Browse Basin Regional Geology

The Browse Basin is an extensional basin that underlies the North West Shelf region (Bradshaw et al, 1988). 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.

The Browse Basin covers an area of approximately 140 000 km², comprising the Caswell, Barcoo and Seringapatam sub-basins, the Scott Plateau, and the Yampi and Leveque shelves (Figure 1). The main depocentre is the Caswell Sub-basin that contains a Paleozoic, Mesozoic and Cenozoic sedimentary succession in excess of 15 000 m thick. The stratigraphy of the basin is shown in Figure 2.

Over 150 petroleum exploration wells have been drilled in the basin since 1970. Coverage by 2D seismic surveys is extensive. Nearly complete coverage by 3D seismic surveys exists in the Caswell Sub-basin. The entire basin is covered by potential field data, including a high resolution aeromagnetic survey flown in 2013 (Hackney and Costelloe, 2014).

The Browse Basin contains several large gas accumulations which are either in the planning or development phases for LNG and condensate production. Hydrocarbon discoveries and accumulations are hosted within Mesozoic reservoirs.

Petroleum production facilities, petroleum fields, and pipeline infrastructure are shown in Figure 3. Current permits and operators in the Browse Basin are shown in Figure 4. Figure 5 shows the Commonwealth Marine Reserve Zones, and Key Ecological Features. 2017 Acreage Release Areas are shown in Figure 1, Figure 3, and Figure 4.

The Browse Basin is one of a series of extensional basins that together form the Westralian Superbasin (Bradshaw et al, 1988; Willis, 1988; Struckmeyer et al, 1998). Structural elements of the Browse Basin (Figure 1) are based on the terminology used by Willis (1988), Elliot (1990), O’Brien et al (1993), Hocking et al (1994), Symonds et al (1994) and Struckmeyer et al (1998). Two shallow basement elements, the Yampi and Leveque shelves, define the eastern and southeastern boundary of the basin. The central Browse Basin is divided into the Caswell and Barcoo sub-basins. The Scott Plateau and Seringapatam Sub-basin together form the outboard deep-water part of the basin.

Yampi and Leveque shelves

The southeastern margin of the Browse Basin is underlain by eroded shallow basement onlapped by Permian and Mesozoic sediments. This area is termed the Yampi Shelf in the eastern part of the basin, and the Leveque Shelf in the southeast (Figure 1). The Leveque Shelf forms the offshore continuation of the King Leopold Mobile Zone, and the northern margin of the mobile zone marks the boundary between the Yampi and Leveque shelves. The basinward boundary of the Leveque and Yampi shelves is defined by a ‘hinge’ where the dip of the basement changes from relatively flat lying to gently basinward-dipping. Beyond this hinge, the Prudhoe Terrace forms a fault-bounded terrace along the southeastern flank of the Caswell and Barcoo sub-basins.

Caswell and Barcoo sub-basins

The Caswell and Barcoo sub-basins  are the major depocentres of the Browse Basin. In the Caswell Sub-basin, the Paleozoic to Cenozoic succession is over 15 000 m thick, whereas in the Barcoo Sub-basin the sediments do not exceed 12 000 m. The Caswell Sub-basin is also significantly wider (200 km) than the Barcoo Sub-basin (100 km), from which it is separated by a north to north-northeast trending structural zone, the Brecknock-Scott Reef Trend.

Scott Plateau and Seringapatam Sub-basin

The Scott Plateau is a deep water (1500–3000 m) marginal plateau located to the west and northwest of the Barcoo and Caswell sub-basins. In this area up to 3000 m of Mesozoic to Cenozoic sedimentary rocks have accumulated over Paleozoic and older basement (Stagg and Exon, 1981). The Seringapatam Sub-basin is a Callovian to Aptian graben overlain by an Aptian to Holocene prograding and aggrading sedimentary wedge (Hoffman and Hill, 2004).

The Browse Basin stratigraphic scheme presented here is based on that from the AGSO Browse Basin Project Team (1997) and Rollet et al (2016a). Formation boundaries and unconformity-bounded sequences have been calibrated to the timescale of Ogg et al (2016; Figure 2).

