Abstract

As part of a program to improve the geological understanding and petroleum prospectivity of Australia's frontier offshore basins, Geoscience Australia has acquired new data and undertaken integrated geoscience studies of several poorly known or unexplored areas. Underpinned by these studies, new exploration opportunities have and will become available to explorers via an annual release of offshore acreage based on a system of competitive work program bids.

Four exploration areas are available in the shallow water northern Arafura Basin (bids close 10th May 2007). A new geological framework study and a hydrocarbon seepage survey jointly provide strong evidence for an active Palaeozoic petroleum system in the undrilled northern portion of the basin. Deposition in the Arafura Basin commenced in the Neoproterozoic during a period of upper crustal extension that resulted in the formation of large NE-SW trending half graben. The overlying Palaeozoic section is more or less structurally conformable, despite long periods of non-deposition and erosion. Potential source rocks were deposited in the Middle Cambrian, Late Devonian and Late Carboniferous to Early Permian, in shallow-marine and deltaic environments. In comparison with the highly deformed Goulburn Graben to the south, the northern Arafura Basin has undergone only minor deformation, and previously identified exploration risks in the Goulburn Graben (timing of generation and expulsion, and reservoir quality) are reduced in the northern region. Evidence for hydrocarbon generation is provided by oil and gas shows and interstitial solid bitumens in wells in the Goulburn Graben, and SAR anomalies and shallow gas indications in sub-bottom profile, side-scan sonar and echosounder data in the northern Arafura Basin.

Four shallow water offshore exploration areas are proposed for release in the Offshore Canning Basin in April 2007. The Offshore Canning Basin is a major northwest-trending, multi-phase, pericratonic basin of Early Ordovician- Carboniferous age, overlain by a northeast-trending Late Carboniferous- Cainozoic succession (part of the North West Shelf Westralian Superbasin). The release areas overlie the offshore extension of the Fitzroy Trough and Broome Platform which host several onshore oil and gas discoveries and minor oil production. The offshore Fitzroy Trough, also known as the Oobagooma Subbasin, comprises up to 5.5 km of Palaeozoic sediments and 4.5 km of Mesozoic- Cainozoic sediments. It contains only six wells, all drilled between 1970 and 1984. The older Palaeozoic section is poorly known offshore, but onshore contains proven Ordovician (Darawillian), Late Devonian (Frasnian) and Early Carboniferous (Tournaisian) source rocks that are likely to extend into the offshore area. Early Permian potential source rocks are also expected to be mature in offshore areas. Based on new biomarker analysis, multiple oil shows in the offshore Perindi-1 well correlate with onshore oils at Lloyd, Sundown and West Terrace, and are probably sourced from the Early Carboniferous Laurel Formation. Geoscience Australia conducted a hydrocarbon seepage survey of the region in 2006, and although active fluid escape pockmarks and fluid vents were identified, analysis of sediments recovered from the pockmarks show no evidence of thermogenic hydrocarbons. Acquisition of high-resolution aeromagnetic data is planned to support the release of these new exploration areas.

The Mentelle Basin is a large, deep-water, unexplored basin in close proximity to the oil- and gas-producing Perth Basin on Australia's southwest margin. The basin formed near a triple junction between Australia, Antarctica and India during the breakup of eastern Gondwana in the Middle Jurassic to Early Cretaceous. In 2004- 2005 Geoscience Australia acquired 1450 km of regional 2D seismic data as well as sub-bottom profile, swath bathymetry, dredge samples and SAR data. This new data has been integrated with a regional tectonostratigraphic framework of the southern Perth and Mentelle basins to assess their petroleum prospectivity. Preliminary results indicate that the Mentelle Basin comprises several large depocentres with apparent extensional architecture and a total sediment thickness up to 9 km. The western, deep water (2000–3500 m), depocentre contains at least 6 km of inferred Middle-Late Jurassic to Early Cretaceous synrift sedimentary section. A series of smaller extensional depocentres in the shallower water (500–1500 m) eastern region contain at least 4 km of synrift sedimentary section. Source potential is interpreted to be similar to that of the southern Perth Basin where Jurassic and Berriasian coals, coaly mudstones and lacustrine shales have generated both oil and gas. Regional basin architecture favours eastward migration of hydrocarbons from western source kitchens updip into structural and stratigraphic traps in shallower water eastern areas. Geoscience Australia will acquire new long-cable seismic data, potential field data and seabed samples prior to the proposed inaugural release of exploration areas in this frontier basin.

Presented at: 2007 South East Asia Petroleum Exploration Society (SEAPEX) Conference, Singapore, 2007