AGSO's 1995-96 Petrel Sub-basin Study was undertaken within AGSO's Marine, Petroleum and Sedimentary Resources Division (MPSR) as part of MPSR's North West Shelf Project. The study was aimed at understanding the stratigraphic and structural development of the basin as a framework for more effective and efficient resource exploration. Specifically, the study aimed to: - define the nature of the major basement elements underlying the Petrel Sub-basin and their influence on the development of the basin through time, - determine the nature and age of the events that have controlled the initiation, distribution and tectonic evolution of the basin; - define the nature and age of the basin fill, and the processes that have controlled its deposition and deformation; and, importantly, - determine the factors controlling the development and distribution of the basin's petroleum systems and occurrences.

The carbon and hydrogen isotopic data of natural gases provide a crucial tool to interpret the origin, occurrence and inter-relationships of natural gases. The CF-GC-IRMS is a convenient system to separate gas mixture and obtain continuous, on-line isotopic data of individual compounds. With CF-GC-IRMS system, the abundance of target components is crucial. For an accurate result, there should be enough target compound going through the furnace to be measured as CO2 using isotopic ratio mass spectrometry. For carbon isotopes, a m/z 44 response below 0.3 V (or over 7V) is regarded as unreliable. For high abundant compounds, there is no difficulty in attaining a voltage over 0.3V with a normal injection of under 100ul with adjusted split flow. However, the acquisition for the low concentration component is problematic since "normal" injection would not produce a strong enough signal. In this presentation, we demonstrated the techniques used to obtain low concentration components occurring in the Australian natural gases and how we apply the results in gas comparison studies. Cryogenics (liquid nitrogen trap) is applied to trap and concentrate low amount of compounds other than methane (C1), including CO2, C2 and above. With this method, extreme low concentration of C2 from very dry gases was obtained with large volume injection of 10ml. Back-flash is used together with cryogenics. For analyses for only C4 and C5 compounds, cryogenics was not needed, since they focus at the front of the column at 40oC and elute from the column under oven temperature programming as single peaks. Neo-pentane (neo-C5) is generally the least abundant wet gas component. Its concentration is enhanced in the gases which are biodegraded, wherein the other gas components have been selectively removed by microbial activity. Neo-pentane is extremely resistant to biodegradation and shows no isotopic alteration even in severely biodegraded gas. In such cases, neo-C5 is the only gas component that can be confidently used in gas-gas correlation. Neo-pentane is an example where we employ injection of a large volume (e.g. to 40ml for hydrogen isotopes), combining a back-flashing technique for compounds eluting before C4 (inclusive) and C5 compounds. The neo-C5 elutes between nC4 and i-C5. Under the current GC conditions, there is a time "window" of less than 40 seconds to capture neo-C5. A manual operation to set back-flash to straight flow to allow capture neo-C5 just after n-C4 elutes and then back to back-flush to eliminate interference of C5's compounds. Mass balance estimation indicates that there is no loss of neo-C5 during the large volume injection and repeatability is excellent.

Terracorp under its facilities management agreement with ANSIR was contracted to conduct the 1999 Yilgarn Seismic Survey located in the Kalgoorlie Goldfields region of Western Australia. In total 194.64 kms of 60 and 120 fold, 240 channel data was recorded between 19th August and 3rd September 1999. This high resolution and regional seismic profiles were acquired in order to image the major structural features of the region, particularly the highly mineralised Bardoc-Boorara Shear and to provide three-dimensional information about the relationship between the greenstones and the granites. Raw data for this survey are available on request from clientservices@ga.gov.au

Seismic reflection survey has been conducted to help identify the possible oil-bearing structures, which were revealed by two residual gravity anomalies in a geophysical survey made by the Bureau of Mineral Resources. Good reflections were obtained in some parts of the area, but the quality was not consistent. The seismic results appear to confirm a small closure near one of the gravity anomalies. No definite closure is shown near the other anomaly.

A seismic reflection traverse on portion of the Giralia anticline was conducted by the Bereau of Mineral Resources (BMR). This work constitutes a part of a general programme of investigation which the BMR, Geology and Geophysics, has been carrying out on Permit areas held by Ampol Petroleum Ltd. in this area. The purpose of the survey is to idenfy the proposed geology structure in this region based on the previous geology surveys.

A refleotion traverse was shot across the centre part of the Giralia Anticline in the Carnarvon Basin of Western Australia, in an attempt to verify the unconformity between Mesozoic and Palaeozoic sediments shown by a previous traverse across the northern part of the anticline. Shallow seismic events recorded were of good quality and correlated very well with surface geology. They also indicated two faults in places where steep dips in surface beds might, by anaJogy with the northern end, be expected. Deep events were in general of poor quality and inconsistent. However, over approximately a mile of the traverse, they were of good quality and indicated nonconformity with the shallow events.

