2016 Release Areas

Perth Basin Regional Geology

The Perth Basin is a large north to north-northwest trending, onshore and offshore sedimentary basin extending about 1300 km along the southwestern margin of the Australian continent. This basin formed during multiple episodes of rifting between Australia and Greater India from the early Permian to the Early Cretaceous. The basin is structurally complex and comprises many tectonic elements, including large depocentres containing in excess of 15 km of sediments, terraces, ridges and intrabasin highs. The Perth Basin is an established hydrocarbon producing region, with 20 commercial oil and gas fields and numerous additional significant discoveries of varying size, dominantly in the onshore northern part of the basin. The basin is close to petroleum industry infrastructure, including two major gas pipelines and trucking facilities to an oil refinery near Perth.

Basin outline

The Perth Basin is a large (172 300 km²), elongate, north to northwest-trending sedimentary basin extending about 1300 km along the southwestern coast of Australia and encompassing both onshore and offshore regions (Figure 1 and Figure 2). The Perth Basin was originally named by Andrews (1938), and has been described and mapped by Playford et al (1976) and Hocking (1994). The maximum extent of the offshore part of the Perth Basin is placed at the limit of basin fill ranging in age from Cisuralian (early Permian) to Early Cretaceous (Bradshaw et al, 2003).

Crystalline basement beneath the Perth Basin comprises Proterozoic igneous and metamorphic rocks of the Pinjarra Orogen, which formed as an intercontinental mobile belt between the Australian and Indian parts of eastern Gondwana (e.g. Collins, 2003; Fitzsimons, 2003; Cawood and Korsch, 2008; Bodorkos et al, 2016). The Darling Fault system forms the eastern boundary of the Perth Basin (Figure 2), controlling its overall north-south orientation. It originated as a shear zone during the Archean (Blight et al, 1981; Dentith et al, 1994) and was reactivated to form a major rift-border fault to the incipient Perth Basin during the Cisuralian (Crostella and Backhouse, 2000). The Perth Basin is separated from the Mentelle Basin in the southwest by the Leeuwin Complex and Yallingup Shelf (Figure 2), and from the Wallaby Plateau to the northwest by thick and extensive volcanic successions that formed during the Early Cretaceous break-up of Australia and Greater India (Colwell et al, 1994; Symonds et al, 1998; Sayers et al, 2002).

The offshore part of the basin is composed of four sub-basins (Figure 2). The Abrolhos and Houtman sub-basins in the north contain a thick succession (8–13 km) of Permian to Lower Cretaceous sediments, overlain by an Upper Cretaceous to Cenozoic post-break up succession (up to 1 km thick; Figure 2 and Figure 5; Bradshaw et al, 2003). The Zeewyck Sub-basin in the northwest and Vlaming Sub-basin in the south are predominantly Mesozoic depocentres containing 7–14 km of Middle Jurassic to Lower Cretaceous strata (Nicholson et al, 2008; Rollet et al, 2013a, 2013b). The boundaries between the sub-basins have been interpreted to have formed through oblique-slip motion in a transtensional setting (Marshall et al, 1989a). In the northern Perth Basin, the offshore and onshore depocentres are separated by intrabasin highs comprising the Beagle Ridge and Dongara Terrace (Figure 2, Figure 3 and Figure 4). Descriptions of the onshore structural elements are given by Iasky (1993), Hocking (1994), Mory and Iasky (1996), Crostella and Backhouse (2000) and Thomas (2014).

Northwest trending accommodation zones have been proposed as controlling the structural compartmentalisation of rift systems in the Perth Basin. For example, the Abrolhos and Cervantes transfer zones are interpreted to divide the northern Perth Basin into regions of significantly different structural character (Figure 2; Mory and Iasky, 1996).

Both oil and gas are produced from a number of fields in the onshore Perth Basin and, since 2006, from the offshore Cliff Head oil field (Figure 8). The Cliff Head oil field is the first commercial oil discovery in the offshore Perth Basin. It had produced approximately 14.5 MMbbl (2.3 Gl) up to 2015 (Elixir Petroleum Ltd, 2015). There is an established gas pipeline network in Western Australia servicing the Perth Basin (Figure 1).

Tectonic development

Onshore northern Perth Basin

The Perth Basin formed through pre-breakup continental extension between the southwestern continental margin of Australia and Greater India (Matte et al, 1997; Ali and Aitchison, 2005). The tectonic and palaeogeographic history of the Perth Basin has been documented in several studies (Smith and Cowley, 1987; Marshall et al, 1989b, 1993; Harris, 1994; Quaife et al, 1994; Mory and Iasky, 1996; Song and Cawood, 2000; Crostella and Backhouse, 2000; Gorter and Deighton, 2002; Bradshaw et al, 2003; Norvick, 2004, Mory, 2017). These authors generally describe an early phase of rifting in the Permian, followed by a long period of widespread post-rift subsidence and a multi-phase extension in the Middle Jurassic to Early Cretaceous, culminating in the breakup.

Early to late Permian extension

Early Permian extension resulted in the formation of a series of half-graben in the Perth Basin that are separated by saddles and characterised by en-echelon border fault relationships Figure 2, Figure 3, Figure 4, and Figure 5; Norvick, 2004; Thomas, 2014). This graben complex extends at least from the Southern Carnarvon Basin in the north to the Bunbury Trough in the south. In many cases, but not exclusively, the original rift basins were aligned north-south (Quaife et al, 1994). Active faulting slowed in the north from the Sakmarian onwards (Norvick, 2004).

Initially, the basins were filled with glacial (Nangetty and Mosswood formations) to pro-glacial marine (Holmwood Shale, and High Cliff and Woodynook sandstones) and deltaic sediments (Irwin River and Rosabrook coal measures; Mory and Iasky, 1996; Norvick, 2004; Mory, 2017). Deglaciation commenced in the Sakmarian and the rifts continued to fill from the south to north with deltaic (Ashbrook Sandstone, Redgate Coal Measures and Willespie Formation) to progressively more marine sediments (Carynginia Formation; Norvick, 2004).

Late Permian uplift and erosion

In the northern Perth Basin, the end of the early Permian is marked by a regional angular unconformity associated with uplifted tilted fault blocks that underwent subaerial erosion (Roc, 2004). This uplift was followed by late Permian to Early Triassic rifting, and the deposition of coarse-grained alluvial deltas (Mory and Iasky, 1996). Deposition of proximal fan to coarse-grained deltaic sediments (Dongara Sandstone; Mory and Iasky, 1996) occurred during approximately the same period (Mory, 2017).

Triassic to Middle Jurassic post-rift subsidence

A rapid marine transgression at the beginning of the Triassic resulted in deposition extending over a much larger area than in the late Permian (Figure 2, Figure 5; Norvick, 2004). This regional marine transgression occurred throughout the Perth Basin (Kockatea Shale) and in basins to the north (Carnarvon, Canning and Bonaparte basins). Sandier facies in the southern part of the Perth Basin (Sabina Sandstone) suggest that the basin was closed and filling from this direction (Norvick, 2004). During a regression that followed the regional marine transgression maximum in the Early Triassic, deltaic and fluvial facies were deposited throughout the Perth Basin in the Middle to Late Triassic (Woodada Formation and Lesueur Sandstone; Figure 3, Figure 4; Mory and Iasky, 1996). Palaeocurrent data show that the Triassic rivers flowed from the south or southwest (Mory and Iasky, 1996) and the catchment covered both the Perth Basin and much of the Yilgarn Craton (Norvick, 2004).