The Browse Basin fill has comprises six tectonostratigraphic phases (Struckmeyer et al, 1998):

1. Mississippian to Cisuralian (middle Carboniferous–early Permian) extension (Extension 1)
2. Cisuralian to Late Triassic thermal subsidence (Thermal Subsidence 1)
3. Late Triassic to Early Jurassic inversion (Inversion 1)
4. Early to Middle Jurassic extension (Extension 2)
5. Late Jurassic to Cenozoic thermal subsidence (Thermal Subsidence 2)
6. Middle Miocene to Holocene inversion (Inversion 2)

The basin was initiated as a series of intracratonic half-graben during the Mississippian to Cisuralian (Symonds et al, 1994). Upper crustal faulting resulted in characteristic half-graben geometry with large-scale normal faults compartmentalising the basin into distinct sub-basins. Structures resulting from this late Paleozoic extensional event controlled the location of subsequent reactivation events and the distribution and nature of the sedimentary fill (Struckmeyer et al, 1998; Lawrence et al, 2014).

A few wells located on the Yampi Shelf and eastern Caswell Sub-basin have intersected the Permo-Carboniferous succession. The Carboniferous succession is dominated by fluvio-deltaic sediments, while the Permian sediments (mainly limestones and shales) were deposited in a marine environment. The oldest Triassic rocks intersected in the Browse Basin are marine claystones, siltstones and volcaniclastic sediments that were deposited during a regional Early Triassic marine transgression. The overlying Triassic succession includes fluvial and marginal- to shallow-marine sandstone, limestone and shale.

The Permo-Triassic thermal subsidence phase was terminated by compressional reactivation in the Late Triassic to Early Jurassic, resulting in partial inversion of Paleozoic half-graben and the formation of large scale anticlinal and synclinal features within their 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 Brecknock-Scott Reef Trend developed at this time (Struckmeyer et al, 1998).

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 and along the adjacent outer margin of the Prudhoe Terrace. The Heywood Graben, located along the inboard Caswell Sub-basin, formed during this period (Struckmeyer et al, 1998). The Lower–Middle Jurassic syn-rift sediments (Plover Formation) comprise sandstone, mudstone and coal that accumulated in deltaic and coastal plain settings, and contain both reservoir and source rocks. Widespread erosion and peneplanation in the Callovian coincided with continental breakup and the initiation of sea-floor spreading in the Argo Abyssal Plain.

From the Late Jurassic to the Cenozoic, accommodation was controlled by the interplay of thermal subsidence, minor tectonic reactivation events, and eustasy. Upper Jurassic interbedded sandstones and shales (Vulcan Formation) onlap and drape the pre-Middle Jurassic structures, providing a thin, regional seal, and potential source rocks across the basin (Figure 2, Figure 6, Figure 7 and Figure 8).

The Cretaceous interval comprise a series of six, predominantly marine, supersequences (K10-K60, Figure 2 and Figure 9) that prograded into the basin and onlapped the basin margin (Abbott et al, 2015; Rollet et al, 2015, 2016a, 2016b). The K10 (Figure 9a) comprises the upper Vulcan Formation mudstone and sandstone, and Brewster Member sandstone. The K20, K30 and K40 supersequences prograded successively further into the basin (Figure 9b-d), filling the Caswell and Barcoo depocentres. These sequences are dominated by mudstone (Echuca Shoals Formation and Jamieson Formation), as the rate of relative sealevel rise trapped sandstone facies (e.g. Heywood Formation) along the inboard basin margin. The Asterias Member (Echuca Shoals Formation, K30 supersequence) is attributed to a submarine fan environment of deposition. A major flooding event, associated with the Cretaceous eustatic peak, resulted in a landward shift in deposition. Progradation resumed with deposition of K50 mudstone and marl (Woolaston and Brown Gannet formations, Figure 9e). The basal K60 sequence boundary is incised by several submarine canyon systems (Figure 9f-g). Progradation continued with deposition of the K60 supersequence, which comprises the mudstone-prone Fenelon and Wangarlu formations, and the sandstone-prone Puffin Formation  (Figure 9f-g). Submarine fans are a characteristic feature of the K60 supersequence (Lech et al, 2015). Rollet et al (2016c) used the Cretaceous sequence stratigraphic framework to formulate stratigraphic plays, revise the extent of potential and effective source rocks, and refine the understanding of petroleum systems in the basin.

Basin-wide inversion commenced in the middle to late Miocene as a result of the convergence of the Australia-India and Eurasia plates (Shuster et al, 1998).

