W10-20 - Department of Resources, Energy and Tourism

eleAse AreA geology
local tectonic setting
the Exmouth Plateau is a deep water marginal Plateau that represents the western structural
element of the northern carnarvon basin (Figure 3). most of the plateau is underlain by 10–15 km
of generally flat-lying and block faulted, tilted lower cretaceous, Jurassic, triassic and older
sedimentary section (Figure 4 and Figure 5) that was deposited during the periods of extension
that preceded breakup of Australia and Argo land in the middle Jurassic, and then Greater india in
the Early cretaceous (stagg et al, 2004). the dominant fault trend on the Exmouth Plateau is northsouth,
swinging to northeast-southwest near the northern and western outer margins of the plateau,
and along the inner margin adjacent to the Rankin Platform and Exmouth–barrow–dampier subbasins
(Figure 3).
structural evolution and depositional history of the sub-basin
the lower triassic of the carnarvon basin is marked by a regional marine transgression that
represents the sag phase of a previous Paleozoic rift cycle. the marine locker shale (below td of
wells on the Exmouth Plateau) unconformably overlies the Permian section and grades upwards
into the middle–upper triassic mungaroo formation (Figure 4). the mungaroo formation was
deposited in a broad, low relief, rapidly subsiding fluvio-deltaic coastal plain that extended across
the Exmouth Plateau. during marine transgression in the latest triassic (Rhaetian), carbonate patch
reefs developed on the Wombat Plateau (von Rad et al, 1992a; Williamson et al, 1989) and probably
extended across the northern – and western-central parts of the Exmouth Plateau. coeval marls,
siltstones and thin sandstones (brigadier formation) were also deposited at this time on the Exmouth
Plateau.
As rifting proceeded between Australia and Greater india, several phases of faulting occurred in
the Jurassic. in the Pliensbachian, rifting inboard of the Exmouth Plateau formed the Exmouth,
barrow and dampier sub-basins. several kilometres of marine Jurassic sediments were deposited in
these troughs that are the equivalent of condensed sections on the central Exmouth Plateau (dingo
claystone equivalents). the major movement on extensional faults on the Exmouth Plateau occurred
in the callovian, and continuous oceanic crust was created in the Argo Abyssal Plain to the north
in the late Oxfordian, and in the Gascoyne and cuvier Abyssal Plains to the west and south in the
Valanginian (norvick, 2002). Rift and breakup volcanics are widespread along the outer margins of
the Exmouth Plateau, and probably include late triassic, Oxfordian/callovian and Early cretaceous
volcanism (stagg et al, 2004).
On the eastern portion of the Exmouth sub-basin, late Jurassic deposition of sandy shelfal facies
occurred within restricted shallow depocentres (including the Oxfordian Jansz sandstone reservoir
at the giant Jansz-io gas accumulation). during the Early cretaceous the barrow Group delta
prograded northward across the southern portion of the plateau to form a major sediment lobe with
the shelf edge arching through or near the investigator 1 and Zeepard 1 well locations (boote and
Kirk, 1989); basin floor fans of this succession form the reservoir at the scarborough gas field.
As the newly formed oceanic crust of the flanking Argo, Gascoyne and cuvier abyssal plains rapidly
subsided, the Exmouth Plateau also foundered and was progressively transgressed throughout the
cretaceous by shallow marine mudstone (muderong shale) and siltstone (Gearle siltstone), midouter
shelf marl and chalk (toolonga calcilutite), and finally cenozoic bathyal chalk and ooze.
Geology of Release Areas W10-20, W10-21 and W10-22 | www.petroleum-acreage.gov.au 2