Perth Basin

The Perth Basin is 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 developing 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. Sinistral and dextral movement, respectively, are inferred along the Darling Fault during these phases.

The main faults were rejuvenated by breakup tectonism, which caused horizontal displacements, wrench-induced anticlines, and further faults.

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Petroleum exploration commenced in the Perth Basin around 1951, when the BMR conducted gravity surveys in the northern onshore area. WAPET was the first company to explore the acreage with gravity and seismic surveys. Both the BMR and WAPET drilled stratigraphic wells across the onshore northern Perth Basin in the late 1950s, leading WAPET to drill the first wildcat hole, Eneabba 1, in 1961.

Drilling activity has concentrated in the onshore part of the basin with 280 wells drilled to date, compared with 42 wells offshore. Of these wells, 28 are oil producers and 42 are gas producers. Three-quarters of these wells and the majority of the known hydrocarbon accumulations are in the northern part of the basin.

The exploration of the Perth Basin has led to the discovery of 13 commercial hydrocarbon fields and numerous additional significant discoveries.

WAPET was responsible for the discovery of the majority of the fields. Also notable were the discoveries of the Woodada gasfield by Hughes and Hughes Oil and Gas, and of the Beharra Springs gasfield and a new pool at the Mount Horner oilfield by Barrack Energy.

Exploration in the Perth Basin has been revitalised in recent years by the discoveries of the Hovea oilfield and Elegans gasfield by ARC Energy, the Beharra Springs North gasfield and the Jingemia oilfield by Origin Energy, and the offshore Cliff Head oilfield by ROC Oil.

Producing fields in 2006 were Cliff Head, Dongara, Tarantula, Woodada, Beharra Springs, Xyris, Xyris South, Eremia, Evandra, Hovea, and Jingemia. A series of extended production tests at Jingemia have produced oil at rates up to 318 kL/d (2000 bbl/d).

The gas in the north of the basin is mainly dry, with minimal condensate production, and the oil is highly paraffinic.

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At least 15 commercial hydrocarbon fields have been discovered in the northern Perth Basin, of which Dongara is by far the largest, with original in-place gas of 14.3 Gm3 (508 Bcf) and original in-place oil of 16.6 GL (104 MMbbl).

Additional discoveries have been made both in the northern and southern parts of the Perth Basin, some of which are currently being delineated. Petroleum-system analysis indicates that mature source rocks are widespread, reservoirs are abundant, and structures are well-timed for hydrocarbon entrapment.

Petroleum systems of the Perth Basin are defined as Transitional and Gondwanan. A critical factor is considered to be seal, due 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 strata. The main source for oil is the marine Lower Triassic basal Kockatea Shale, with reservoirs in Lower Triassic and Permian sandstones.

Oil has also been 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 three-dimensional seismic surveys have 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.

No commercial fields have been discovered in the onshore southern Perth Basin, even though hydrocarbon shows have been encountered in several wells. Gas has flowed on test from the Permian Sue Coal Measures in wells near the Whicher Range.

The Permian to Cretaceous stratigraphic and structural evolution of the southern Perth Basin is similar to that of the northern Perth Basin, but marine intervals are not present in the south, where continental depositional environments dominated until the late Neocomian. Consequently, thick regional shales are absent and the area has poor sealing potential. On the other hand, potential reservoirs, source rocks for both gas and oil, and anticlinal traps are well documented.

There are many untested hydrocarbon prospects in the Perth Basin. The logistics and economics of potential oil and gas discoveries are very positive, particularly since the deregulation of Western Australian gas markets in 1988.

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