In at least some parts of the Perth Basin, the prolonged phase of post-rift subsidence appears to have been interrupted by a minor phase of west-northwest–east-southeast extension during the latest Triassic to Early Jurassic (Figure 3, Figure 4; Song and Cawood, 1999, 2000; Bradshaw et al, 2003). This extension reactivated a number of faults in the basin, with syn-tectonic deposition of non-marine red beds (Eneabba Formation). This mild tectonism was followed by deposition of delta-top swamp deposits (Cattamarra Coal Measures), attributed to a regional transgression (Norvick, 2004). This transgression peaked in an extensive but short-lived marine flooding event in the Bajocian which resulted in deposition of the marine shales of the Cadda Formation.

Middle Jurassic to Early Cretaceous rifting and breakup

An abrupt resumption of fluvial sedimentation in the Bajocian (Yarragadee Formation) was accompanied by the onset of major extensional faulting (Figure 3 and Figure 4). In the Tithonian to Berriasian, there was a poorly delineated change to red bed sedimentation (Parmelia Group), which also probably occurred in a syn-rift setting (Norvick, 2004). The deeper water parts of the basin may have received deltaic or marine equivalents of these sediments at that time.

The northwest-southeast extension from the Middle Jurassic to earliest Cretaceous culminated in the break-up of Australia and Greater India during the Valanginian, and produced much of the final structural architecture of the Perth Basin (Bradshaw et al, 2003). Early Cretaceous breakup was associated with a widespread uplift, erosion and volcanism (Figure 1 and Figure 5).

Cretaceous to Cenozoic post-breakup

The post-breakup subsidence in the Early Cretaceous (Valanginian–Aptian; Quaife et al, 1994) has been associated with widespread volcanic activity in some parts of the Perth Basin (Gorter and Deighton, 2002) and on the Wallaby Plateau (Symonds et al, 1998). Marine sediments, including turbidites were deposited as a result of localised subsidence (Norvick, 2004) in deeper parts of the basin (Warnbro Group).

Late Cretaceous and Cenozoic sedimentation occurred under stable, passive margin conditions and produced a thin cover of predominantly marine carbonates. In some places, during the Neogene to Quaternary, there was a transition from cool-water ramp sedimentation to reefal platform development (Houtman Abrolhos coral reefs) near the shelf edge (Collins et al, 1998).

Offshore northern Perth Basin

Recent studies of the offshore northern Perth Basin (Jones et al, 2011a, 2011b; Jorgensen et al, 2011; Hall et al, 2013; Rollet et al, 2013a, 2013b; Thomas, 2014; Borissova et al, 2017) have highlighted both similarities and differences in tectonic evolution of the onshore and offshore depocentres.

The revised interpretation of the offshore northern Perth Basin has confirmed that it had a complex history of extension and reactivation which controlled sedimentation. Regional offshore tectonic events include:

Three phases of rifting: Permian; Early to Middle Jurassic; and Late Jurassic to Early Cretaceous (during the deposition of the Yarragadee Sequence)
Three periods of subsidence: late Permian to Early Jurassic (includes deposition of the Dongara, Woodada and Lesueur sequences); Middle to Late Jurassic; and Early Cretaceous to Cenozoic
Two periods of uplift: middle to late Permian and Early Cretaceous
Three periods of magmatism: late Permian to Early Triassic; Late Triassic to Early Jurassic; and Early Cretaceous

Permian extension

As in the onshore Perth Basin, early to middle Permian extension led to the formation of northwest-oriented half-graben separated by saddles and characterised by en-echelon border faults (Norvick, 2004; Jones et al, 2011b; Rollet et al, 2013a, 2013b; Bernardel and Nicholson, 2013; Thomas, 2014; Borissova et al, 2017). Pfahl (2011) noted that the timing of rifting in the offshore northern Perth Basin is not significantly different to rifting in the onshore parts of the basin. Northwest to southeast extension has been proposed for Permian rifting in the offshore northern Perth Basin (Pryer et al, 2002; Borissova et al, 2017), although the stress regime remains poorly understood.

Although based on only three wells, the study by Pfahl (2011) indicates that the timing of Permian rifting appears to coincide with the deposition of thicker sediments in the Abrolhos Sub-basin. Wells drilled in the Houtman Sub-basin have not reached the Permian sequences. Syn-rift Permian sediments, however, are interpreted (Borissova et al, 2017) in the northern Houtman Sub-basin, where they are up to 10 km thick. Sedimentary units deposited between the Permian rifting event and the first regional uplift event, are discussed by Rollet et al (2013a, 2013b).

Permian uplift and erosion

Similar to the onshore northern Perth Basin, the offshore part of the basin also underwent regional tectonic uplift resulting in tilted fault blocks being exposed to sub-aerial erosion, and the formation of a major angular unconformity (Roc, 2004). Based on limited well data along the edge of the basin, Pfahl (2011) suggested that this uplift was relatively minor (~100 m). Erosion was most prevalent on the Beagle Ridge and in parts of the Abrolhos Sub-basin, where the Beekeeper and Carynginia sequences were totally or partially removed (Figure 3 and Figure 4).

Late Permian to Early Jurassic subsidence

Late Permian to Early Jurassic thermal subsidence resulted in the formation of a westward-thickening sag. Seismic interpretation suggests that a thick succession was deposited across the entire offshore Perth Basin reaching 4-5 km in the northern Houtman Sub-basin (Borissova et al, 2017). In the Early Jurassic, in the eastern Abrolhos Sub-basin, subsidence rates show some variations, initially with greater subsidence at Leander Reef 1, and then an increase of subsidence farther north, recorded at Cliff Head 1, Frankland 1 and Dunsborough 1 (Figure 2 and Figure 8; Pfahl, 2011; Rollet et al, 2013a, 2013b).

Deposition of the upper Permian Dongara Sequence or equivalents occurred during this sag period. The Dongara Sequence transgressive facies filled incised valleys formed during the previous lowstand on the Beagle Ridge and in the Abrolhos Sub-basin. Continued transgression and a maximum flooding event in the Early Triassic led to deposition of the Kockatea Shale. Initial slow deposition resulted in the accumulation of the organic-rich sediments of the Hovea Member, which reaches a thickness of 45.7 m in Lilac 1. This was followed by rapid deposition of the remainder of the Kockatea Shale (435 m thick in Batavia 1). The Kockatea Shale thickens to the north, reaching 1800 m in the northern Houtman Sub-basin (Borissova et al, 2017).

A late Permian to Early Triassic intrusive event is recorded in Edel 1; igneous rocks associated with this event are interpreted to be present below Geelvinck 1 (Gorter and Deighton, 2002). Gradual regression from Early to Late Triassic led to south-north deposition of axial fluvio-deltaic systems including the Woodada and Lesueur sequences (Mory and Iasky, 1996; Norvick, 2004).

Early to Middle Jurassic extension

Onshore thickening of the Eneabba Formation in the hanging wall of a series of reactivated master faults has been interpreted as an evidence of an Early Jurassic extensional phase (Song and Cawood, 2000; Gorter et al, 2004). Offshore, seismic interpretations do not show rift-related thickening of the Eneabba Sequence, apart from locally in the Houtman Sub-basin (Gorter et al, 2004). Similarly, tectonic subsidence analysis does not show evidence of an offshore rifting event in the Early Jurassic (Pfahl, 2011). This phase of rifting seems to be restricted to the onshore part of the basin and areas north of the Abrolhos Sub-basin.

The Early to Middle Jurassic rifting phase (181–169 Ma) shifted outboard, becoming restricted to the Houtman Sub-basin with the deposition of the Cattamarra and Cadda sequences (Rollet et al, 2013a, 2013b; Borissova et al, 2017). This migration of the locus of deposition is further supported by seismic interpretations that record an increase in deposition in the northern Abrolhos and Houtman sub-basins. The Middle Jurassic to Lower Cretaceous syn-rift succession is thickest (about 4 km) in the central part of the Houtman Sub-basin and gradually thins to the north, where only the lower part of the Jurassic succession is present. Jurassic extension reactivated the Permian fault systems in the Abrolhos and Houtman sub-basins. Fault architecture across the basin is characterised by en-echelon fault networks separated by relay ramps at a range of scales.

