Perth Basin

Note: refer to the Acreage Release basins page for updated information on the Perth Basin

Basin Details and Geological Overview

The Perth Basin is a north to north-northwest trending, onshore and offshore sedimentary basin extending about 1300km along the southwestern margin of the Australian continent. This is a large (172 300km²), structurally complex basin that formed during the separation of Australia and Greater India in the Permian to Early Cretaceous. It includes a significant onshore component and extends offshore to the edge of continental crust in water depths of up to 4500m.

The structural architecture of the Perth Basin is the product of oblique rifting during the Permian, Late Triassic to Early Jurassic and Middle Jurassic to Early Cretaceous, superimposed over pre-existing basement terrains. Extension during the Permian produced a series of deep (up to 15km), north-south trending rift basins (Bunbury Trough and Dandaragan Trough) along the western margin of the Yilgarn Craton. The Abrolhos Sub-basin represents a northwestern branch of the Permian rift system formed along the southwestern margin of the Northampton Complex, which is separated from the Dandaragan Trough by an intra-basin high represented by the Beagle Ridge, Dongara Terrace and Greenough Shelf. The Houtman Sub-basin is a major depocentre for Triassic and Jurassic sedimentary rocks that formed as a westward thickening sag basin across a hinge zone during the Middle Triassic to Middle Jurassic, and was extensively faulted during Late Jurassic to Early Cretaceous rifting. The Vlaming Sub-basin is the major Middle Jurassic to Early Cretaceous rift basin in the Perth Basin and is characterised along its northern extent by a very large and deep half graben that dips to the west. The footwall block of this half graben consists of a series of shallow tilted fault blocks containing mainly Permian and older strata from the Edward's Island Block.

Breakup during the Early Cretaceous (Valanginian) was associated with widespread inversion, erosion, strike-slip tectonics and volcanism, which significantly modified the structural architecture of the Perth Basin. Major structural elements associated with breakup tectonism include an area of inverted Permian half graben in the Turtle Dove Ridge and the Zeewyck Sub-basin which formed in a zone of strike-slip faulting along the Turtle Dove Transfer.

Summary of exploration status and petroleum system elements

ProvExplorer

• Perth Basin Details and Geological Overview
• Abrolhos Sub-basin Details and Geological Overview
• Houtman Sub-basin Details and Geological Overview
• Turtle Dove Ridge Details and Geological Overview
• Vasse Shelf Details and Geological Overview
• Vlaming Sub-basin Details and Geological Overview
• Zeewyck Sub-basin Details and Geological Overview

Stratigraphy

The stratigraphy and petroleum system elements of the Perth Basin developed during the tectonic evolution of the basin and vary significantly from north to south. Initial rifting established a series of Permian to Early Triassic depocentres for fluvial and marine siliciclastics with minor carbonates and coals in the north, while in the south fluvial siliciclastics and coals dominated. These Permian- and Early Triassic-age rift-sag deposits are associated with the major petroleum system in the north Perth Basin, particularly the Kockatea Shale which forms an important oil source rock and regional seal to underlying reservoirs.

A second phase of rifting in the Late Triassic and Early Jurassic was associated with widespread fluvial and deltaic deposits, including a thick succession of siliciclastics and coals (Cattamarra Coal Measures), which are overlain by Middle Jurassic marine shales (Cadda Formation) in the north Perth Basin. These Early to Middle Jurassic strata form a second proven petroleum system in the north Perth Basin. The final rift and breakup phase in the Middle Jurassic to Early Cretaceous was associated with deposition of fluvial and marine siliciclastics (Yarragadee Formation, Parmelia Group and Warnbro Group), which form the main petroleum system in the Vlaming Sub-basin.

It is uncertain which petroleum system elements are associated with the Zeewyck Sub-basin. However, it may contain Middle Jurassic to Early Cretaceous age source rocks. Late Cretaceous and Tertiary sedimentation has occurred under stable passive margin conditions and produced a thin cover of predominantly marine carbonates. Potential traps include tilted fault blocks, anticlines, compressional rollovers and structural/stratigraphic traps.

Regional Cross-sections

Seven regional cross-sections have been selected to provide examples of the structural architecture of the Perth Basin. Examples from the onshore Perth Basin are provided courtesy of the Geological Survey of Western Australia.

Petroleum Systems and Hydrocarbon Potential

Petroleum exploration

The exploration status of the Perth Basin varies from submature in the northern onshore area and immature to frontier in most offshore areas.

Initial exploration for hydrocarbons in the Perth Basin began in the late 1940s with an onshore field survey and evaluation of water drilling commissioned by Ampol and Richfield Oil companies and gravity surveys by the Bureau of Mineral Resources. The onshore portion is extensively explored with over 200 wells drilled.

Offshore exploration began in 1965. The offshore portion is under-explored with only 32 wells drilled (Dec 2003). Seismic coverage over the offshore Perth Basin varies in density and seismic quality ranges from poor in the older surveys to good in more recent data. Ten offshore exploration permits are currently active (WA-226-P, WA-286-P, WA-325-P, WA-326-P, WA-327-P, WA-328-P, WA-349-P, WA-336-P, WA-339-P and WA-337-P), all of which are in the Abrolhos and Houtman Sub-basins.

To date, twenty-six petroleum accumulations have been discovered in the Perth Basin, thirteen of which have proven to be commercial. Most discoveries are from the onshore north Perth Basin. The first discovery was the Yardarino gas field in 1964, while the most significant discovery has been the Dongara oil and gas field (initial reserves of 15.3 billion cubic metres of gas and 10 million barrels of oil).

Hydrocarbon production has occurred from Dongara (gas and some oil), Yardarino (gas and some oil), Beharra Springs (gas), Mondarra (gas), Mt Horner (oil), Woodada (gas), Walyering (gas) and North Yardanogo (oil) in the onshore north Perth Basin. Of these, Dongara, Beharra Springs, Woodada and Mt Horner are still producing.

Oil and gas has been produced from Jurassic coals within the Cattamarra Coal Measures, from Triassic sandstones within and underlying the Kockatea Shale and from the Late Permian Dongara Sandstone and Wagina Sandstone.

Commercial quantities of gas have also produced from the Permian Carynginia Formation and Irwin River Coal Measures. Important recent discoveries in the onshore Perth Basin include Beharra Springs North (gas), Hovea (oil), Jingemia (oil) and Eremia (oil).

In the onshore south Perth Basin, Gingin 1 has produced gas but is now depleted. Whicher Range contains 46.4 billion cubic metres of gas that was originally considered uneconomical, but is currently being tested to see if economic gas flow rates can be sustained.

Limited exploration in the offshore Perth Basin has identified three discoveries. These include a non-commercial gas accumulation in the Lower Carnac Member at Marri 1 (Vlaming Sub-basin) and a non-commercial oil accumulation in the Gage Sandstone at Gage Roads 1 (Vlaming Sub-basin).

The most significant offshore discovery to date is the Cliff Head oil field, located on the Beagle Ridge just east of the Abrolhos Sub-basin. Cliff Head contains estimated recoverable reserves of 20-30 million barrels of oil within Permian reservoirs that are sealed by the Kockatea Shale. The Cliff Head discovery has stimulated renewed interest in exploration for oil in the offshore Perth Basin.

Key References