arthur Creek-Hagen(!) petroleum system is evident at Elkedra-7A while there is a mixed Thorntonia Limestone and Arthur Creek source contribute to the oil stain at Ross-1.

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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.

The relative distributions and stable carbon isotopic compositions of aliphatic and certain aromatic biomarkers from eleven torbanites from Scotland, South Africa and Australia covering the Late Carboniferous to Late Permian rich in B. braunii fossils are stringently scrutinised for any evidence of molecular features which may be characteristic of palaeogeographical location of deposition. Torbanites from South Africa and Australia (Temi) were deposited in the Permian and Late Carboniferous when Gondwana was covered by an extensive ice sheet. Australian (Newnes) torbanites were deposited in the Late Permian when the climate began to warm up changing from glacial to temperate. Other samples from Scotland are of Carboniferous age and were deposited when Laurasia was located within the equatorial zone. All torbanites are composed of abundant n-alkanes and novel macrocyclic alkanes. These compounds are ascribed a B. braunii algeanan origin based on similar ?13C. All samples are also characterised by a high cyanobacterial input. Differences in other biomarker distributions and stable carbon and isotopic compositions can be used to classify the torbanites into 3 groups (A, B and C), which also correlate to three different palaeogeographical locations: Group A - southern Africa and eastern Australia (Temi) torbanites are characterised by a high relative abundance of cyanobacterial hopanoids, methylotrophic hopanoids and abundant branched hydrocarbons (including 13C enriched novel monomethylalkanes) (ii) Group B - eastern Australia (Newnes and Glen Davis) torbanites contain relatively high amounts of drimanes and abundant 13C enriched novel monomethylalkanes (iii) and Group C - Scottish torbanites (Torbane Hill and Westfield) contain high relative amounts of cyanobacterial hopanoids, methylotrophic hopanoids and abundant branched aliphatics (but no 13C enriched novel monomethylakanes).

Abundant, micron-scale, spherical aggregates of 2?5 nm diameter sphalerite (ZnS) particles formed within natural biofilms dominated by relatively aerotolerant sulfate-reducing bacteria of the family Desulfobacteriaceae. The biofilm zinc concentration is about 106 times that of associated groundwater (0.09 ? 1.1 ppm Zn). Sphalerite also concentrates arsenic (0.01 wt %) and selenium (0.004 wt %). The almost monomineralic product results from buffering of sulfide concentrations at low values by sphalerite precipitation. These results show how microbes control metal concentrations in groundwater and wetland-based remediation systems and suggest biological routes for formation of some low temperature ZnS deposits.

This report provides a detailed account of several important aspects relating to the organic geochemical analyses of oil, gas and source rocks in the AGSO - Geoscience Australia laboratory. It focuses on the main methods used for the molecular analysis of the sterane, hopane and alkylaromatic biomarkers as well as the stable carbon isotopic (bulk and CSIA) analysis of these materials. In the following description of these methods the areas of sample preparation, instrumental analysis and data processing/reporting are separately addressed.