Middle to Late Jurassic subsidence

During a Late Jurassic regression, fluvio-deltaic sandstones and siltstones were deposited in the lower part of the Yarragadee Sequence. This was followed by coarse-grained fluvial sediments being deposited through a northerly-flowing river system (Mory and Iasky, 1996; Norvick, 2004). According to Pfahl (2011), it appears that the deposition of the Yarragadee Sequence (169–145 Ma) occurred during the post-rift subsidence phase, rather than a syn-rift phase as previously thought (Jones et al, 2011b). Dinocyst assemblages in Houtman 1 and Charon 1 suggest that minor marine incursions also occurred during this time (Jones et al, 2011b; Rollet et al, 2013a, 2013b).

Late Jurassic to Early Cretaceous extension

The commencement of the final rifting phase occurred at about 150 Ma and was largely restricted to the western Houtman and Zeewyck sub-basins (Pfahl, 2011; Rollet et al, 2013a, 2013b). Preceding the separation of Australia and Greater India in the Valanginian (Larson et al, 1979; Veevers et al, 1985; Gibbons et al, 2012), this final rift phase continued the development of a dense north-northwest-striking system of closely spaced, westerly dipping rotated fault blocks (Rollet et al, 2013a, 2013b). A series of small-scale north to north-northwest-trending inversion anticlines and pop-up structures in the Houtman and Abrolhos sub-basins may have formed as a result of transpressional stress during this period. These inversion structures are characterised by well developed fault propagation folds some of which are truncated by the Valanginian unconformity.

Intrusive rocks interpreted in the Yarragadee Sequence in the Houtman Sub-basin, coupled with a thickened Upper Jurassic to Lower Cretaceous succession in the outer Zeewyck Sub-basin, suggest that prior to break up, rifting was restricted mostly to the far western offshore Perth Basin.

Cretaceous uplift and erosion

Tectonic subsidence analysis suggests that regional uplift lasted approximately 6 My after the final rifting (Pfahl, 2011). The basin flanks experienced significant uplift resulting in removal of Lower Cretaceous rocks and erosion of the Upper Jurassic succession throughout most of the Abrolhos Sub-basin, Wittecarra Terrace, Beagle and Turtle Dove ridges and the northern part of the Houtman Sub-basin (Figure 3).

Early Cretaceous to Cenozoic subsidence

Breakup was followed by passive margin subsidence and regional westward tilting in the Early Cretaceous (Valanginian–Aptian). Diachronous breakup between segments of the southwest margin (Hall et al, 2013) influenced the timing and amount of subsidence in the Houtman and Zeewyck sub-basins (Figure 7; Rollet et al, 2013a, 2013b; Hall et al, 2017). Plate reconstruction models (Gibbons et al, 2012) indicate that active transform motion lasted from approximately 137‑124 Ma outboard of the Zeewyck Sub-basin, continuing until 115 Ma outboard of the northern Houtman Sub-basin (Hall et al, 2013). A ridge jump occurred at around 128 Ma (Barremian) in the Cuvier Abyssal Plain, transferring the Wallaby Plateau to the Australian Plate (Gibbons et al, 2012). In the northern Houtman Sub-basin adjacent to the Wallaby Saddle, a volcanic margin evolved with a large volume of igneous rocks being deposited on the western flank of the basin. Successions deposited during this period feature Seaward-Dipping Reflector Sequences, lava flows and individual volcanoes clearly imaged by the seismic data. This led to significant differences in thermal subsidence patterns between the southern and northern parts of the Houtman Sub-basin (Rollet et al, 2013a, 2013b; Borissova et al, 2017).

Miocene inversion

Evidence of the Miocene convergence between the Australian and Eurasian plates is seen in the offshore northern Perth Basin as late-stage fault reactivation and inversion, in association with minor folding of Cretaceous and younger strata. This reactivation and inversion is primarily evident along some of the major basin-bounding fault systems of the offshore northern Perth Basin (Gorter et al, 2004; Borissova et al, 2017).

Regional hydrocarbon potential

Regional hydrocarbon systems

Several hydrocarbon families are recognised from the geochemistry of oils, gases and oil shows from wells across the offshore Perth Basin: Permian, Triassic, mixed Permian-Triassic, Jurassic and Upper Jurassic/Lower Cretaceous (Figure 8). The majority of oils and condensates from the northern onshore and offshore Perth Basin are sourced from the sapropelic interval of the Hovea Member of the Lopingian-Lower Triassic Triassic Kockatea Shale. However, there are locally important hydrocarbons accumulations that are sourced solely or partially from older and younger successions.

Source rocks

The source rock potential of key critical units is described in the following sections. The potential for oil and gas generation of offshore source rocks has been described previously by Jones et al (2011b) and Rollet et al (2013a, 2013b).

Cisuralian Irwin River Coal Measures

The Permian Irwin River Coal Measures intersected in wells on the Beagle Ridge and in the central Abrolhos Sub-basin show good organic richness with Total Organic Carbon (TOC) >2% for the majority of samples (up to 16.9% in Cliff Head 4) and is predominantly gas-prone with Hydrogen Index (HI) below 200 mgHC/gTOC. A few carbonaceous sediments from the Beagle Ridge and inner Abrolhos Sub-basin (Dunsborough 1 and Frankland 1) show TOC >4% and HI in the range 150–250 mgHC/gTOC, with the potential to generate gas and some liquids.

Cisuralian to Guadalupian Carynginia Formation

More than 80% of the Permian Carynginia Formation samples show good organic richness with TOC greater than 1%. However, consistently low HI values (<100 mgHC/gTOC) indicate only limited present-day source potential for gas.

Lopingian-Lower Triassic Kockatea Shale

The Lopingian-Lower Triassic Kockatea Shale, long recognised as the primary source of oil onshore (Summons et al, 1995; Thomas and Barber, 2004), is intersected in all offshore wells of the Abrolhos Sub-basin. In the Houtman Sub-basin where it has not been intersected, revised seismic interpretation suggests that it is likely to be present at depth. The Hovea Member, which forms the base of the Kockatea Shale, and straddles the Permian–Triassic boundary, is identified in all wells (17) where the Kockatea Shale is encountered, with the exception of South Turtle Dove 1B and Wittecarra 1 (Jorgensen et al, 2011). Onshore, the Hovea Member was shown to begin with an Upper Permian portion characterised by mostly inert material (the inertinitic interval) followed by oil-prone sediments of excellent source quality (the sapropelic interval) and terminated by a thin limestone unit (Thomas and Barber, 2004). Offshore, the inertinitic and sapropelic intervals are also recognised and sediments from the sapropelic interval are found to be organically rich, with 2% mean TOC, and of predominantly oil-prone character (HI >300 mgHC/gTOC). The best Hovea Member source rocks for oil are found on the Beagle Ridge and in its vicinity. The source potential gradually declines further north and outboard with only fair potential for mixed oil and gas observed in wells of the northern Wittecarra Terrace and very limited potential for gas in outboard wells Batavia 1 and Geelvink 1A.

In contrast to the Hovea Member, the overlying Lower Triassic Kockatea Shale is organically lean with TOC mainly less than 1% and consequently has only fair potential for both oil and gas generation.

Lower to Middle Triassic Woodada Formation

The Lower to Middle Triassic Woodada Formation shows good source richness in some sediments of Batavia 1 and Wittecarra 1 (TOC >1%) with the potential for both oil and gas generation (HI 131‑ 446 mgHC/gTOC).

Jurassic units (Eneabba, Cattamarra, Cadda and Yarragadee)

Jurassic units (Eneabba, Cattamarra, Cadda and Yarragadee) have variable source quality. The Eneabba Formation has organic-rich sediments throughout the section in Gun Island 1 in the Houtman Sub-basin, with the potential to generate both oil and gas. Its source potential in the Abrolhos Sub-basin is, however, limited. Most shaly intervals have TOC < 1%, except for a few from Batavia 1 and Geelvink 1A that show gas generating potential.

