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
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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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