Regional petroleum systems

Geochemical analyses of oils, oil stains, fluid inclusion oils, condensates, gases and source rocks from the Browse Basin have been undertaken by AGSO and Geotech (2000), Boreham et al (1997, 2001), Blevin et al (1998a, 1998b), Edwards et al (2000, 2004, 2006, 2014, 2016), Edwards and Zumberge (2005), Grosjean (2015, 2016), Le Poidevin et al (2015) and Volk et al (2005). Stable 13C isotopic data of gases and oils have been used to determine the different sources of hydrocarbons in this basin (Edwards et al, 2014, 2016; Grosjean et al, 2015, 2016). These isotopic datasets, together with molecular analyses, provide evidence that four Mesozoic hydrocarbon families/petroleum systems are present in the Browse Basin (Barrett, 2004; Kennard et al, 2004; Edwards et al, 2014, 2016; Rollet et al, 2016a; Palu et al, 2017):

• A basin wide dry gas-prone system where the gas is mostly reservoired within the Plover Formation and sourced from mixed terrestrial and marine organic matter deposited in a fluvio-deltaic environment. The gas accumulations in Jurassic reservoirs on the Scott Reef Trend (Brecknock and Torosa), and potentially those further along its northerly extent (Crown/Proteus and Poseidon), as far as Argus, belong to this petroleum system and have condensate to gas ratios that range from 39.27 to 196.35 cm³/m³ (Edwards et al, 2016). The Plover-reservoired gases within the central Caswell Sub-basin at the Ichthys and Concerto accumulations are also attributed to this petroleum system (Rollet et al, 2016a). The most likely source for these gases is the Lower–Middle Jurassic Plover Formation (Edwards et al., 2014, 2016; Grosjean et al., 2015), although an alternative, and less preferred scenario, is a Permo-Triassic source which has been modelled by Belopolsky et al. (2006). In addition, Plover-sourced gas is also found to have migrated onto the basin’s easterly margins based on geochemical evidence obtained from the dry gases recovered from shallow Cretaceous reservoirs on the Yampi Shelf (Grosjean et al, 2016). It is likely that gas in Psepotus 1 and gas shows in Leveque 1 also represent Plover-sourced gases but no data are available to confirm this.
• A central sub-basin wet gas-prone system reservoired in the Brewster Member of the upper Vulcan Formation and including the Ichthys and Prelude/Concerto accumulations (condensate to gas ratio = 337 cm³ /m³). The Burnside gas accumulation, southwest of Ichthys, represents an extension of this system (Grosjean et al, 2015), as could the interpreted condensate-rich gas at Echuca Shoals (Rollet et al, 2016a). These accumulations are most likely sourced from within the marine Jurassic Vulcan Formation (Grosjean et al, 2015; Edwards et al, 2016).
• An oil- and gas-prone petroleum system sourced from mixed marine and terrestrial organic matter within the Lower Cretaceous sediments of the Echuca Shoals Formation and referred to as the Westralian (W3) Petroleum System (Blevin et al, 1998b). This petroleum system is believed to have sourced the oil within the Caswell structure in the Caswell Sub-basin and the greater Cornea and Gwydion structures on the Yampi Shelf. It also sourced the gas within the Kalyptea, Rondo and Adele structures in the Caswell Sub-basin (Edwards et al, 2016). A thermogenic hydrocarbon signal found in a shallow core sediment along the southern flank of the Ashmore Platform was found to be geochemically similar to Echuca Shoals sourced oils and may represent an extension of this petroleum system to the northwest (Stalvies et al, 2017).
• The Crux gas condensate discovery in the far northeastern portion of the basin (Heywood Graben) is interpreted to be sourced from predominantly terrestrial organic matter within Jurassic source rocks (Edwards et al, 2004, 2014, 2016; Grosjean et al, 2015) and represents a fourth hydrocarbon family in the basin. The satellite Libra and Octans gas accumulations, and the nearby Hippolyte gas accumulation, may also belong to this petroleum system.

Source rocks

A comprehensive assessment of the source rock potential of the Browse Basin was undertaken by Boreham et al (1997), and the results summarised by Blevin et al (1998a, 1998b). These studies recognised organic-rich rocks with fair to moderate oil potential at numerous stratigraphic levels within the Permian to Lower Cretaceous succession. Although many possible source units within this succession have liquid potential with Hydrogen Index (HI) values of >200 mg hydrocarbons/gTOC, they contain less than 2% TOC. At these low-to-moderate TOC levels, any generated oil may not be expelled and could be subsequently cracked to gas at higher maturities.

Local, thin coals and pro-delta shales with high source potential occur within the thick succession of Lower–Middle Jurassic Plover Formation sediments that extend throughout the Caswell Sub-basin and reach a maximum penetrated thickness within the Barcoo Sub-basin (920 m in Barcoo 1). This section is dominated by fluvio-deltaic sediments, including pro-delta shales and coastal plain shaly coals that have significant source potential (Blevin et al, 1998b). Hydrocarbons generated from this succession are likely to be dominated by gas rather than oil.