The Cattamarra Coal Measures and Cadda Formation exhibit good organic richness with a TOC average of 3.2% and 3.3%, respectively, with the potential for generating mostly gas (HI <200) and only minor liquids.

The Yarragadee Formation is characterised by highly carbonaceous sediments, with an average TOC of 11%, and the potential for both oil and gas generation. Carbonaceous shales and coals (TOC from 5% up to 64%) are highly oil-prone.

Hydrocarbon families

Permian sourced hydrocarbons

A number of gas/condensate discoveries in the Perth Basin have been charged by Permian coal measures. The Whicher Range 1 (Figure 8) condensate (and by inference, the associated gas) from the Bunbury Trough in the southern Perth Basin is believed to have been sourced locally from coal measures in the Permian Sue Group (Summons et al, 1995). It has an isotopic signature enriched in ¹³C (d¹³C ~ -25‰) and a dominant land-plant biomarker assemblage (e.g. high pristane/phytane and C₂₉/C₂₇ sterane ratios) that are distinctive for Permian-sourced hydrocarbons. Onshore, similar isotopically ¹³C-enriched gas derived from Permian source rocks was recovered from the Irwin River Coal Measures in the Elegans Field (Boreham et al, 2011) and from the Kingia/High Cliff sandstones in the recently discovered Waitsia Field (Tupper et al, 2016) (Figure 8). Other gases believed to have been partially sourced from Permian coal measures include gases from the Dongara field onshore and gases from the offshore wells, Dunsborough 1, Frankland 1 and Perseverance 1 (Jones et al, 2011b; Rollet et al, 2013a). A contribution from the Permian Irwin River Coal Measures is also proposed for the Woodada 3 and Yardarino 1 oils (Jones et al, 2011b) as well as for a fluid inclusion oil in the Carynginia Formation of Leander Reef 1 (Volk et al, 2004).

Triassic sourced hydrocarbons (Hovea Member, Kockatea Shale)

Extensive oil charging from the Hovea Member source in the northern Perth Basin is revealed from the geochemical composition of reservoired and migrated hydrocarbons, as well as from oil fluid inclusions. Oils from the Dongara Terrace and immediate surrounds in the onshore northern Perth Basin share distinct geochemical features with Hovea Member source rock extracts, including an anomalously high abundance of C₃₃ n‑alkylcyclohexane and ¹³C-depleted carbon isotopic values (<-31‰) (Summons et al, 1995). C₃₃ n‑alkylcyclohexane was proposed as a marker for the end-Permian mass extinction event, as its concentration increases sharply at the onset of the extinction interval in several marine P–T sections (Grice et al, 2005). More recent discoveries of Hovea Member–sourced oils have extended beyond the Dongara Terrace to the accumulations at Eremia, Hovea, Jingemia, and offshore at Cliff Head (Thomas and Barber, 2004; Jones et al, 2011b) and Dunsborough (Grosjean et al, 2011). These specific geochemical fingerprints are also observed in oil stains recovered from the Permian Dongara sandstone below the Kockatea seal in Flying Foam 1, Frankland 1, Hadda 1, Livet 1 and Morangie 1 (Grosjean et al, 2011). These oil shows may be partly remnants of palaeo-oil columns which have been recognised in these wells on the basis of fluid inclusion data (Kempton et al, 2011).

The majority of gases from the northern Perth Basin show ¹³C depletion, consistent with sourcing from the marine Lower Triassic Kockatea Shale (Boreham et al, 2011; Grosjean et al, 2016). Carbon isotopic analysis of gases from offshore wells Dunsborough 1, Frankland 1 and Perseverance 1 also show correlation to the Hovea Member source, although with a possible contribution from the Permian Irwin River Coal Measures as mentioned above.

The typing of these oils, oil stains and gases to the Hovea Member demonstrates that the Permo-Triassic petroleum system is not limited to the Beagle Ridge but extends further outboard, over much of the Abrolhos Sub-basin, reaching the northern part of the Wittecarra Terrace (Livet 1 and Morangie 1) and the outer Abrolhos (Batavia 1 and Geelvink 1A). The Hovea Member was shown to be of limited quality and only marginally mature in wells of the northern Wittecarra Terrace, suggesting a non-local charge from a fully mature source kitchen, potentially located to the west in the adjacent Houtman Sub-basin (Rollet et al, 2013b).

Jurassic sourced hydrocarbons

Onshore, liquid hydrocarbons recovered from the Lower to Middle Jurassic Cattamarra Coal Measures in the Coomallo Trough (condensate at Walyering 1 and 2) and Beermullah Trough (oil at Gingin 1 and 2, and Bootine 1) are locally sourced from the Cattamarra Coal Measures, as initially suggested by Thomas (1984) (Figure 8). Geochemical evidence for this correlation is provided by ¹³C-enriched carbon isotopic compositions and the high proportion of retene, a conifer-derived aromatic biomarker generally considered to be diagnostic of Jurassic age (Summons et al, 1995; Gorter et al, 2004). Condensates at Walyering may have been sourced from a more marine-influenced facies lower in the Cattamarra Coal Measures based on their intermediate carbon isotopic composition, lower pristane/phytane ratio and the presence of the diagnostic marine C₃₀ desmethylsterane (Summons et al, 1995; Gorter et al, 2004). A marine interval within the Jurassic Cattamarra Coal Measures is also interpreted to be the source of oil reservoired in the Lower Jurassic Eneabba Formation in Cataby 1 in the Beermullah Trough (Gorter et al, 2004; Figure 8). The recent gas discoveries in Jurassic strata (e.g. Gingin West 1 gas reservoired in the Cattamarra Coal Measures and Warro 3 gas reservoired in the Upper Jurassic Yarragadee Formation) also show ¹³C-enriched carbon isotopic compositions for the C₁−C₅ hydrocarbons and are considered to be locally derived from Jurassic source rocks (Boreham et al, 2011).

The presence of a working Jurassic petroleum system in the offshore northern Perth Basin is supported by fluid inclusion data, which revealed a paleo-oil column in the Cadda Formation in Houtman 1. Detailed analysis of fluid inclusions from this interval suggests that it is most likely sourced from Jurassic strata (Volk et al, 2004).

Oil recovered from Gage Roads 1, in the Vlaming Sub-basin, is waxy, isotopically heavy (d¹³C_oil ~ -24 ‰), and has a relatively high content of conifer-derived aromatic hydrocarbons (Summons et al, 1995). In addition, it is devoid of marine markers (Summons et al, 1995) and has the lightest hydrogen isotopic content (dD) of any Perth Basin oil (Dawson, 2006). These characteristics are consistent with a terrestrial (including lacustrine) source, potentially within either the Upper Jurassic Yarragadee Formation or Lower Cretaceous Parmelia Group (Summons et al, 1995; Volk et al, 2004; Figure 8).

Reservoirs and seals

Potential reservoirs are present throughout the Permian to Lower Cretaceous sequences in the offshore Perth Basin (Figure 3 and Figure 4). Within the Permian succession, reservoirs include the High Cliff Sandstone, the Irwin River Coal Measures, intra-formational sands of the Carynginia Formation, and the Dongara Sandstone. Permian sandstones intersected in the offshore wells return porosities ranging up to 25% (O’Neill et al, 2016). Although the overall prospectivity of these Permian reservoirs is thought to decrease at depths greater than 2.5 km, due to porosity degradation by silica cementation, their viability may be maintained at depth (>3 km), where cementation has been inhibited by grain-coating clay, preserving intergranular porosity (Ferdinando et al, 2007). Onshore, this process has resulted in preservation of excellent primary porosity and permeability at significant depths for the Wagina Sandstone in the Beharra Springs Field (Tupper et al, 1994), and for the Kingia and High Cliff sandstones in the recently discovered Waitsia Field (Tupper et al, 2016). In the offshore northern Perth Basin, potentially good reservoirs with porosities of up to 30%, are present within the Triassic (Woodada Formation, Lesueur Sandstone) and Jurassic to Lower Cretaceous successions (Eneabba Formation, Cattamarra Coal Measures, Yarragadee Formation; Jones et al, 2011b; Rollet et al, 2013a, 2013b). Porosities of the Jurassic sandstones decrease with increasing burial depth to about 10‑15% at 3000‑3500 m (Gorter et al, 2004).

