Areas W07-16 and W07-17
Exmouth Plateau, Carnarvon Basin
Release Area Geology
Tectonic setting
Area W07-16 lies wholly within the Exmouth Plateau as defined by Geoscience Australia, whereas Area W07-17 lies within the Exmouth Plateau, the northeastern part of the Rankin Platform, the northern part of the Dampier Sub-basin, and the southwestern part of the Beagle Sub-basin.
One of the prominent structural elements within this part of the Exmouth Plateau is commonly referred to as the Brigadier Trend (Figure 2).
Structures on the boundary between the Dampier and Beagle sub-basins are low relief and difficult to clearly define. The Mutineer/Norfolk and Exeter accumulations are located here.
Stratigraphy
The Exmouth Plateau contains a thick sequence (up to greater than 10 km) of Mesozoic to Cainozoic sediments. The Mesozoic stratigraphy of the adjacent Beagle sub-basin is summarised in Figure 3 (modified after Blevin et al, 1994a, 1994b and Woodside, 1999).
The Early Triassic section is marked by a widespread marine transgression and deposition of the Locker shale in shallow shelf and shoreline facies. The overlying Mungaroo Formation consists of numerous fining-upward cycles of thick fluvio-deltaic sandstones with minor interbeds of siltstones, shales and coals.
The Early Jurassic Brigadier Formation comprises numerous fining- and coarsening-upward cycles of thinly bedded sandstones, siltstones and claystones deposited in low energy, marginal to shallow marine environments. The North Rankin Formation comprises thick clean sandstones deposited in a high energy shallow marine setting, and minor interbedded siltstone and claystone units.
The Early–Middle Jurassic Legendre Formation is characterised by a series of stacked coarsening-upward fluvio-deltaic cycles of sandstones, siltstones, shales and minor coals, capped by massive sandstones. The cycles are up to 60 m thick and are interpreted as sand-dominated north-to-northwest prograding delta-front deposits separated by fine-grained interdistributary and overbank deposits. The Athol Formation and Murat Siltstone represent marine facies outboard of the Legendre delta.
The Callovian Calypso Formation disconformably overlies the Legendre Formation and consists of glauconitic claystone and thinly interbedded sandstones and siltstones. It represents a widespread transgressive marine unit associated with the onset of continental breakup and sea-floor spreading in the Argo Abyssal Plain to the north. The top of the unit is marked by the Oxfordian and/or amalgamated base Cretaceous unconformity.
Late Jurassic sediments are generally thin or locally absent in the Exmouth Plateau, but thicken markedly into the Dampier Sub-basin to the south and southeast where they are dominated by organic-rich restricted marine claystones of the Dingo Claystone. Prograding fan sands of Tithonian age (Angel Formation), and locally more restricted fans of Oxfordian age (Eliassen Formation), occur along the flanks of horst blocks in the southern portion of the sub-basin, and down-dip of the main basin-margin fault. These sands consist of good quality reservoir sands and thin claystone interbeds.
The Berriasian to early Valanginian Forestier Claystone is a marine claystone that drapes and onlaps the horst blocks and basin margin. The claystone thickens into the troughs between the uplifted blocks, and forms a regional seal across the sub-basin, although seal quality deteriorates towards the southeastern margin.
The Valanginian–Aptian Muderong Shale is a thick marine claystone that occurs throughout the sub-basin, and provides an important regional seal where the underlying Forestier Formation is thin or absent.
The mid-Cretaceous to Cainozoic section comprises claystones, marls and
calcilutites deposited on a progradation, increasingly carbonate-dominated,
open marine passive ramp margin.