The Upper Jurassic Vulcan Formation is generally thin throughout the Browse Basin, with major sediment thickening occurring towards the Heywood Graben in the northeast, where restricted marine source facies are likely to be best developed. Localised thickening of Upper Jurassic sediments also occurs on the Leveque Shelf and Prudhoe Terrace, but here the section is dominated by deltaic facies with poorer quality terrigenous organic matter.

Thick marine claystones within the Lower Cretaceous Echuca Shoals and Jamieson formations occur within both the Caswell and Barcoo sub-basins and contain mixed marine and terrestrial organic matter with moderate source potential. Rollet et al (2016c) have used a sequence stratigraphic framework to refine the mapped distribution of potential and effective Cretaceous source rock pods.

Reservoirs and seals

Caswell Sub-basin

Exploration activity has focused on the Caswell Sub-basin, where the Upper Jurassic–Lower Cretaceous upper Vulcan and Lower Cretaceous Echuca Shoals and Jamieson formations form the regional seal. The thick (500–600 m) Callovian–Turonian claystone seal exceeds the throw of the faults within the underlying reservoirs, ensuring an adequate lateral seal across much of the basin. Sections within the lower Vulcan Formation also form adequate seals for Plover Formation reservoirs. Potential intraformational seals occur within the Plover Formation (Blevin et al, 1998b), while marls and mudstones provide potential seals for Campanian–Maastrichtian turbidites and submarine fan sandstones in the Puffin Formation (Benson et al, 2004). The influence of basement controlled drainage patterns on the Kimberley Block hinterland has had a profound effect on the distribution of reservoirs and seals (Tucker, 2009).

The Lower–Middle Jurassic Plover Formation and the Berriasian Brewster Member of the Upper Vulcan Formation are the most laterally extensive reservoirs across the Caswell Sub-basin. Oil and gas shows also occur in locally developed sandstones of the Middle–Upper Jurassic Montara Formation, and in submarine fans and turbidites of Barremian, Campanian and Maastrichtian age (Le Poidevin et al, 2015).

The Ichthys, Prelude, Concerto and Mimia gas accumulations are collectively reservoired within the Plover, Montara and upper Vulcan (Brewster Member) formations. The Plover Formation at these locations comprises a fluvial-deltaic sandstone and mudstone succession with marine affinities towards the top. The main reservoir is characterised by cross-bedded channel sandstone. The top of the Plover Formation is associated with the regional Callovian unconformity; however, at Ichthys, the unconformity is poorly expressed with the estuarine to paralic sandstones of the Plover Formation passing into lagoonal and barrier island clastics of the ‘Ichthys Formation’, and then into the shoreface and shelfal sandstones of the basal Montara Formation and lower Vulcan Formation (Ban and Pitt, 2006; Le Poidevin et al, 2015). The Montara Formation is a thinner, secondary reservoir of localised extent, consisting of prograding fan-delta systems. The Brewster Member is a thick sequence of clean, relatively high net/gross sands that often exhibits poor to moderate reservoir properties.

The Crux structure within the Heywood Graben in the northeastern Caswell Sub-basin hosts gas within the Upper Triassic Nome Formation, Lower–Middle Jurassic Plover Formation and Middle–Upper Jurassic Montara Formation (Le Poidevin et al, 2015). The Nome Formation was deposited in a high-energy fluvial environment and comprises quartz-rich, medium to very coarse grained sandstone with interbedded siltstone, mudstone, and minor coal. Unlike those of the Nome Formation, the Plover Formation sandstones are fine to very fine grained and are interbedded with occasional medium to coarse grained sandstones, siltstones, claystones and minor coals.

Brecknock-Scott Reef Trend

The upper Plover Formation is the main reservoir at the Brecknock and Calliance accumulations while the lower Plover Formation acts as the main reservoir at Torosa. The older sandstones were deposited in a fluvial-dominated upper delta plain with the younger sandstones reflecting deposition in a more tidally influenced lower delta plain environment. Tuffaceous volcanics are also present at some locations within the lower Plover Formation (Tovaglieri et al, 2013).