The Kockatea Shale is proven to be an effective regional seal across the northern Perth Basin (Jones et al, 2011b). This sequence is regionally extensive with thicknesses generally sufficient to provide robust vertical and cross-fault seal, unless breached by subsequent reactivation. The Middle Jurassic Cadda Formation may also act as a regional seal, reaching thicknesses of up to 400 m in offshore well intersections (Jorgensen et al, 2011). Intraformational seals are potentially present in the Eneabba Formation, Cattamarra Coal Measures and Yarragadee Formation (Gorter et al, 2004; Jones et al, 2011b; Robertson et al, 2011).

Charge history

Petroleum systems modelling of the onshore northern Perth Basin by Thomas and Barber (2004) suggested that peak hydrocarbon generation for the sapropelic Hovea Member of the Kockatea Shale occurred in the Late Jurassic to Early Cretaceous. Further, the timing of oil generation from this interval is virtually coincident with gas generation from the underlying Irwin River Coal Measures, indicating that in most onshore areas, the oil charge from the Lower Triassic is in direct competition with gas being generated from the Permian.

Petroleum systems modelling of the Houtman Sub-basin by Gorter et al (2004) indicates that the basal Kockatea Shale could have expelled oil in the Early to Middle Jurassic. More recent petroleum systems analyses have shown that the timing and level of maturity of the Hovea Member varies significantly across the offshore northern Perth Basin (Pfahl, 2011; Rollet et al, 2013b; Hall et al, 2017). In most of the Houtman Sub-basin the Hovea Member is modelled to have entered the main oil window in the Triassic, with a subsequent further rapid increase in maturity during the Jurassic and Early Cretaceous (Pfahl, 2011; Hall et al, 2017). In contrast, throughout much of the Abrolhos Sub-basin, including the Wittecarra Terrace and along the inboard margin of the northern Houtman Sub-basin, the Hovea Member is modelled to have entered the oil window during the final Late Jurassic–Early Cretaceous phase of rifting (Pfahl, 2011; Hall et al, 2017). The exception to this is in the outer western Abrolhos Sub-basin, where the Hovea Member is modelled to have reached the late oil/wet gas window (Pfahl, 2011).

In the central and southern Houtman Sub-basin modelled oil and gas expulsion from the Cattamarra Coal Measures occurred in the latest Jurassic to earliest Cretaceous (135–150 Ma), immediately preceding uplift and erosion. In the northern Houtman Sub-basin, Lower Jurassic source rocks (Eneabba, lower Cattamarra) reached the oil window in the outer part of the basin, with peak hydrocarbon generation occurring in the Early Cretaceous (Hall et al, 2017). However, Middle Jurassic and younger source rocks (upper Cattamarra, Cadda and Yarragadee formations) remain predominantly immature in the northern Houtman sub-basin, although increased heat-flow associated with Early Cretaceous volcanism (Borissova et al, 2017) may have resulted in localised hydrocarbon generation.

Although peak generation for all source intervals generally occurred pre-breakup, some additional generation and expulsion from multiple source rocks is modelled to have occurred after Valanginian breakup resulting from the deposition of 1-1.5km of Upper Cretaceous-Cenozoic overburden (Gorter et al, 2004; Hall et al, 2017).

Exploration history

Petroleum exploration in the Perth Basin commenced in the late 1940s with an onshore field survey and evaluation of water well drilling commissioned by Ampol and Richfield Oil companies and gravity surveys by the Bureau of Mineral Resources (BMR) (Mory and Iasky, 1996). Oil shows recorded in BMR wells drilled on the Beagle Ridge in 1959-60 led West Australian Petroleum Pty Ltd (WAPET) to drill the first wildcat hole, Eneabba 1, in 1961. The first discovery in the Perth Basin was the Yardarino gas field in 1964, with mixed Permian and Triassic sourced hydrocarbons reservoired in the Wagina Formation (Crostella, 1995). The Dongara oil and gas field, with oil and gas sourced from the Lower Triassic Kockatea Shale and additional gas sourced from lower Permian shales and coals, was discovered in 1966 and commenced production in 1971 (Crostella, 1995). The discovery of gas and oil in Gingin 1 and 2, drilled between 1964 and 1966, although uneconomic, was significant in that it proved the effectiveness of sources from the Jurassic succession (Crostella, 1995).

Hydrocarbon discoveries in the onshore Perth Basin have continued to the present. The most recent example is the Waitsia Field, discovered in 2014 in the onshore northern Perth Basin, which is believed to be the largest onshore conventional gas discovery in Australia in the last 30 years (Tupper et al, 2016). The Waitsia Field commenced production in 2016 and is estimated to have a total 2P reserve of 327 Bcf (9.25 Bcm) in interpreted good quality reservoirs, with a further 2C contingent resource associated with tight reservoirs of 157 Bcf (3.74 Bcm; Tupper et al, 2016). Other recent discoveries include: Gingin West 1 gas (7.5 MMcf/d, 0.212 Mm³/d) and condensate (375 bbl/d, 59.6 kL/d) discovery in 2010 (Empire Oil and Gas NL, 2011); Red Gully 1 gas (11.7 MMcf/d, 0.33 Mm³/d) and condensate (837 bbl/d, 133 kL/d) discovery (Department of Mines and Petroleum, 2011); and Warro 3 and 4, which reported excellent gas shows within the deep Yarragadee Formation (Department of Mines and Petroleum, 2012a, 2012b). Evandra 1 and 2 encountered strong oil shows, over 20 m and 14 m respectively, in the Wagina Sandstone, but were deemed uneconomic (Department of Mines and Petroleum, 2012c). Senecio 2 achieved a flow rate of 1.35 MMscf/d (0.038 Mm³/d) from the Dongara and Wagina sandstones in the onshore northern Perth Basin (AWE, 2012).

Offshore exploration commenced in 1965, with the acquisition of the Abrolhos and Perth Marine Seismic Surveys in the northern Abrolhos and southern Vlaming sub-basins, respectively. Seismic data have been acquired consistently since that time, giving a coverage that is a mixture of older regional reconnaissance grids and more detailed surveys and scientific studies (Soames et al, 2014). Seismic coverage is most concentrated in the Abrolhos Sub-basin, the Wittecarra Terrace and the central Vlaming Sub-basin. The southern Houtman and southern Vlaming sub-basins are also covered by relatively tight (<1-5 km line spacing) industry grids. Four 3D seismic surveys were acquired in 2003 and 2004, and one later in 2008, all in the northern Perth Basin. A series of regional 2D lines were acquired over the frontier northern Houtman and Zeewyck sub-basins, as part of Geoscience Australia seismic survey 310 in 2008-09 (Foster et al, 2009). In 2008-10, Geoscience Australia commissioned the reprocessing of approximately 11 665 line km (180 lines) of seismic data from 17 surveys with vintages spanning 1976-2003. Recent 2D seismic acquisition has been undertaken in the basin by Spectrum Geo Pty Ltd and Geoscience Australia, with the acquisition of 8292 line km (Rocket 2D) and 3455 line km (GA349) respectively (Borissova et al, 2015, 2016; Owen, 2016).

