Areas W07-8, W07-9, W07-10 and W07-11
Yampi-Leveque Shelf, Browse Basin
Browse Basin Geology
Regional Geology
The Browse Basin is a northeast–southwest-trending, Palaeozoic to Cenozoic depocentre situated entirely offshore in the Timor Sea region of Australia’s North West Shelf (Figure 2). It covers an area of approximately 140,000 km2 and contains in excess of 15 km of Palaeozoic, Mesozoic and Cenozoic sedimentary section (Struckmeyer et al, 1998). It hosts significant, but as yet undeveloped reserves of gas and condensate. Oil discoveries are focussed on the Yampi Shelf (ie Cornea area and Gwydion), although oil was discovered at Caswell 2 in the central Caswell Sub-basin.
The basin is one of a series of northwest-trending extensional basins that formed part of the Westralian Superbasin underlying the North West Shelf region (Bradshaw et al, 1988; Willis, 1988). Struckmeyer et al (1998) divided the Browse Basin into three major sub-basins; Caswell, Barcoo and Seringapatam. The basin is flanked to the west by the Scott Plateau, and the eastern and southeastern elements of the basin are the Yampi and Leveque shelves (Maung et al, 1994; Struckmeyer et al, 1998). The basin is contiguous with the Rowley Sub-basin of the Roebuck Basin to the southwest, and the Vulcan Sub-basin and Ashmore Platform of the Bonaparte Basin to the northeast (Figure 2).
Basin evolution and tectonic development
The Browse Basin has experienced a multi-stage structural history with six major phases of basin development (Struckmeyer et al, 1998):
- Late Carboniferous to Early Permian extension.
- Late Permian to Triassic thermal subsidence.
- Late Triassic to Early Jurassic inversion.
- Early to Middle Jurassic extension.
- Late Jurassic to Cenozoic thermal subsidence.
- Middle to Late Miocene inversion.
Since no formal lithostratigraphy has been established for the basin, formation-equivalent names from the Vulcan Sub-basin (Bonaparte Basin) to the north have been adopted by Blevin et al (1988a) and Struckmeyer et al (1998) and are used in this review (Figure 3).
The basin was initiated as a series of intracratonic extensional half graben during the Late Carboniferous to Early Permian (Symonds et al, 1994). Further to the west this extensional event is thought to have led to breakup and separation of Sibumasu from northwest Australia in the Early Permian (Metcalfe, 1990). The upper-crustal faulting resulted in a characteristic half-graben geometry with large-scale normal faults compartmentalising the basin into distinct sub-basins. Structures resulting from the Late Palaeozoic extensional event controlled the location of subsequent reactivation events and the distribution and nature of the sedimentary fill (Struckmeyer et al, 1998).
The Carboniferous section is dominated by fluvio-deltaic sediments, while the Early Permian sediments (mainly limestones and shales) were deposited in a marine environment. The Late Permian section consists of sandstone grading into shale and limestone. The oldest Triassic rocks intersected in the Browse Basin are marine claystones, siltstones and volcaniclastic sediments (eg, Echuca Shoals 1) deposited during a regional Early Triassic marine transgression. Overlying Triassic rocks include fluvial and marginal to shallow-marine sandstones, limestones and shales.
The Permo-Triassic sag phase was terminated by compressional reactivation in the Late Triassic to Early Jurassic, resulting in partial inversion of Palaeozoic half graben and the formation of large-scale anticlinal and synclinal features within the hanging walls. This event is marked by a regional unconformity that is correlated with the Fitzroy Movement in the Canning and Bonaparte basins (Etheridge and O’Brien, 1994). The arcuate Buffon–Scott Reef–Brecknock anticlinal trend (Figure 2) developed at this time.
The Early to Middle Jurassic extensional phase resulted in widespread small-scale faulting and the collapse of the Triassic anticlines. Extensional faulting was concentrated in the northeastern part of the Caswell Sub-basin (Figure 4) and along the adjacent outer margin of the Prudhoe Terrace (Figure 2: Struckmeyer et al, 1998). The Heywood Graben also formed during this period. The Early–Middle Jurassic syn-rift sediments (Plover Formation) comprise sandstones, mudstones and coals deposited in deltaic and coastal-plain settings. Widespread erosion and peneplanation occurred in the Callovian, associated with continental breakup and the initiation of sea-floor spreading in the Argo Abyssal Plain.
From the Late Jurassic to the Cenozoic, accommodation space was controlled by the interplay of thermal subsidence, minor reactivation events and eustasy. Late Jurassic interbedded sandstones and shales onlap and drape the pre-Callovian structures, and provide a thin, regional seal across much of the basin. An overall transgressive cycle began in the Early Cretaceous and peaked in the mid-Turonian, with open marine conditions established throughout the basin by the Aptian (Figures 4 and 5). Thick marine claystones deposited during this period (Echuca Shoals and Jamieson/Heywood formations), provide a regional seal and contain potential source rocks, with particularly high total organic carbon (TOC) values recorded at the maximum flooding surfaces of several Early Cretaceous transgressive cycles (Figure 6: Blevin et al, 1998b).
The Turonian–Cenozoic section records a major progradational (regressive) cycle in which the shelf edge migrated northwestwards to the outer limits of the Buffon–Scott Reef–Brecknock anticlinal trend. The development of submarine canyons on the Yampi Shelf and deposition of turbidite mounds within the central Caswell Sub-basin occurred during the middle to late Campanian (Benson et al, 2004). Inversion in the Late Miocene occurred as a result of the convergence of the Australia-India and Eurasia plates (Shuster et al, 1998).