Yampi Shelf

The Cornea oil and gas accumulation on the Yampi Shelf is reservoired within the Cretaceous Heywood Formation, in which fine to very fine grained sandstones of Albian age were deposited in a shoreface environment. The primary seal is a marine Albian claystone; however, it shows elevated gas readings at some locations and only provides a partial seal. Gwydion 1 discovered three gas-bearing zones and one oil- and gas-bearing zone in shallow marine sandstones of the Echuca Shoals Formation to upper Heywood Formation in a small closure draped over a prominent basement high. A follow-up well, Caspar 1A, discovered gas and residual oil in a small structure on the migration path to the Gwydion accumulation (Le Poidevin et al, 2015).

Timing of generation

Hydrocarbon expulsion modelling (Kennard et al, 2004) indicates multiple effective source units for gas expulsion in the basin, whereas effective oil charge is largely restricted to the Heywood Graben in the northeast, the central and southern Caswell Sub-basin, and possibly the rift section beneath the deep-water Scott Plateau and Seringapatam Sub-basin.

Modelling suggests significant quantities of gas were expelled from the Plover Formation throughout the Browse Basin, including the southern, outer and northeastern Caswell Sub-basin, Seringapatam Sub-basin and parts of the Barcoo Sub-basin. This gas expulsion occurred during the late Early–Late Cretaceous and Neogene. Further to the northeast, oil was expelled from Jurassic sediments in the Plover Formation in the Heywood Graben during the Paleogene and Neogene and the lower Vulcan Formation during the Neogene (Kennard et al, 2004). Fluid inclusion analysis indicates that these Jurassic sediments are the likely source of the thick palaeo-oil columns interpreted in Heywood 1 and Crux 1 (Eadington and Middleton, 2000; Brincat et al, 2004). Lesser quantities of oil are modelled to have been expelled from the Vulcan Formation in the central and southern Caswell Sub-basin. An investigation of the fluid inclusions in the gas reservoirs of the Browse Basin has shown that the hydrocarbon charge consisted of an early oil charge, filling only the crests of the structures before being either displaced or absorbed by gas (CSIRO Petroleum, 2002). Only relatively minor gas expulsion, but no oil, is predicted to have occurred in the Barcoo Sub-basin where source rocks are generally leaner (Kennard et al, 2004). If the Jurassic units of the Seringapatam Sub-basin contain good quality source rocks, then significant quantities of oil and gas could have been expelled during the Paleogene (Kennard et al, 2004).

Hydrocarbon generation and expulsion studies of Lower Cretaceous (Echuca Shoals and Jamieson formations) source rocks using Small Angle Neutron Scattering (SANS), confirm the existence of potential source rocks that are sufficiently thermally mature to generate both oil and gas, but which show either little or no evidence of expulsion or effective regional charge (Radlinski et al, 2004). Similarly, fluid inclusion analysis provides no evidence of an effective regional oil charge of Cretaceous reservoirs in the Caswell Sub-basin (CSIRO Petroleum, 2002; Brincat et al, 2004). In a recent petroleum systems study, Rollet et al (2016c) applied a seismic sequence stratigraphic approach to map potential and effective source rocks within the Lower Cretaceous succession (K10- K40 supersequences, Figure 2). Source rock intervals are condensed section deposits associated with seismic clinoform bottomsets. The number and distribution of source intervals may be greater than recognised in previous studies. Burial history and hydrocarbon expulsion models indicate that the K10-K30 source intervals contribute to several effective petroleum systems within the basin.

Effective oil charge from parts of the Echuca Shoals Formation is confirmed by geochemical analysis of the Cornea, Gwydion and Caswell accumulations (Boreham et al, 1997; Blevin et al, 1998a; Edwards and Zumberge, 2005). The Echuca Shoals Formation is also the source of the gas in Adele 1 (Grosjean et al, 2015; 2016). More recently, an updated compilation of source rock geochemical data was integrated into a pseudo-3D basin model to provide new insights into source rock maturity, generation and expelled fluid composition (Palu et al, 2017).

The Browse Basin is one of the richest hydrocarbon-bearing basins in Australia. The Caswell Sub-basin and the Leveque and Yampi shelves lie in shallow to intermediate water depths and are mature in exploration terms, hosting significant gas accumulations and discoveries of gas and, to a lesser extent, oil. This contrasts with the Barcoo Sub-basin and deep water Scott Plateau and Seringapatam Sub-basin, which are underexplored. Over 150 wells have been drilled in the basin and over 180 000 km of 2D and 46 000 km² of 3D seismic data has been acquired, most of which is open file (Department of Mines and Petroleum, 2014).