The first well drilled in the offshore Perth Basin was Gun Island 1, a stratigraphic well drilled by BP Petroleum Development Australia Pty Ltd in 1968. This was followed by the first hydrocarbon discovery in the offshore Perth Basin at Gage Roads 1 by WAPET in 1969. A total of 98.5 barrels (15.66 m³) of oil were recovered from the Stragglers Member of the Carnac Formation at Gage Roads, which remains the only accumulation found in the Vlaming Sub-basin. A series of wells were then drilled in a campaign by WAPET from 1971-1978, including Roe 1, Warnbro 1, Charlotte 1, Sugarloaf 1, Bouvard 1 and Challenger 1 in the Vlaming Sub-basin, South Turtle Dove 1B on the Turtle Dove Ridge, and Geelvink 1A in the Abrolhos Sub-basin.

A general lack of success in the WAPET drilling campaign subsequent to the Gage Roads discovery led to a period of decreased exploration activity in the basin, with only 10 wells drilled in the 1980s and 1990s, by companies such as Esso Exploration and Production Australia Ltd and Ampol Exploration Limited. In the offshore northern Perth Basin there was an 11 year hiatus between the drilling of Wittecarra 1 in 1985 and Livet 1 in 1996.

Between 2001 and 2008, ROC Oil (WA) Pty Ltd drilled 11 new field wildcat wells in the northern Perth Basin. The first of these exploration wells discovered the Cliff Head oil field. The Cliff Head discovery, and a series of onshore oil discoveries in the early 2000s (Hovea, Jingemia, Eremia), changed the perception of the northern Perth Basin from being marginally prospective for gas to one that is highly prospective for gas and oil (Buswell et al, 2004).

Cliff Head 1 intersected a 4.8 m waxy (31.6°API) oil column in the Irwin River Coal Measures immediately beneath the Kockatea Shale regional seal (Jones and Hall, 2002). Seismic interpretation of the Cliff Head oil field has shown it to be structurally complex with reverse, strike-slip and listric normal faults mapped at both field and reservoir scale within the Permo-Triassic succession (Hodge, 2005). Five extension/appraisal wells were drilled between 2002 and 2005 to delineate the extent of the Cliff Head field, and seven development/water injection wells were drilled between 2005 and 2006. Production from the Cliff Head field commenced in 2006 with a total of 14.5 MMbbl (2.3 Gl) of oil produced to 2015 (Elixir Petroleum Ltd, 2015) and 0.48 MMbbl (76 Ml) of oil in the 12 months to June, 2016 (AWE, 2016). Production License WA-31-L over the Cliff Head oil field is currently held by Triangle Energy (Global) Ltd. and ROC Oil (WA) Pty Ltd. The Cliff Head field is estimated to have 3.8 MMboe of proven and probable reserves remaining (Energy Quest, 2017).

Three other discoveries were made in 2007 as part of ROC Oil’s drilling campaign, with Frankland 1, Dunsborough 1 and Perseverance 1 all intersecting gas columns, and Dunsborough 1 including a light oil leg. However, the Frankland 2 and Dunsborough 2 appraisal wells returned disappointing results, and the Frankland and Dunsborough discoveries were deemed non-commercial. GOI™ (Grains containing Oil Inclusions) analyses indicate paleo-oil columns in both Frankland 2 and Dunsborough 2, implying loss of oil through breach of structure or redistribution (e.g. tilting or displacement by gas; Kempton et al, 2011), or both.

The most recent drilling in the offshore northern Perth Basin was a three-well program (Koel 1, Cisticola 1, Munia 1) in exploration permit WA-481-P, one of only two active offshore exploration permits in the offshore Perth Basin (Figure 9). Undertaken by Murphy Oil, with joint venture partners KUFPEC Australia Pty Ltd and Samsung Oil and Gas Australia Pty Ltd, all three wells were dry holes (Kelemen, 2016). WA-481-P has since been acquired by Pilot Energy Ltd with joint venture partner Key Petroleum Ltd (Pilot Energy, 2016; Key Petroleum, 2016) and Pilot Energy have subsequently issued a prospectivity update and undertaken a prospective resource assessment (Pilot Energy, 2017). The only other active offshore exploration permit in the offshore Perth Basin is WA-512-P. Since being awarded to AWE in 2015, AWE has reprocessed 535 km²of 3D and 4048 km² of 2D seismic data and identified a number of Permo-Triassic leads (Energy News Bulletin, 2016).

Between 2009 and 2013, Geoscience Australia undertook work in the northern Perth Basin as part of the Australian Government’s Offshore Energy Security Program. As a part of this program, numerous studies were conducted in an attempt to reduce exploration risk to industry by providing pre-competitive data for the northern Perth Basin. These studies include an offshore northern Perth Basin well folio of 23 composite well logs incorporating new biostratigraphic data and a sequence stratigraphic interpretation (Jones et al, 2011a; Jorgensen et al, 2011; Robertson et al, 2011); and organic geochemical analyses (Grosjean et al, 2012, 2013). Additional collaborative studies on the detection of paleo-oil columns in wells in the offshore northern Perth Basin (Kempton et al, 2011) and trap integrity (Langhi et al, 2012) were also undertaken. Further to this, a marine survey was conducted in 2011 aboard the RV Southern Surveyor and included the use of a remotely operated vehicle (ROV). Underwater video footage taken by the ROV identified a dark coloured fluid proximal to sidescan flares (Jones et al, 2012b). A range of geophysical studies has provided additional insights into the geology of the northern Perth Basin (Hackney, 2012; Johnston and Goncharov, 2012; Johnston and Petkovic, 2012; Petkovic, 2012; Hackney et al, 2014). Basin geology and prospectivity compilations have been produced by Jones et al (2011b; 2012a, 2012b), Rollet et al (2013 a, 2013b) and Totterdell et al (2014).

Recent work by Geoscience Australia in the northern Houtman Sub-basin includes a prospectivity study following acquisition of a 3455 line km 2D seismic survey, GA349, in 2015-16. The GA349 data has allowed imaging of the full sedimentary sequence as well as the Moho and deep basement structures, providing insight into the architecture and evolution of this sub-basin and the wider Western Australian margin (Sanchez et al, 2016; Borissova et al, 2017). Interpretation of the new data has been combined with regional mapping of the northern Perth Basin, geophysical modelling and petroleum systems analysis to assess prospectivity. To date, results of this ongoing work indicate substantial prospectivity in this region. Also undertaken in 2015 was the Rocket 2D multi-client seismic survey by Spectrum Geo with acquisition of 8292 line km covering much of the Houtman sub-basin and Southern Carnarvon Basin (O’Neill et al, 2016).

Exploration for conventional oil and gas plays continues in the onshore parts of the Perth Basin, and there has been a recent upsurge in unconventional gas exploration (tight gas, shale gas and coal seam gas). The primary targets for unconventional gas exploration are the Kockatea Shale, Carynginia Formation and Irwin River Coal Measures. Recent onshore drilling highlights include discoveries at Senecio 3, Irwin 1 and Red Gully North 1 and positive results from appraisal wells at Waitsia (Waitsia 1 and 2) and Warro (Warro 5, 5 ST1 and 6) (Department of Mines and Petroleum, 2016a, 2016b; Kelemen, 2016)

Production status

Producing fields within the onshore and offshore Perth Basin in 2014 were Beharra Springs, Beharra Springs North, Cliff Head, Dongara, Gingin West, Hovea, Redback, Red Gully and Tarantula (Department of Mines and Petroleum, 2016b). Most recently, the Waitsia Field commenced production in 2016 within the onshore Perth Basin (Evans et al, 2016).

Marine and environmental information

This section provides an overview of the environmental characteristics of the Acreage Release areas and the general setting of the region overlying the northern Perth Basin (Figure 10), together with information focused on the environmental geoscience characteristics of individual acreage release blocks.

The northern Perth Basin is located in the south west margin of Australia and the offshore part primarily includes continental slope, terrace and isolated canyon geomorphic features.

