Areas W07-18, W07-19, W07-20 and W07-21

Beagle Sub-basin, Carnarvon Basin

Carnarvon Basin Geology

Regional Geology

The Carnarvon Basin is the southernmost component of the Late Palaeozoic to Cenozoic Westralian Super-basin that underlies the northwestern continental margin of Australia from North West Cape in the south to the Arafura Sea in the north. The basin contains a series of major Mesozoic depocentres, most of which lie offshore (Figure 1). The southern part of the basin consists of Palaeozoic depocentres, with Palaeozoic strata out cropping onshore.

The northern, offshore part of the Carnarvon Basin evolved from a pre-rift, broadly sag basin in the Late Palaeozoic, through tectonically active syn-rift sub-basins in the Jurassic, to a passive margin carbonate shelf in the Cenozoic (Figure 2). The geological evolution of the basin has been discussed in detail by many authors, and the summary presented below is derived from the work of Kopsen and McGann (1985), Boote and Kirk (1989), Hocking (1990), Jablonski (1997), Westphal and Aigner (1997), Tindale et al (1998), Bussell et al (2001), Norvick (2002), and Longley et al (2002).

The offshore part of the Carnarvon Basin comprises the Exmouth, Barrow, Dampier and Beagle sub-basins, the Exmouth Plateau (including the Investigator Sub-basin) and the Rankin Platform (Figure 1). The tectonic elements of the region are dominated by a northeasterly trend that developed as a result of rift tectonism initiated in the Early Jurassic and continuing until the Late Jurassic. Inboard basin-bounding faults are similarly oriented, and subsequent tectonic movements have variably inherited this structural alignment. The last major rift-related tectonism occurred in the Valanginian, preceding the final continental separation of Greater India from Australia.

As a result of the Jurassic and Early Cretaceous rift tectonism, the Barrow and Dampier sub-basins form a northeast-trending graben, bounded on the outboard side by the buried fault escarpment of the Rankin Platform and the Exmouth Plateau. The outboard side of the Exmouth Sub-basin and Exmouth Plateau is bounded by oceanic crust.

The Palaeozoic evolution and stratigraphy of the northern offshore part of the Carnarvon Basin is poorly known, but it is relatively unimportant for petroleum geology. The Mesozoic and Cenozoic successions are divided into several megasequences, variably influenced by tectonic phases associated with major rifting and sea-floor spreading. A generalised stratigraphy of the basin is shown in Figure 2 and comprises the following megasequences:

Depositional environments and hydrocarbon generation, migration and entrapment are strongly controlled by rift-related tectonics in the basin. During the pre-rift active margin phase, marine and fluvio-deltaic sediments were deposited in an extensive basin. This basin subsequently fragmented into smaller depocentres in which marine and deltaic sediments were deposited during the early syn-rift phase, with restricted marine sediments being deposited during the main syn-rift phase. During the late syn-rift phase, various marine sediments were deposited within the framework of the large-scale Barrow Delta system. In the post-rift active phase, transgressive shaly marine deposition prevailed. During the subsequent passive margin phase, a variety of marine carbonate sediments were deposited in open marine shelfal environments.

Clastic sediments were sourced from the southeastern cratonic flank throughout the Mesozoic and Cenozoic evolution of the basin (Longley et al, 2002). In addition, tectonic uplift in the Jurassic and syn-depositional inversions in the Cretaceous provided depocentres with reworked sediments from highstanding areas within the basin.


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Pre-rift active margin

A regional marine transgression, as a result of post-rift sagging of the previous Palaeozoic rift cycle, commenced at the beginning of the Triassic, depositing the Locker Shale unconformably over Permian sediments (Figure 2). The Locker Shale was deposited in broad, relatively unfaulted downwarps and it grades upwards into the fluvio-deltaic Mungaroo Formation, which is composed of thick sequences of sandstones and claystones with some coals. The fluvio-deltaic system prograded to the northwest, to cover much of the offshore part of the Carnarvon Basin. The Mungaroo Formation sediments were deposited in a broad, low relief, rapidly subsiding coastal plain, which included an extensive swamp system cross-cut by multiple rivers. The Mungaroo Formation also contains limestone units.

