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U. S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY


A TOTAL PETROLEUM SYSTEM OF THE BROWSE BASIN, AUSTRALIA:
Late Jurassic, Early Cretaceous-Mesozoic

by

Michele G. Bishop1


 

Open-File Report 99-50-I
 
 
 

1999



 
 
PETROLEUM OCCURRENCE
Triassic and Jurassic reservoirs account for more than 3130 million barrels of oil equivalent (MMBOE) of recoverable reserves in the Browse Basin and 77 MMBOE in the 3913 province portion of the Vulcan sub-basin. Cretaceous reservoirs account for more than 2.6 billion barrels of oil equivalent (BBOE) in the Browse Basin and 10 MMBOE in the Vulcan sub-basin.

Cornea-1, drilled in 1997, was reported to have found from 600 million barrels of oil (MMBO) to 2.6 billion barrels of oil (BBO) in place (Williamson, 1997; DPIE, 1998). This discovery is considered to be the first commercially producible oil in the Browse Basin. It confirms a new play trend located along the far eastern edge of the basin that was first indicated by Gwydion-1 drilled in 1995. The Cornea discovery proves that very large volumes of oil have been generated in the mature central portions of the basin and it confirms there has been long-range migration and charge. Gwydion-1 tested a compaction structure draped over a basement high similar to Cornea and located roughly along the same trend. Both discoveries are visible as seismic anomalies and hydrocarbons are detected in the water column at Cornea.

The Scott Reef gas discovery targeted a high fault block structure on the western edge of the Browse Basin. Gas and condensate are present in Jurassic rocks with shows in Triassic rocks. North Scott Reef-1 is an extension of the same structure and Brecknock-1 gas discovery is located to the south along the same trend. The Brecknock-1, Brewster-1A, and Echuca Shoals-1 gas discoveries are Jurassic rocks formed into drape anticlines over fault blocks.

Arquebus-1, just southwest of Lombardina, was drilled in 1991 in the southern portion of the Browse Basin. This gas discovery tested middle to upper Jurassic sandstones, which are situated in a three-way closure. Testing confirmed a 51 m gross oil column with a 45 m gross active oil column and a possible 105 m gross oil column interpreted from logs (Haston and Farrelly, 1993).

In the central Browse Basin oil shows are from Cretaceous sandstones in Caswell-1 and -2.

Several oil and gas discoveries have been made in the Vulcan sub-basin that trends southwest-northeast and continues northeast into Province 3910. O’Brien and others (1993) suggest that Vulcan sub-basin discoveries appear to be located along northwest and north-south trending faultswhere these trends intersect northeast/east-northeast trending structural grain. This intersection is thought to be a result of complex interactions of the Proterozoic/Late 

 Carboniferous-Early Permian fault sets with the varying Mesozoic stress directions. Discoveries in the province portion of the Vulcan Graben are: 1) Puffin, oil in Cretaceous shallow marine sandstones; 2) Skua, oil and gas in Cretaceous and Jurassic intervals; 3) Montara oil and gas; and 4) Tahbilk, gas in Jurassic deltaic rocks.

Skua Field was discovered in 1986 and confirmed in 1987. Hydrocarbons occur in high quality, submarine fan sandstones of Early Jurassic age and are trapped in a steeply dipping fault block. The field is trapped against the bounding fault of the graben, which offsets Early Jurassic age rock against Cretaceous Santonian age rock. A gas cap of 28 m and an oil leg of 46.5 m in reservoirs of 22 % porosity was reported by Osborne, (1990). Cumulative production reported in 1995 was 18.8 MMBO and since then production facilities have been moved off site due to rising production costs (World Oil, 1997). This is the only production to date from this province.

Caswell–1, drilled in 1977, was the first recorded oil discovery in the Browse Basin. Initial production (IP) was reported as 201 barrels of oil per day (bopd) of 46° API gravity oil (Butcher, 1989). The Scott Reef-1 discovery was reported as 49° -54° API gravity hydrocarbons at an IP of 100 m3 of gas/condensate per day (Willis, 88). Porosity at Gwydion-1 averages 26%, and 30.5° API oil was recovered (Spry and Ward, 1997). Barcoo-1 in the far southeastern part of the province, has total organic carbon (TOC)s as high as 4.7wt%, and hydrogen index (HI)s from 23-269 reported by Bradshaw and others (1994).

In the Vulcan sub-basin, Puffin oil has an API of 40° and reservoir porosity of 21.5% and Skua field oil has an API of 43.4° , reservoir porosity of 16-20% and permeability of 950 millidarcies (mD).

Migration paths to discoveries in the eastern parts of the Browse Basin are described by Spry and Ward (1997) as vertical up the marginal fault system and lateral within Cretaceous sands. Hydrocarbons migrate updip to traps draped over basement highs and stratigraphic pinchouts against basement. Migration paths within the Vulcan sub-basin would primarily be along faults to structural traps that overlie and are adjacent to mature source rocks within the graben or to sandstones within the source rock intervals. Accumulations in the western parts of the basin may have been fed across and along faults that control the structural highs of Scott Reef from source rocks in the Caswell sub-basin to the east or possibly the outer sub-basin to the west and north. If discoveries at Scott Reef are associated with possible source rocks of Permian and Triassic age (Stephenson and Cadman, 1994) vertical migration of hydrocarbons along fault would be likely.

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U. S. Geological Survey Open File Report 99-50I