Geraldton and Fremantle are the closest industrial ports and towns.

Climate of the region

Climate statistics for Geraldton indicate a mean annual rainfall of 440.9 mm and mean annual maximum and minimum temperatures of 25.9°C and 13.6°C, respectively (average of 72 years, www.bom.gov.au). Mean maximum and minimum temperatures and precipitation for Carnarvon Airport are; 27.3°C and 17.2°C, and 224.6 mm of rainfall per annum (average over 71 years, www.bom.gov.au). Most precipitation occurs between May and August (www.bom.gov.au). Sea surface temperatures in this region have an annual average of approximately 24°C and are predicted to increase 0.25-0.5°C per decade in the immediate future (Foster et al 2014).

Oceanic regime

The oceanic regime of the Perth Basin is characterised by general poleward flow of surface currents in the offshore. The seasonal poleward Leeuwin current is strongest in autumn, and diminishes in strength during the wet monsoon season. On the shelf, currents are more spatially variable, although these are also aligned generally alongshore, and are directed towards both the north and south (James et al, 1999). In contrast to the monsoonal climate of the north-west, the wettest months for most of the Perth Basin are June and July. Surface waters, generally sourced from the north, are warm and in summer grade from 25°C offshore from Carnarvon to 22°C offshore from Perth, and winter surface waters are generally 4°C cooler (Pearce, 1991). These waters are nutrient poor and of slightly lower salinity compared to normal marine values (Waite et al, 2007). The Leeuwin Current strongly influences the types and ranges of biota in this region (Richardson et al, 2005). Tidal ranges are microtidal. Oceanic swells are from the south-west, and wave heights are significant, at approximately 2.5 m in the offshore northern Perth Basin, although maximum wave heights of 8‑14 m have been recorded (Richardson et al, 2005).

Seabed environments: regional overview

The area of interest (abutting release areas W17-9 and W17-10) is situated in 150 m to over 3000 m water depth, within the northern Perth and Southern Carnarvon Basins, where the Northern Australian tropical, and the southern Australian warm temperate provinces overlap (Collins, 2010). The continental shelf, which lies landward of the Release Areas, is relatively narrow at only 4 km, and widens rapidly to 100 km further south. The sea floor morphology is primarily continental slope (slope = 0.02), dissected by a series of canyons, and the extensive Carnarvon Terrace (Heap and Harris, 2008). The Carnarvon Terrace is a geographic term for an arcuate shallow portion of the Carnarvon Basin on the upper continental slope (Symonds and Cameron, 1977; Harris et al, 2003). Carbonate fringing and isolated reef systems have developed discontinuously along the length of the Western Australian margin extending south into the predominantly warm temperate area of interest, enabled by the warm, southerly directed Leeuwin Current (Collins, 2010).

Potter et al (2008) reported on 139 grainsize and/or carbonate data points within the northern Perth Basin and Southern Carnarvon region, with better sample density on the shelf adjacent to the area of interest. Sand and mud distribution are essentially controlled by water depth and proximity to the shoreline and thus display a shelf parallel distribution: gravelly sand dominates the adjacent continental shelf, while slightly gravelly sandy mud and sandy mud dominate the continental slope and terrace (Richardson et al, 2005; Potter et al, 2008; MARS: http://www.ga.gov.au/oracle/mars). Tidal sand waves/sandbanks are present in nearby Shark Bay (Baker et al, 2008) which is updrift of the area of interest (James et al, 1999).

On the adjacent shelf, sediment comprises predominantly carbonates (80-100%), which slightly reduce in percentage in an offshore direction (continental slope and terrace carbonates: 60–100%) (Potter et al, 2008). While James et al (1999) primarily described sediment facies for the continental shelf, their descriptions do extend slightly further offshore to the eastern edge of the northern Perth Basin, and Southern Carnarvon Basin. Here they described pelagic sand and silt in ~150 m water depth, well washed pelagic sand at slightly greater depths (150–200 m), and carbonate silt on the continental slope at depths up to ~600 m. Recent skeletal shelf sediment components have attributes of both warm and cool water carbonates. Hypersaline outflow from Shark Bay moves westward and southward onto the shelf resulting in significantly reduced diversity in the carbonate biota (James et al, 1999). However, rates of mass accumulation of biogenic materials in the sediments were higher during the last glacial maximum (McCorkle et al, 1994). This indicates that some weak upwelling was occurring on the western margin of Australia at that time, thereby implying a weakening of the southward flowing Leeuwin Current, which allowed some upwelling of nutrient rich waters and increased oceanic productivity compared to the present day (Heggie et al, 1993; McCorkle et al, 1994). Holocene sedimentation rates on the Carnarvon Terrace, at water depths of approximately 2700 m, have varied between 0.7 cm/kyr and 1.5 cm/kyr (Heggie et al, 1993).

Ecology

The northern Perth Basin is predominately in the North-West Marine Bioregion, with the southernmost part of Release Area W17-10 in the South-West Marine Bioregion. The North-West Bioregion has comparatively low rates of endemism but high biodiversity and is home to globally significant populations of internationally threatened species (Commonwealth of Australia, 2008). At the smaller scale of provincial bioregions (IMCRA, 2006), the northern Perth Basin is included in the Central Western Transition, which is an area of overlap - meaning it does not have distinct fauna, flora and ocean conditions compared to adjacent areas (Commonwealth of Australia, 2005).

The Wallaby Saddle in Release Area W17-10 is an area in which Sperm Whales aggregate, possibly in response to baitfish attracted to productive waters (Commonwealth of Australia 2008). Limited underwater imagery showed no suspension feeders at two locations along the Wallaby Saddle, although these organisms were observed further west on the edge of the Cuvier (Wallaby) Plateau (Daniell et al, 2009).

The northern Perth Basin includes part of the northwest slope over which several studies have investigated biodiversity patterns (Poore et al, 2015). The slope has been shown to support the second richest demersal fish assemblage in Australia (Last et al, 2005). On a cross-shelf gradient, assemblages of benthic invertebrate megafauna from the upper and mid-slope differ. Assemblages of benthic invertebrate megafauna differ among upper and mid-slope (Williams et al, 2010).

There has been no targeted survey of the vast abyssal plains of this region. Hence, our understanding of this environment is limited to that from other regions, notably the assumption that the abyssal plain supports cosmopolitan deep-water communities with patches of highly diverse and specialised communities supported by rocky outcrops, hydrothermal vents and whale falls (Levin et al, 2001). The deep ocean seafloor of the northern Perth Basin likely supports meiofauna, infauna and patches of epibenthic communities (Brewer et al, 2007), as well as mobile animals such as holothurians, decapods, polychaetes, cephalopods and bentho-pelagic fish (Daniell et al, 2009).

Recent geological history

The area of interest is primarily situated within the northern Perth Basin (Permian to Early Cretaceous), but also intersects the Southern Carnarvon Basin (Ordovician-Permian). The Southern Carnarvon Basin is an epicratonic, faulted and folded basin, elongated north-south, and extends along 700 km of the westernmost Australian coast. The younger Perth Basin is 1300 km in length and forms a complicated north-south trending series of sub-basins, troughs and highs (Stagg et al, 1999). The Perth Basin formed during Permian-Early Cretaceous rifting (Harris, 1994; Bradshaw et al, 2003). Overlying Cenozoic marine carbonates are evidence of subsequent passive margin conditions that continue to the present day (Bradshaw et al, 2003). The low relief of the Western Australian hinterland has resulted in little deposition of terrigenous sediments during most of the Cenozoic (Quilty, 1977).

