The Officer Basin extends 1500 km from the southeastern flank of the Pilbara Craton to the central-western part of South Australia and, within Western Australia, occupies an area of about 310,000 km2.
The basin contains five major westerly- and northwesterly-trending depocentres. The basin fill is predominantly Neoproterozoic, overlying older Proterozoic to Archean sedimentary, igneous and metamorphic rocks. The Gunbarrel Basin, an overlying succession of Cretaceous and younger strata, influences the maturity of source rocks in Officer Basin petroleum systems.
Recent work, including stratigraphic coring and re-evaluation by the Geological Survey of Western Australia (GSWA), has contributed to the understanding of this frontier basin. To date, a few thin, good to excellent source rocks have been identified in each of the main stratigraphic units.
Geological setting
The Officer Basin is a Neoproterozoic intracratonic basin with a total sedimentary thickness of up to 8 km. The basin infill is a mixed carbonate, silty and sandy siliciclastic, and evaporitic succession dominated by shallow marine to coastal deposition. The Officer Basin sedimentary succession preserves three of the four supersequences common to central Australian Neoproterozoic basins. Only the second supersequence appears to be absent in Western Australia, except near the South Australian border.
The structural configuration of the basin is largely determined by major salt deposits, which mobilised during several tectonic episodes.
Exploration history
There have been several periods of stratigraphic and exploration drilling in the Officer Basin since 1965. A consortium including Hunt Oil drilled five wells in 1965–66, encountering minor oil and gas shows within the Browne Formation in Browne 1 and 2. Shell Australia drilled three wells in 1980–84, and found an algal-sourced oil show in Kanpa 1. Eagle Corporation Limited and others drilled two stratigraphic wells in 1982 in the northwest of the basin. Subsequent exploration drilling in the Amadeus Basin and in the Officer Basin in South Australia led to several oil and gas shows, and the discovery of the Dingo gasfield in the Northern Territory.
Approximately 6,500 km of good-quality seismic data were acquired between 1980 and 1984. The Japanese National Oil Company (JNOC) reprocessed most of the modern seismic data in the Yowalga area in 1996. Geoscience Australia gravity data cover the basin on an 11-km grid, and GSWA acquired semi-detailed gravity surveys on 2–3-km grids over parts of the Savory and Waigen areas in 1995 and 1998, respectively.
Petroleum prospectivity
Neoproterozoic sedimentary rocks are known to have sourced commercial accumulations of oil and gas in Russia, Oman, and in the Amadeus Basin in west-central Australia. Hydrocarbon shows (in the form of minor oil staining and bitumen in intergranular pores, fractures and vugs) are recorded in the northwest Officer Basin in Mundadjini 1, Boondawari 1, and LDDH 1, and in the adjoining Paleoproterozoic Scorpion Group in OD 23. Elsewhere in the western Officer Basin, hydrocarbon shows are recorded in NJD 1, Kanpa 1A, Browne 1, Browne 2, Dragoon 1, Hussar 1, and Vines 1.
Results from these wells indicate reservoirs with porosity greater than 20%, and permeabilities of hundreds of millidarcies. Halite beds more than 10 m thick in the Browne Formation, and shales more than 10 m thick in the Browne, Hussar, Kanpa, and Lupton Formations, provide potentially effective seals. Thin, but potentially effective, source rocks are found in the Browne, Kanpa, and Hussar Formations. The close association of laminae-scale source rocks with good quality reservoir and seal horizons indicates the presence of at least the basic physical elements of a petroleum system. Geochemical modelling indicates that most potential source rocks in the Officer Basin first entered the oil-maturation window after formation of substantial structural traps, and much of the section remains in that window today.
Find out more about the petroleum prospectivity of the Officer basin (page 51)