Exploration commenced in the Browse Basin in 1967, when the North West Shelf Joint Venture acquired 1 600 km of regional seismic data (Department of Mines and Petroleum, 2014). The first well drilled in the Browse Basin was Leveque 1 (1970), a stratigraphic test of the sedimentary succession on the Leveque Shelf. This was followed by the discovery of gas at Scott Reef 1 in 1971. This well intersected a thick sequence of gas-bearing reservoirs within Lower–Middle Jurassic Plover Formation sandstones and Upper Triassic sandy dolostones of the Nome Formation on the southern culmination of a faulted anticline (Willis, 1988). Two appraisal wells, Scott Reef 2A in 1977 and North Scott Reef 1 in 1982, were drilled to further delineate the extent of the accumulation (Bint, 1988). No net hydrocarbon pay was assigned to the Scott Reef 2A well, but North Scott Reef 1 encountered a Jurassic hydrocarbon reservoir with an inferred net thickness of 122.9–134.2 m. In 1979, Brecknock 1 tested a broad anticlinal feature 40 km southwest of Scott Reef. The well penetrated 68.3 m of net gas sandstone in Lower to Middle Jurassic sediments (Bint, 1988).

Discoveries during the early 1980s include Brewster 1A ST1 (1980), Caswell 2 ST2 (1983) and Echuca Shoals 1 (1983). Between 1984 and 1994, exploration was focused largely on the northern Caswell Sub-basin with the drilling of Gryphaea 1 (1987), Asterias 1 ST1 (1987), Discorbis 1 (1989) and Kalyptea 1 ST1 (1989), and along the Leveque and Yampi shelves with the drilling of Trochus 1 ST1 (1991), Arquebus 1 ST1 (1991), Sheherazade 1 (1993), Copernicus 1 ST1 (1993) and Yampi 2 (1994). Many of the wells reported minor hydrocarbon shows from either Upper Jurassic or Lower Cretaceous reservoirs (Maung et al, 1994).

Evidence of the oil potential of the basin was demonstrated by the Gwydion 1 (1995) and Cornea 1 (1997) oil and gas discoveries, both located on the Yampi Shelf. These discoveries challenged the previous perception that the basin was gas-prone (Stein et al, 1998). Gwydion 1 intersected three gas-bearing zones and one oil and gas-bearing zone in Lower Cretaceous (Barremian to Albian) shallow marine sandstones draped over a prominent basement high (Spry and Ward, 1997). The Cornea 1, 1B and 2 wells encountered a 25 m gas column overlying an 18 m oil column in a Lower Cretaceous (Albian) reservoir (Ingram et al, 2000), and were followed by nearby oil occurrences at Cornea South 1 and 2 ST1, Focus 1 and Sparkle 1, and gas at Stirrup 1 and Macula 1 throughout 1998. In the same year, Psepotus 1 and Caspar 1A discovered small gas accumulations within Lower Cretaceous sandstones on the Leveque Shelf and Yampi Shelf, respectively. In the Caswell Sub-basin, Adele 1 (1998) discovered gas in the lower Jamieson and Echuca Shoals formation sandstones and Columba 1A ST1 (1999) discovered gas in the Upper Vulcan Formation.

Drilling in 2000 resulted in the discovery of several major gas accumulations, as well as the extension of previously recognised gas provinces. These included Brecknock South 1, located on the Brecknock-Scott Reef Trend, 19 km south of Brecknock 1, and Argus 1 to the north of this trend. Further drilling on the Brewster structure, including the Titanichthys 1, Gorgonichthys 1 and Dinichthys 1 wells, better defined the extent of gas within the Plover (Ichthys) and Montara formations, as well as the Brewster Member of the Vulcan Formation (Ban and Pitt, 2006). Crux 1, drilled in the Heywood Graben in the northeastern Caswell Sub-basin, encountered a 280 m gross gas column in the Upper Triassic Nome Formation, with secondary reservoirs being found within the Plover and Montara formations (Kaoru et al, 2004).

In 2001-02, exploration targeting Lower Cretaceous lowstand fans and ponded turbidite for oil within the Caswell Sub-basin was unsuccessful (Carbine 1 and Firetail 1), apart from an interpreted possible gas pay zone in Marabou 1 ST1 (Benson et al, 2004).

In 2003, BHP Billiton Petroleum drilled the Maginnis 1, 1A, 1A ST1, 1A ST2 wells to test the potential of the outer deep-water Caswell Sub-basin and eastern Seringapatam Sub-basin. Instead of the predicted Plover Formation reservoir, a thick volcanic section was intersected and no hydrocarbon shows were encountered (BHP Billiton Petroleum Pty Ltd, 2003). A second deep-water well, Warrabkook 1, was drilled by BHP Billiton Petroleum Pty Ltd in 2007 in the western Barcoo Sub-basin.