Richardson et al (2005) summarised previous work on Quaternary shelf palaeoshorelines and reef growth in the region, and is briefly summarised here. Quaternary sea level fluctuations resulted in distinct inner and outer shelves and upper continental slope terraces, caused by subaerial erosion during periods of low sea level (Collins, 1988; James et al, 1999). The strength of the Leeuwin Current varied with changing sea levels; during the last glacial maximum the Leeuwin Current was greatly reduced or absent, which allowed oceanic upwelling to occur along the coast. At this time the Western Australian Current was stronger and increased nutrient levels from upwelling enhanced cool-water carbonate production and microphyte algae growth, creating a similar environment to the one experienced today on the southern shelf (Collins et al, 1991; James et al, 1999). In contrast, a stronger Leeuwin Current during the last interglacial at ~125 ka has been inferred from the species of large tropical foraminifera found in Pleistocene sediments as far east as Spencer and St. Vincent gulfs (South Australia) (Cann and Clarke, 1993; McGowran et al, 1997; James et al, 1999). At that time, seas were ~2°C warmer than today and sea level was approximately 2 m above the present (Kendrick et al, 1991; Murray-Wallace and Belperio, 1991; Belperio, 1995).

Deposits from the nearby Houtman Abrolhos reef platforms show at least three generations of coral reef growth which are linked to the presence of the Leeuwin Current (Collins et al, 1991; Collins et al, 1996). Last interglacial reef growth is thought to have lasted for 10 000-15 000 years, from ~132 ka to ~117 ka, and Holocene reef growth commenced at around 10 ka (Collins et al, 1993). The last interglacial platforms outcrop as central platforms that sit 2-3 m above present sea level, and reflect the higher sea level at that time. As a result, no Holocene reef growth has occurred on these outcropping central platforms and reef growth has been more active on leeward margins.

Commonwealth Marine Reserves

Carnarvon Canyon Commonwealth Marine Reserve

http://www.environment.gov.au/topics/marine/marine-reserves/north-west/carnarvon-canyon

Major Conservation Values:

Contains the whole of the Carnarvon Canyon - a single channel canyon with representations of slope, continental rise and deep holes and valleys
The Carnarvon Canyon ranges in depth from 1500 m to over 5000 m and hence provides a wide range of habitats for benthic and demersal species
Examples of the ecosystems of the Central Western Transition provincial bioregion, the reserve lies in a biogeographic faunal transition between tropical and temperate species.

The reserve is currently zoned Habitat Protection Zone IUCN Category IV. Management plans for the North-west Commonwealth marine reserves network are not yet in place.

Shark Bay Commonwealth Marine Reserve

http://www.environment.gov.au/topics/marine/marine-reserves/north-west/shark-bay

Major Conservation Values:

Foraging areas adjacent to important breeding areas for several species of migratory seabirds
Includes part of the migratory pathway of protected Humpback Whales
Adjacent to the largest nesting area for Loggerhead Turtles; the largest in Australia
Provides protection to shelf and slope habitats as well as a terrace feature
Examples of the shallower ecosystems of the Central Western Shelf Province and Central Western Transition provincial bioregions including the Zuytdorp meso-scale bioregion
The reserve also provides for connectivity between the inshore waters of the Shark Bay World Heritage Area and the adjacent deeper waters.

The reserve is currently zoned as a Multiple Use Zone IUCN Category VI. Management plans for the North-west Commonwealth marine reserves network are not yet in place.

Abrolhos Commonwealth Marine Reserve

http://www.environment.gov.au/topics/marine/marine-reserves/south-west/abrolhos

Major Conservation Values:

Important foraging areas for: the threatened Australian Lesser Noddy, northernmost breeding colony of the threatened Australian Sea Lion, migratory Common Noddy, Wedge-tailed Shearwater, Bridled Tern, Caspian Tern and Roseate Tern
Important migration habitat for the protected Humpback Whale
Second largest canyon on the west coast, the Houtman Canyon
Examples of the northernmost ecosystems of the Central Western Province and South-west Shelf Transition (including the Central West Coast meso-scale bioregion)
Examples of the deeper ecosystems of the Abrolhos Islands meso-scale bioregion
Examples of the shallower, southernmost ecosystems of the Central Western Shelf Province provincial bioregion including the Zuytdorp meso-scale bioregion
Examples of the deeper ecosystems of the Central Western Transition provincial bioregion
Examples of diversity of seafloor features including: southernmost banks and shoals of the Northwest region; deep holes and valleys; slope habitats; terrace and shelf environments

The reserve is currently zoned Marine National Park Zone IUCN Category II, Habitat Protection Zone IUCN VI, Multiple Use Zone IUCN Category VI and Special Purpose Zone IUCN Category VI. Management plans for the South-west Commonwealth marine reserves network are not yet in place.

Western Australian Marine Parks

Shark Bay Marine Park

The marine park protects seagrass meadows, Dugongs, Humpback Whales and the Bottlenose Dolphins of Monkey Mia. Turtles, prawns, scallops, sea snakes, corals, sponges and other invertebrates are found in the park. Within the park there are nine sanctuary zones, three recreation zones, six special purpose zones and a large general use zone.

Hamelin Pool Marine Nature Park

The marine nature park protects the stromatolites in Hamelin Pool, within the Shark Bay World Heritage area.

Fisheries

The following Commonwealth fisheries occur in the Perth Basin area:

Southern and Bluefin Tuna fishery. The fishing season runs for 12 months, starting on the 1 December. (http://www.afma.gov.au/fisheries/southern-bluefin-tuna-fishery/)
Western Tuna and Billfish fishery operates in the Australian Fishing Zone and adjacent high seas. The fishing season runs for 12 months, starting on the 1 February. (http://www.afma.gov.au/fisheries/western-tuna-and-billfish-fishery/)
Western Skipjack Tuna fishery is not active. (http://www.afma.gov.au/fisheries/skipjack-tuna-fishery/)
Western Deepwater Trawl fishery operates in waters deeper than 200 m from Exmouth to Augusta. The fishery boundary has recently changed. (http://www.afma.gov.au/fisheries/western-deepwater-trawl-fishery/ https://www.legislation.gov.au/Details/C2017G00026)

Western Australian Fisheries

The Perth Basin crosses the Western Australian West Coast and Gascoyne Coast bioregions. The boundary between the bioregions runs along latitude 27°S. Three of Western Australia’s most valuable fisheries occur in the Gascoyne Coast bioregion, namely Shark Bay Prawn, Exmouth Gulf Prawn and Shark Bay Scallop fisheries. Other important fisheries include: Gascoyne Demersal Scalefish Fishery, Shark Bay Beach Seine and Mesh Net Fishery and Mackerel Managed Fishery. Invertebrate species targeted include: Prawns, Scallops, Blue Swimmer and Deep Sea Crabs and Pearl Oysters (Fletcher and Santoro, 2015). The main commercial fishery targets rock lobsters but there are commercial fisheries for Scallops, Abalone, Blue Swimmer Crabs and Octopus. Fish species targeted in the area include: shark, Dhufish, Snapper, Baldchin Groper, Emperor, Whitebait, Mullet and Whiting (Fletcher and Santoro, 2015).

Information resources

Bureau of Meteorology http://www.bom.gov.au/

National Conservation Values Atlas http://www.environment.gov.au/webgis-framework/apps/ncva/ncva.jsf

Carnarvon Canyon Commonwealth Marine Reserve http://www.environment.gov.au/topics/marine/marine-reserves/north-west/carnarvon-canyon

Shark Bay Commonwealth Marine http://www.environment.gov.au/topics/marine/marine-reserves/north-west/shark-bay

Abrolhos Commonwealth Marine Reserve http://www.environment.gov.au/topics/marine/marine-reserves/south-west/abrolhos

Western Australian Marine Parks and Reserves. https://www.dpaw.wa.gov.au/management/marine/marine-parks-and-reserves#

Commonwealth Fisheries http://www.afma.gov.au/fisheries/

Heritage https://www.environment.gov.au/topics/heritage

References

ALI, J.R. AND AITCHISON, J.C., 2005—Greater India. Earth-Science Reviews, 72, 169–188.

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