SOURCE
ROCK
In the Browse Basin Province 3913 the Late Jurassic,
Early Cretaceous-Mesozoic TPS is characterized by source rocks of Late
Jurassic and Early Cretaceous age and potential and proven reservoir rocks
that range in age through the entire Mesozoic (Fig. 5).
Willis (1988) and Haston and Farrelly (1993) report potential
source rocks throughout the Middle Jurassic to Lower Cretaceous section
and speculate the source potential of the Triassic rocks is probably gas.
Bradshaw and Sayers, (1996) map an extension of deep-water, anoxic, marine
conditions that existed in the Late Jurassic, from the north, between the
exposed Ashmore Platform and the exposed Londonderry High. A Cretaceous-Jurassic
petroleum system has been suggested with hydrocarbon sources in the Jurassic
upper and lower Vulcan formations and the Cretaceous Echuca Shoals Formation
(DPIE, 1998). This possible petroleum system would source hydrocarbon accumulations
in the Triassic Nome Formation at Scott Reef, Jurassic Plover Formation
at Brecknock, North Scott Reef, and Skua, Montara Formation at Montara,
and Tahbilk, Cretaceous Echuca Shoals Formation at Cornea and Gwydion,
and the Cretaceous Puffin Formation at Puffin (DPIE, 1998) (Fig. 3). Stephenson
and Cadman (1994) suggest that Middle to Late Triassic rocks may have sourced
gas accumulations at Scott Reef.
Source rocks of late Middle Jurassic (Callovian) to Early
Cretaceous (Valanginian) time are mainly confined to the Vulcan sub-basin
and were deposited during Vulcan subsidence in a low energy restricted
marine environment (Pattillo and Nichols 90).
Scott (1994) notes two occurrences of source rocks in
the Browse Basin. Marine shelf and basin sediments of Late Jurassic-earliest
Cretaceous are characterized by total organic carbon (TOC) of 1-5 wt%,
S2 of 2-15 mg/g, and hydrogen index (HI) of 100-400. Estimated
thickness of this source rock section varies from 100 m to more that 1000
m. Alluvial plain and deltaic facies account for another possible source
rock in the Early-Middle Jurassic Plover Formation. This mixed marine and
alluvial section is greater than 500 m thick with TOCs of 1-70 wt%, S2
of 2-250 mg/g, and HI of 100-600 (Scott, 1994). Bradshaw and others, (1994)
indicate the highest levels of TOC are present in rocks of Early Jurassic
(Pliensbachian to Toarcian) age and Middle Jurassic (Bajocian to Bathonian)
age with associated HIs of approximately 250. The Late Jurassic (Kimmeridgian)
is not as thick or as rich in TOC as is found in other areas of offshore
Australia, however, the interval is considered as possible source rock.
A Lower Cretaceous source interval was suggested for the Browse Basin by
Wilmot and others at AGSO (Bradshaw and others, 1994).
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Using present-day geothermal gradients,
the Upper Cretaceous claystone section may be mature in west-central and
outer parts of the basin, the Lower Cretaceous (Neocomian) rocks may be
mature over most of the central basin and the Lower to Middle Jurassic
section may be mature across the entire basin (Fig. 6) (Willis, 1988; Butcher,
1989). The mid-Cretaceous section is shown by vitrinite reflectance data
to be in the peak oil generative stage west of the Prudhoe Terrace (Bradshaw
and others, 1994). Stephenson and Cadman (1994) proposed that the northern
portion of the basin is mature for hydrocarbon generation and that maturity
decreases to the southwest.
OVERBURDEN ROCK
During the Cretaceous, open ocean circulation was established
and a passive margin style of tectonic stability prevailed where carbonate
deposition built the present shelf and reef configuration. The 1,511 m
thick Cretaceous section in Arquebus-1, next to Lombardina (Fig. 2), includes
marine shales of open marine to shelfal marine and carbonate (Haston and
Farrelly, 1993). The Tertiary section in this well is 918 m of bioclastic
calcarenite interbedded with dolomite and sandstones of limited areal extent.
TRAP TYPES
Compactional drapes over fault blocks and tilted fault
blocks delineate the proven trap style in the central and western portion
of the Browse Basin. These tilted fault blocks are aligned in trends that
are roughly parallel to and located between the sub-basin trends (Willis,
1988; Stephenson and Cadman, 1994)(Fig. 4). The drape structures appear
as anticlines on figure 2. Triassic tilted strata are faulted then draped
and onlapped by Jurassic strata (Fig. 4a, b). Continued faulting along
these trends involved the Jurassic strata which were then overlain by upper
Jurassic and younger strata. Offshore extension of lineaments and folds
from the onshore Kimberley Block (PESA, 1996), are interpreted to influence
accumulations on the Yampi Shelf where recent discoveries are trapped in
compactional drape anticlines formed over paleo-topography and in depositional
pinchout against basement.
Tilted fault-blocks formed across the basin during Late
Triassic to Early Jurassic tectonics. An unconformity of Middle to Late
Jurassic (Callovian-Oxfordian) age was characterized by Late Jurassic lava
flows and deposition of volcaniclastic sediments. Regionally extensive
Cretaceous claystones sealed traps (Fig. 3). These traps have generally
been preserved since the Cretaceous. Drape anticlines and faulted anticlines
result from compaction of Jurassic and Cretaceous sediments over tilted
Triassic fault blocks. These traps are also found in the Vulcan sub-basin
where two parallel graben formed during Middle to Late Jurassic (Callovian-Kimmeridgian)
subsidence. Minor faulting
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