The fluvial sandstones of the Mungaroo Formation form the principal reservoir rocks of the giant gas accumulations on the Rankin Platform. The uppermost, marine part of the Mungaroo Formation consists of shoreline sandstones and claystones. The uppermost part of the Mungaroo Formation is absent in the well explored eastern part (around the North Rankin gas field) of the Rankin Platform. In contrast, this part of the formation is well preserved on the western outboard portion of the Rankin Platform where it is one of the major reservoirs in the Gorgon, Geryon, Maenad and Orthrus gas accumulations (Figure 3).

Throughout much of the Triassic, the onshore portions of the Carnarvon Basin and the onshore Pilbara Block were undergoing active uplift and erosion, providing sediment sources for the Locker Shale and Mungaroo Formation. However, this abundant supply was disrupted by the end of the Triassic (Hocking, 1990).

Marking rapid subsidence at the onset of the Early Jurassic, the transgressive Brigadier Formation and Murat Siltstone were deposited in a marine shelf environment and comprise thinly bedded marine siltstones, claystones and marls. The Brigadier Formation is well preserved below a widespread Late Jurassic unconformity in the outer part of the Carnarvon Basin, and the top of the formation represents the maximum flooding surface of the Early Jurassic marine transgression. Within the Kangaroo Syncline in the southern Exmouth Plateau, the preserved Early to Middle Jurassic section, including the Brigadier Formation, is thicker than on the Rankin Platform (Bussell et al, 2001). Thin, reservoir-quality sandstones on some horst blocks along the Rankin Platform are known as the North Rankin Formation.

Early syn-rift

The pre-rift active margin to syn-rift transition is represented by the rift-onset Pliensbachian Unconformity (JP1 seismic horizon; Figure 2). Extensional rift faulting and warping produced northeast-trending tilted fault blocks, horsts and graben (Barber, 1988). The development of the Exmouth, Barrow and Dampier sub-basins and Rankin Platform was initiated during this early syn-rift phase, and these elements remained tectonically active throughout the Jurassic.

The early syn-rift megasequence (mid-Pliensbachian to mid-Callovian) comprises restricted marine claystones of the Athol Formation and deltaic sandstones of the Legendre Formation. The Legendre Delta developed in the early Bathonian in the Dampier Sub-basin, but sedimentation ceased by the early Callovian.

Main syn-rift

The mid-Callovian unconformity surface (JC seismic horizon; Figure 2) defines the boundary between the early syn-rift (mid-Pliensbachian to mid-Callovian) and main syn-rift (mid-Callovian to latest Tithonian) megasequences. This unconformity represents the onset of the continental breakup of the northwest Australian margin (Jablonski, 1997). Claystones of the transgressive Callovian Calypso Formation were deposited in the Barrow and Dampier sub-basins over the surface of the unconformity.

Major rift faults developed along the northern edge of the Exmouth Plateau in the Callovian, but through-going oceanic crust was not created until the late Oxfordian (Norvick, 2002). The basal Oxfordian Unconformity (‘Breakup Unconformity’; JO seismic horizon; Figure 3) represents this phase of continental breakup and the onset of sea-floor spreading to form the Argo Abyssal Plain.

The term ‘Main Unconformity’ (MU seismic horizon; Figure 2) has been used widely to refer primarily to the basal Oxfordian (JO) unconformity. In practice, however, this horizon is often a composite sequence boundary, ranging in age from basal Jurassic in one part of the basin to Aptian in another (Jablonski, 1997). For instance, in some areas on the Rankin Platform, the Norian Mungaroo Formation underlies the Albian Windalia Radiolarite or Gearle Siltstone, indicating that the Main Unconformity represents a 92 million year hiatus (Newman, 1994). A large and inconsistent time break at the Main Unconformity has led to confusion regarding the nature and timing of erosion. Because of the diachronous nature of the unconformity surface, the composite Main Unconformity is also called the ‘Intra-Jurassic Unconformity (IJU seismic horizon; Figure 2)’ (Sibley et al, 1999).

