Subsidence and thermal history analysis of 31wells and 25 depocentre sites has been undertaken to examine the generation and expulsion history of Jurassic and Early Cretaceous petroleum systems in the Browse Basin. The models incorporate new palaeo-bathymetric estimates of the deepwater post-Valanginian to Early Tertiary succession based on seismic geometries and detailed benthic foraminiferal data, as well as new kerogen kinetic data for multiple potential source units. The models suggest multiple effective source units for gas expulsion in the basin, whereas effective oil charge is largely restricted to the Heywood Graben in the northeast, and the rift section in the deepwater Serringapatam Sub-basin. Significant quantities of oil are modelled to have been expelled from Jurassic sediments (Plover and Lower Vulcan formations) in the Heywood Graben during the Tertiary and Late Tertiary, respectively. These charges are likely to have sourced the thick palaeo-oil columns interpreted at Heywood-1 and Crux-1. Modelled oil expulsion in the deepwater Seringapatam Sub-basin is highly sensitive to the organic richness and quality of interpreted Jurassic rift sediments. Provided these units contain good quality source rocks comparable to those within Jurassic rifts elsewhere on the North West Shelf, then significant quantities of oil and gas are modelled to have been expelled from this graben during the Late Tertiary. Evidence of such an oil charge may be provided by thin zones of elevated GOI data in Jurassic reservoirs at North Scott Reef-1, Scott Reef-2A and Brecknock-1, all located on a major anticlinal trend inboard of the Seringapatam graben. Regional models of the recently identified, oil-prone, Early Cretaceous (Echuca Shoals Formation) petroleum system suggest that these rocks have not expelled significant volumes of oil. However, organic-rich sediments within this succession occur as thin transgressive sheets deposited in response to fluctuating sea level on a gently inclined ramp margin, and thus higher-resolution sequence stratigraphic models are required to more accurately assess their local expulsion history. Local effective oil charge from these sources is confirmed by the Cornea, Gwydion-1 and Caswell-2 accumulations. The expulsion models highlight the importance of Late Tertiary carbonate clinoforms in controlling the generation and expulsion history of effective source rocks in the Browse Basin. The rapid progradation of these carbonates is likely to cause an outward-migrating compaction wave that forces expelled fluids and hydrocarbons to outboard areas. This mechanism may apply elsewhere on the North West Shelf where carbonate clinoforms are similarly well developed, and could provide new exploration opportunities in outboard areas.

27-30 May, 2001, St Johns Newfoundland, Canada.

Conference volume and CD are available through the Petroleum Exploration Society of Australia

The Tanami region of the Northern Territory lies approximately 600 km north-west of Alice Springs and is a major Australian gold province. A thin veneer of Cainozoic sand and gravel largely obscures basement host rocks, posing considerable challenges to mineral explorers looking for buried deposits. Recent multi-disciplinary investigations rely on the integration of geophysical data with more traditional field techniques to map sub-surface structure, stratigraphy and identify potential mineralised targets (Hendrickx et al., 2000a,b, Crispe and Vandenberg, in prep). While two dimensional (2-D) mapping of the subsurface has been relatively straightforward, three dimensional (3-D) reconstruction of the regional structure and stratigraphic architecture is more difficult given the lack of continuous exposure and vast distances between outcrops. In this study 19 2-D geological cross-sections were tested via ModelVision, a 21/2-D potential field modelling package, to a depth of 15 km. Sections were revised and remodelled till geological relationships and geometries across serial sections approached a reasonable level of consistency. The modelled sections were then captured in GOCAD, a 3-D earth modelling system. This enabled the interpolation of fault surfaces, major contacts and granitic bodies to produce a 3-D geological model (Fig. 1). The modelled sections, surfaces and other data sets were then combined in Virtual Reality Modelling Language (VRML) to create 3-D views of the geology (Vandenberg and Meixner, 2004). The resulting model provides a 3-D framework in which various data sets and interpretations can be visualised and compared. The model can evolve as new data is incorporated, and provides a base for the construction of further tests to refine our understanding of the geology and associated mineral systems.

The Bedout High in the offshore Roebuck/Canning Basin at the Australian NW Margin (NWAM) has recently been re-interpreted as a massive impact structure that appears to be associated with the global Permian/Triassic extinction event. This impact may have significantly modified the crustal structure in the region. Depth conversion of the reflection seismic data suggests that depth to basement in this area is ~ 3.5 km, and the Bedout High stands more than 4 km above the surrounding sedimentary basins. The basement and crust in the Roebuck/Canning Basin have a number of features that make it distinctly different from other basins at the NWAM. Rapid crustal thinning (from 34 to 12 km thick crust over a distance of 100 km) and the presence of a thick layer of magmatic underplating in the lower crust are among these features. The protracted thermal/fluid events associated with this underpalting may have rendered the Roebuck Basin ?sterile? of Triassic-Jurassic sourced hydrocarbons which are ubiquitous across other parts of the NW Australian margin. Interval velocities for Top Permian - Lower Carboniferous (pre-impact interval) and for Base Jurassic ? Top Permian (post-impact interval) sections display similar correlation with total sediment thickness: in both cases lower velocities correspond to thinner sediment, unlike at other basement highs in the region. OBS-derived interval velocities appear to be up to 10% higher than the stacking-derived ones, consistent with fine velocity stratification of this part of the section. However, existing seismic data do not allow an accurate estimate of the size of the area affected by the meteorite impact, and to measure any effects or controls it may have exerted on rifting, thermal, sedimentation and hydrocarbon maturation regimes in the area. Further multidisciplinary research is necessary to answer these questions.

3-6 July, 2001, Townsville, Qld.

18th Dec., 1996, CSIRO, Hobart.

24-30th August, 1990, Nottingham, U.K.

A regional seismic survey in north Queensland, with acquisition paremeters set for deep crustal imaging, show a potential geothermal target beneath about 2 km of sediments. Beneath the sedimentary structure there appears to be an area of low seismic reflection signal from about 1 s to 4 s. Combined with the relatively low gravity signature over this location, this area of low seismic reflection signal could be interpreted as a large granite body, overlain by sediments. This body lies near an area of high crustal temperature and suggests a potential geothermal energy target.

Reducing uncertainty at an early stage of resource development is a key necessity to attract project finance. Risk analysis frameworks exist in the petroleum industry for quantifying risk and expected returns (Newendorp, 1975; Suslick et al., 2009). For deep Enhanced Geothermal Systems (EGS) and Hot Sedimentary Aquifers (HSA), there is limited knowledge and experience available from in-the-ground projects to make informed estimates of the likelihood of outcomes for incorporation into a risk analysis framework. Modelling incorporating uncertainty analysis based on a library of EGS and HSA geothermal reservoirs, together with proxy data, could be used to develop a Geothermal Play Systems framework for assessing reservoir risk and ranking prospects.