The Perth Basin extends south from the Southern Carnarvon Basin and covers an area of about 100,000 km2, from the Yilgarn Craton in the east to the edge of the continental shelf in the west. The onshore area is readily accessible, consisting of farming and shrub land in the central region. The undulating northern portion of the basin has relatively simple access from main roads. In the south, forestry and grazing are the main land uses.
The basin is close to petroleum industry infrastructure, including two major gas pipelines and trucking facilities to an oil refinery 30 km south of Perth. The Parmelia Gas Pipeline provides ready access to market and allows economic exploitation of small discoveries.
Geological setting
The Perth Basin forms a north–south elongate rift–trough along the west coast of Australia. The tectonic framework of the basin is dominated by the Darling Fault and Dandaragan Trough in the east, and the offshore Abrolhos and Vlaming Sub-basins in the west. The Dandaragan Trough is a major depocentre up to 12 km thick.
The basin contains mainly continental clastic rocks of Permian and younger age, deposited in a rift system that culminated with the breakup of Gondwana in the Early Neocomian. Two major tectonic phases are recognised: Permian extension in a southwesterly direction, and Early Cretaceous transtension to the northwest during breakup.
Exploration history
Petroleum exploration commenced in the Perth Basin in 1951, when the BMR conducted gravity surveys in the northern onshore area. WAPET was the first private company to explore the acreage with gravity and seismic surveys. Both BMR and WAPET drilled stratigraphic wells across the onshore northern Perth Basin in the late 1950s, leading WAPET to drill the basin’s first wildcat hole, Eneabba 1, in 1961.
Drilling activity concentrated on the onshore part of the basin, with 309 wells drilled onshore to September 2014, compared with 52 wells offshore. Three-quarters of these wells, and the majority of the known hydrocarbon accumulations, lie in the northern part of the basin.
The exploration of the Perth Basin led to the discovery of 20 commercial oil and gas fields and numerous additional significant discoveries of varying size.
Petroleum prospectivity
Of the 20 commercial hydrocarbon fields discovered in the northern Perth Basin, Dongara is by far the largest, with 14.3 Gm3 (508 Bcf) of original in-place gas and 16.6 GL (104 MMbbl) of original in-place oil. Additional discoveries were made both in the northern and southern Perth Basin, some of which are currently being delineated.
Petroleum-system analysis of the basin indicates that mature source rocks are widespread, reservoirs are abundant, and structures are well timed for hydrocarbon entrapment. These petroleum systems are defined as Transitional and Gondwanan. The seal is considered to be a critical factor owing to the intense faulting and high sand-to-shale ratio of the post-Lower Triassic succession.
The main source for gas is the Permian Irwin River Coal Measures, with reservoirs in the Upper Permian and Jurassic. The main source for oil is the base of the marine Lower Triassic Kockatea Shale, with reservoirs in Lower Triassic and Permian sandstones. Oil was also recovered from the Lower Cretaceous reservoir immediately offshore from Perth at Gage Roads 1. Before 2001, the success rate of wells drilled in the northern part of the basin was about one in ten. Since then, several discoveries in 2001 and the application of 3D seismic surveys led to a higher success rate. Major play types include Permian–Triassic and Jurassic anticlines, as well as Permian-Triassic tilted fault blocks and stratigraphic traps.
Read more about the petroleum prospectivity of the Perth Basin (page 54)