Following continental breakup, active faulting continued in the Late Jurassic. This resulted in uplift and tilting of the inboard basin-bounding shelf and Rankin Platform. Reworked sediments were deposited in depocentres adjoining the uplifted areas. Tectonic subsidence rates far exceeded sedimentation rates in regional depocentres, resulting in a thick succession of deep-water Dingo Claystone, which gradually filled the graben depocentres and progressively overlapped the flanks of high blocks (Tindale et al, 1998). This deep-water marine sedimentation was confined to the graben depocentres of the Barrow, Dampier and Exmouth sub-basins. The Oxfordian maximum flooding phase of the graben system provided a favourable depositional environment for high quality, oil-prone source rocks (Norvick, 2002).

Although marine claystones dominate the main syn-rift (mid-Callovian to latest Tithonian) megasequence, paradoxically this is also the time when reservoir-quality turbidite, submarine fan, shoreline and fluvial sandstones were deposited locally at the edge of tectonically active graben.

On the eastern portion of the Exmouth Plateau, Late Jurassic deposition of sandy shelfal facies occurred within restricted shallow basins. The Kangaroo Syncline was also tectonically active during the Late Jurassic across the southern Exmouth Plateau and northern Exmouth Sub-basin, in response to footwall uplift of the Triassic tilted fault blocks on the Rankin Platform. The uplift created a hinterland that provided a source for coarse clastic sediments eroded from the Mungaroo Formation and transported into the shallow marine environment of the syncline. By the Tithonian, the gradual subsidence and peneplanation of the provenance area limited clastic input to the syncline (Jenkins et al, 2003).

Oxfordian shallow-marine sandstone (Jansz Sandstone) is the major reservoir in a stratigraphic trap for the giant Jansz/Io gas accumulation (Jenkins et al, 2003). The Biggada Sandstone, Dupuy and Angel formations are other significant reservoir-quality sandstones deposited in this megasequence. For example, turbidite sandstones of the Angel Formation, which are the major oil- or gas-bearing reservoirs in the Dampier Sub-basin, were deposited in the Tithonian when reactivation of horsts and graben resulted in further erosion of marginal areas with reworking of quartz-rich sandstones (Hocking, 1990).

Further inshore, an Oxfordian shallow-marine sandstone (Linda Sandstone) was deposited in the eastern Barrow Sub-basin, which has traditionally been viewed as a deep-water depocentre. Late Jurassic shore-face and shallow-marine sandstones may be aligned parallel to shorelines elsewhere in the Carnarvon Basin (Moss et al, 2003).


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Late syn-rift Barrow Delta

Latest Jurassic uplift and erosion marked the onset of late syn-rift (latest Tithonian to mid-Valanginian) sedimentation. The large Barrow Delta system abruptly and briefly developed in the Carnarvon Basin during this tectonically quiescent phase. The Barrow Delta was extensive, and its sediments are up to 2500 m thick. The delta prograded in two major phases, and two main delta lobes were developed. The initial depositional phase occurred over the Exmouth Sub-basin in response to a copious supply of sediments from the south. The delta then prograded rapidly to the north over a thick pile of turbidites and pro-delta shales to a maximum northward limit roughly west from Barrow Island across the Exmouth Plateau. On the Exmouth Plateau, the Barrow Group consists of turbidites, basin floor fans and fluvio-deltaic sediments of the lower Barrow Delta lobe. A turbidite fan formed the sandstone complex at the Scarborough gas accumulation to the north of the delta front (Norvick, 2002).
While the delta resumed progradation in the late Berriasian, erosion of the lower delta lobe occurred in the inshore part of the Exmouth Sub-basin. The new depocentre of the delta retreated 250 km to the east and extended inboard beyond the eastern limit of the first phase. As a result, a back-stepped delta (upper Barrow Delta lobe) was developed in the Barrow and Dampier sub-basins. The second phase reached its northern limit around the Gorgon horst structure.