A reconnaissance seismic reflection and refraction survey in the East Otway Basin, Victoria, was carried out by the Bereau of Mineral Resources from mid-February to mid-June 1967. The objective of the survey was to determine whether the gravity low areas of the Torquay Embayment and Port Phillip Sub-Basin in the eastern part of the Otway Basin contain thick Cretaceous sediments like those which has shown potential hydrocarbon source and reservoir characteristics in the western part of the Otway Basin. Nine reflection and five refraction traverses were recorded in the gravity low areas of the Barwon Trough and Port Phillip Sub-basin. Single-coverage reflection results of variable quality were obtained. Evidence for the presence of Tertiary section is provided by shallow reflections of good to fair quality, but the evidence for Cretaceous sediments is tenuous because of the poor quality of the deeper reflections, some of which may be multiples. The presence of several faults, onlappings and pinch-outs is also indicated. The refraction results are considered unreliable because of the difficulty of interpreting the discontinuous profiles and because of the mapped and suspected faults and pinch-outs in the sections.

A reflection seismic survey was made in an area north and north-west of Roma, to find whether there are any domal structures associated with the known occurrences of oil and gas. The general quality of the reflections recorded was poor, and no evidence of an anticline or basement high was found at Hospital Hill or Block 1 6 where oil and gas have been previously found. No targets for drilling were found and further seismic work is not at present recommended.

Paper to accompany presentation of teh 2006 Offshore Petroleum Exploraiton Areas, give at the annual Australian Petroleum Production and Exploration Association (APPEA) conference, Gold Caost 7th to 11th May 2006.

The Browse Basin lies offshore from Western Australia's Kimberley region and hosts vast accumulations of natural gas, some of which are rich in condensate, making it Australia's next major liquefied natural gas (LNG) producing province on the North West Shelf. The Ichthys accumulation is estimated to host 12.8 trillion cubic feet (Tcf) of gas and 527 million barrels (mmbbl) of condensate (condensate:gas ratio (CGR) 60 bbl/MMscf) representing the largest hydrocarbon accumulation with recoverable liquids found in Australia since the discovery of the Gippsland Basin and Barrow Island oil fields in the 1960s. Similar amounts of gas, albeit drier (CGR 2030 bbl/MMscf) are hosted within the Brecknock, Calliance and Torosa accumulations (cumulative 15.9 Tcf gas, 436 mmbbl condensate). Despite the extensive ongoing exploration activity and prior research interest [1, 2 and 3], the basin's petroleum systems (PS) have not been publically updated for a decade. Collating the existing molecular and isotopic datasets for the wet gases and associated hydrocarbon liquids, along with the biomarker and 13C/12C and D/H ratios of the n-alkanes for the crude oils, has enabled the origin and extent of the petroleum systems to be redefined. In doing so, it is apparent that the filling of the gas accumulations within the Caswell Sub-basin and along the Scott Reef-Brecknock trend is complex, with the component gases originating from multiple organic and inorganic sources. Differing degrees of biodegradation are observed in the Cornea and Gwydion oil and gas accumulations. Four preliminary petroleum systems are defined for known accumulations by their 13C n-alkane isotopic profiles (Figure 1). The PloverPlover PS is a basin-wide gas-prone system where the gas is reservoired within the Middle Jurassic Plover Formation (e.g. Brecknock-Torosa, Ichthys) and sourced from mixed terrestrial and marine organic matter deposited in fluvio-deltaic sediments. The Plover/VulcanVulcan PS occurs within the central Caswell Sub-basin at Ichthys and Prelude/Concerto and is a wet gas-prone system reservoired within the Upper Jurassic Brewster Member, upper Vulcan Formation. This PS has a more marine source affinity with the additional hydrocarbons probably being sourced from the lower Vulcan Formation. The Plover/VulcanPlover/Vulcan/Nome PS is a gas-prone system within the Heywood Graben. The complex reservoir at Crux is sourced from mixed terrestrial and marine organic matter deposited that may be sourced from within Jurassic sediments. The Echuca ShoalsHeywood PS is an oil- and gas-prone system (e.g. Caswell, Cornea and Gwydion) sourced by marine algae and bacterial remains within Lower Cretaceous sediments [2]. The oils and gases on the Yampi Shelf vary in their degree of biodegradation. Further work is in progress to confirm these petroleum systems and redefine their extent by correlating the wet gases and oils with their source rocks.