Further appraisal drilling in the Ichthys gas accumulation was undertaken in 2003-04 (Ichthys 1, 1A, Ichthys 2, 2A, 2A ST1, 2A ST2 and Ichthys Deep 1) by INPEX. Gas is primarily reservoired within the upper Vulcan Formation (Brewster Member) and Plover Formation. Gas was also encountered within Callovian sandstones (reported as the Ichthys Formation, but included within the Montara Formation on Figure 2), and in basal Oxfordian sandstones of the lower Vulcan Formation (Ban and Pitt, 2006).

Extensive drilling continued in the central Caswell Sub-basin from 2007 to 2010 (Shell - 14 wells, INPEX - 3 wells and Santos - 2 wells), with gas discoveries at Prelude 1A, Fortissimo 1, Mimia 1, Concerto 1 ST1, Concerto 2 ST1 and Burnside 1 ST1 (Le Poidevin et al, 2015).

Evaluation of the gas accumulations along the Brecknock-Scott Reef Trend also continued in 2005-09, with Woodside drilling the extension/appraisal wells Torosa 1, 2, 3, 4, 5 and 6, Brecknock 2, 3 and 4, and Calliance 1, 2 and 3, as well as the Snarf 1 exploration well on the edge of the Caswell Sub-basin close to the Seringapatam Sub-basin.

Nexus Energy Ltd continued the appraisal of the Crux gas accumulation in 2006–2008 with the drilling of the Crux 2 ST1, 3 and 4 wells, which encountered gas-bearing sands in the Montara, Plover and Nome formations (Nexus Energy Ltd, 2007a, 2011). The Libra 1, Octans 1 and Hippolyte 1 exploration wells tested the potential of the greater Crux area. Libra 1 and Octans 1 intersected gas-bearing sands in an accumulation interpreted to be separate from the Crux accumulation (Nexus Energy Ltd, 2011), and Hippolyte 1 intersected a 55 m gas column in the Jurassic succession (Department of Resources, Minerals and Energy, Petroleum Division, 2011).

Fossetmaker 1 was drilled 7 km east-northeast of Echuca Shoals 1 and intersected an approximate 10 m gas zone, but pressure communication with Echuca Shoals 1 was unable to be determined due to tight reservoir characteristics (Nexus Energy Ltd, 2007b). In 2009, Hawkestone Oil Pty Ltd drilled Braveheart 1 ST1 to test a Barremian submarine fan play which yielded indications of residual hydrocarbons (Exoil Ltd, 2010). Woodside Energy Ltd drilled Omar 1 in 2011 to test lower Jurassic Plover Formation and Triassic Nome Formation targets within a faulted anticlinal trend in the eastern Barcoo Sub-basin (Woodside, 2012). All sandstone intervals were water bearing and the well was plugged and abandoned. Eupheme 1 was spudded in December 2012 in the far south-west of the Caswell Sub-basin to test the stratigraphic section below the ‘Main’ Jurassic unconformity (Murphy Australia WA-423-P Oil Pty Ltd, 2013). The well failed to encounter significant hydrocarbons, and was plugged and abandoned.

In 2009-14, joint venture partners ConocoPhillips (Browse Basin) Pty Ltd and Karoon Gas Australia Ltd completed a multi-well drilling program to the northeast of the Torosa accumulation in the central Caswell Sub-basin. The program was principally designed to test Plover Formation and Montara Formation targets in tilted fault blocks associated with a north-east trending structural high referred to as the Greater Poseidon Trend. Poseidon 1 intersected three gas-bearing sandstone packages of 10 m, 67 m and 140 m gross thickness in the Plover Formation. Poseidon 2 intersected the same three Plover Formation sandstone packages, as well as a new gas reservoir in the overlying Montara Formation (Karoon Gas Australia Ltd, 2010a). Kronos 1 discovered gas in the Plover Formation, which flowed at an equipment-constrained 26 MMscf/d (0.7 MMm³/d; Karoon Gas Australia Ltd, 2010b). Pressure data confirmed the Kronos area is a separate gas accumulation, with a gas-water contact potentially over 200 m deeper than that at Poseidon 1 (Karoon Gas Australia Ltd, 2010b). Kontiki 1 intersected gas saturation in poor reservoir quality sands (Karoon Gas Australia Ltd, 2009).