The sediments of the lower (or western) Barrow Delta lobe are collectively known as the Malouet Formation comprising bottom-set submarine fan sandstones and pro-delta claystones, and those of the upper (or eastern) lobe are known as the Flacourt Formation, comprising basinal turbidites, fore-set claystones and top-set sandstones. The boundary between the two lobes is markedly diachronous and cannot always be picked as a continuous regional seismic horizon (Baillie and Jacobson, 1997). The lower Barrow Delta lobe contains approximately 75 % of the sediments deposited by the Barrow Delta system (Ross and Vail, 1994). Barrow Group sandstones are predominantly quartzose with minor clay matrix and are weakly cemented by calcite, pyrite or siderite. The porosity and permeability of these sandstones tend to be excellent in the outer part of the Carnarvon Basin.

Sandy units within the top Barrow Group are variously named, and their nomenclature is somewhat confusing. They include; the top sandstone of the Barrow Group, top sandstone of the Flacourt Formation, Zeepaard Formation, and Flag Sandstone. The Zeepaard Formation was deposited across wide areas of the Barrow and Exmouth sub-basins, Rankin Platform and Exmouth Plateau as progradational top-set units of the Barrow Delta in front of multiple distributaries at slightly different times in the early Valanginian. In contrast, the Flag Sandstone was deposited as a submarine fan sandstone in the northeastern inboard part of the Barrow Sub-basin, in front of the last fore-set of the Barrow Delta.

The supply of sediment to the Barrow Delta system ceased due to the disruption of a major fluvial distributary system in the Valanginian, when continental breakup commenced to the southwest of the Exmouth Plateau (Hocking, 1990). The Exmouth Sub-basin and Exmouth Plateau were tectonically inverted during breakup, but subsidence and marine sedimentation continued throughout the Barrow and Dampier sub-basins.

Post-rift active margin

After tectonic uplift and faulting associated with the separation of Greater India and Australia in the Valanginian, a large portion of the Carnarvon Basin was subjected to peneplanation. This event was followed by regional post-rift sagging sedimentation in the offshore part of the basin from the mid-Valanginian to mid-Santonian.

Post-rift marine deposition commenced on the Valanginian unconformity surface (KV seismic horizon; Figure 2), and the Birdrong Sandstone and glauconitic Mardie Greensand were deposited in smaller deltas. This localised sedimentation cycle was followed by the basin-wide deposition of the transgressive marine Muderong Shale, Windalia Radiolarite and Gearle Siltstone. The Muderong Shale is a regional seal, but also contains economically important petroleum-bearing marine sandstones such as the M. australis Sandstone (also known as the Stag Sandstone) and Windalia Sandstone in the Barrow and Dampier sub-basins. These sandstones overlie the intra-Valanginian unconformity, and are characteristically glauconitic and diachronous.

The Windalia Sandstone of the Muderong Shale was historically a major exploration target in the Barrow Sub-basin. More than 90 % of the initial oil reserves of the Barrow Island oil field, which is the largest in the Carnarvon Basin, are contained within this sandstone in at least 30 identified oil- or gas-bearing reservoir units in the field (Ellis et al, 1999).

Passive margin

Siliciclastic sedimentation ceased by the mid-Santonian as a result of tectonic stability and a decreasing supply of terrigenous sediments. Shelfal carbonate sediments were deposited on the passive continental margin in the Late Cretaceous and Cenozoic, as the whole region continued to cool and subside after cessation of the rifting process. On the deep-water Exmouth Plateau, sedimentation during this period was relatively thin, as subsidence rates outstripped sediment input. Towards the end of the Cretaceous, however, the Kangaroo Syncline on the Exmouth Plateau became the major depocentre of the Carnarvon Basin.

During the Campanian, uplift of the hinterland resulted in a phase of inversion in the Exmouth Sub-basin and Exmouth Plateau, forming the Exmouth Plateau Arch. This tectonic event also marked the onset of transpressional structural growth of pre-existing rift-related structures within the Barrow and Dampier sub-basins (Longley et al, 2002).

In the Miocene, a major compressional event associated with the collision of the Australia-India and Eurasia plates affected the entire northwest Australian margin, including the Carnarvon Basin (Longley et al, 2002). This event caused tilting, inversion, renewed movement on faults, and the creation of new strike-slip or wrench faults (Malcolm et al, 1991). This is also the time when many structural traps within the Cretaceous and Cenozoic strata were formed.


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