TRAP TYPES
-continued
related to these graben continued through the Cretaceous
and some reactivation began during the Tertiary Timorese collision. Proven
traps along the Yampi Shelf consist of drape over basement relief in combination
with depositional onlap of Cretaceous sediments onto the Kimberley Block.
Tilted fault block traps of Scott Reef were among the
earliest hydrocarbon discoveries in the Browse Basin. These traps are similar
in style to earlier successful discoveries along the Northern Carnarvon
portions of the Australian Northwest Shelf in Province 3948 to the southwest.
RESERVOIR ROCK
The Browse Basin was generally a low energy, shallow
and narrow marine- influenced continental seaway that was open to the north,
northeast and sometimes to the Rowley sub-basin in the southwest and beyond.
The land mass named Argo Land, portions of the eastern Indonesian plate,
other continental plates connected to the Scott Plateau, and the Ashmore
Platform separated the basin from the Tethys Ocean (Bradshaw and others,
1994). Formation of the Argo Abyssal Plain in Middle to Late Jurassic (Callovian-Oxfordian)
time opened the area to the ocean as a thermally subsiding passive style
margin.
The main reservoir at Skua Field is the Jurassic Plover
Formation. The Plover Formation is present as erosional remnants beneath
the late Middle Jurassic (Callovian) unconformity in tilted fault blocks
mainly in the Vulcan sub-basin. These marine to deltaic clastics are absent
on the Ashmore Platform and partially preserved on the Londonderry High
(Pattillo and Nichols, 1990). Coarse fan deltas of the Montara Formation
fringe the sub-basin on the southeast and extend into the axis. The Vulcan
Formation consists of restricted marine mudstone and deep-water fans that
were derived from the graben flanks.
Late Cretaceous rocks, deposited as deep-water fans, in
the central basin (PESA 1996) are the reservoir rocks in Caswell-1. This
well tested oil from thin sandstone of Albian age surrounded by shale.
The main reservoir sequence in the Scott Reef field and
at Brecknock-1 is described as Jurassic interbedded sandstones, shales
and volcanics that were deposited in nearshore marine to fluviodeltaic
environments. Porosity ranges from 11-14%. The Triassic gas bearing interval
at Scott Reef is dolomite and dolomitic sandstones (Bint, 1988).
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Lower Cretaceous (Hauterivian to
Barremian) oil and gas bearing reservoirs in Gwydion-1 are described as
clean quartz sandstones that were deposited in high energy upper shoreface
conditions and retain porosities of 24-27% (Spry and Ward, 1997). Other
gas bearing strata in this well are glauconitic sandstones of Early Cretaceous
(Barremian to Albian) age.
The Jurassic section drilled at Arquebus-1, next to Lombardina
(Fig. 2), is 827 m thick and has an interpreted hydrocarbon column of 105
m (Haston and Farrelly, 1993). The Lower Jurassic section contains spilitic
volcanics that are interpreted to have erupted along the basin margin fault
into shallow marine environments where deposition of carbonates, sandstones,
siltstones, claystones, and volcaniclastics occurred. The Middle Jurassic
strata contains shallow marine to non marine, massive quartzose sandstone
with porosity of 14%, interbedded with siltstone, claystone and carbonate.
The Upper Jurassic is described by Haston and Farrelly (1993) as shallow
water claystone to sandstone with porosity ranging from 6-12%.
SEAL ROCK
Jurassic intraformational shale and claystone seals are
described by Bradshaw and others (1994). Local and regional seal rocks
consisting of lower to upper Cretaceous (Valaginian-Cenomanian) claystones
are reported to reach a thickness of 1.5 km on the highs of the Scott Reef
area and thicker in the sub-basins (Stephenson and Cadman, 1994). These
claystones are a result of the post breakup establishment of a passive
margin continental shelf and slope of generally low energy and low sediment
accumulation rate (Spry and Ward, 1997).
UNDISCOVERED PETROLEUM
BY ASSESSMENT UNIT
The U. S. G. S. Browse Basin Province (3913) contains
one TPS (391301) with one assessment unit (39130101) (Fig. 1). Exploration
targets are numerous and are under-explored in the Browse Basin. The entire
basin is underlain by either Permian or Triassic/Jurassic tilted fault
blocks. Where primary porosity is preserved, reservoir quality is good,
although quartz overgrowths occlude much of the porosity at some locations
in the deeper sedimentary rocks. Miller and Stuart (1992) describe possible
deep-water fans of Early Cretaceous (Valanginian) age, that have been identified
on seismic. These fans occur in the southwestern portion of the Browse
Basin and are similar to the reservoir rocks at Caswell-1. Depositional
onlaps of progressively younger sediments onto the Kimberley Block, all
along the Yampi
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