A second phase of the ConocoPhillips (Browse Basin) Pty Ltd and Karoon Gas Australia Ltd joint venture commenced in 2012 with Boreas 1 which flowed gas from the primary Plover Formation reservoir (Karoon Gas Australia Ltd, 2012). In Zephyros 1, 108 m of core was cut through gas-bearing sandstones that were interpreted to have high mobility values (Karoon Gas Australia Ltd, 2013a). Wireline logging in Proteus 1 ST1 indicated multiple gas-charged reservoirs within the Jurassic, with an 87 m gross reservoir with high net pay (Karoon Gas Australia Ltd, 2013b). The sidetrack measured flow rates of up to 7.3 MMscf/d (0.207 MMm³/d) through a 16/64” choke at 4457 psi (30 730 kPa) and condensate gas ratios of 106.59–123.42 cm³/m³. Production wells are predicted to flow at commercial rates in excess of 100 MMscf/d (2.831 MMm³/d; Karoon Gas Australia Ltd, 2013c). Poseidon North 1 encountered Jurassic gas-bearing sands across a 20 m gross, 12 m net reservoir interval, although pressure data was inconclusive (Karoon Gas Australia Ltd, 2014b). Pharos 1 (2014) intersected movable hydrocarbons in the gas-charged Montara Formation across a 53 m gross interval with 34 m interpreted net pay (Karoon Gas Australia Ltd, 2014c). Grace 1, located to the north of the Greater Poseidon Trend, intersected Jurassic volcanics, and no significant hydrocarbons were encountered (Karoon Gas Australia Ltd, 2014a).

Santos Ltd and joint venture partners tested a number of faulted structural high prospects in the central Caswell Sub-basin between 2012 and 2014. The main target was Jurassic reservoirs within Triassic-Jurassic horsts. Crown 1 intersected 61 m of net gas pay in the Jurassic Montara and Plover reservoirs (Santos Ltd, 2012). Bassett West 1, operated by joint venture partner Total E&P Australia, intersected 7.5 m of gas pay in Jurassic sandstones (Santos Ltd, 2013a). Jurassic gas in the Plover Formation was the target of Dufresne 1 (Total E&P Australia, 2014) but the well was plugged and abandoned (Santos Ltd, 2013b). Lasseter 1 was a gas-condensate discovery, with 78 m confirmed net gas/condensate pay in the Jurassic lower Vulcan and Plover formations (Santos Ltd, 2014).

Hunt Oil spudded Schooner 1 in 2014 to the west of the Burnside gas field (Hunt Oil Australia Permit 425 Holding Company Pty Ltd, 2014). The main target was a stratigraphic pinchout of the Brewster Member associated with a roll-over anticline. Secondary targets were other Vulcan Formation sandstones and the Plover Formation. The well was plugged and abandoned after Jurassic altered volcanics were intersected. Pryderi 1 was spudded by CalEnergy Resources on the Yampi Shelf near Gwydion 1 in October 2014 to test an interpreted extension of the Barremian M. australis sandstone play (Energy-pedia, 2014; CalEnergy Resources, 2015). The well was dry with some residual oils shows. A hydrocarbon discovery was made in 2015 by Shell drilling Auriga West 1 in (OE Digital, 2015) in the northern Caswell Sub-basin near the Crux gas field. Additional information about the Auriga West 1 discovery has not been published. The proposed well Cronus 1, located within AC/P52 north of the Argus gas field, was scheduled for drilling in 2015 (Shell Australia Ltd, 2015) but remains undrilled as at February 2017. The well was intended to target Montara, Plover and Nome formation reservoirs (Triassic-Jurassic) within the Cronus structure (Finder Exploration, 2016).

Regionally significant 2D seismic surveys acquired in recent years are NWS Australia Geostreamer (PGS, 2017a) and WestraliaSPAN 2D (ION, 2017). These are deep regional surveys that extend across the North West Shelf and each includes several lines within the Browse Basin.

Much of the 3D seismic survey acquisition in the Browse Basin in recent years has focussed on undrilled or sparsely drilled areas with little or no 3D coverage. Lord 3D, Admiral 3D, Curt 3D and Rafter 3D (all commissioned by Woodside Energy Limited) occur within, or extend into, the Barcoo Sub-basin. Sandman 3D and Kraken 3D (both commissioned by Karoon Gas Australia) are located in the Seringapatam Sub-basin.

Other recent 3D surveys were acquired in the Caswell Sub-basin to improve resolution in areas with existing 3D coverage. Rosebud 3D (Shell Development Australia) is located over the Torosa gas field. Caswell 3D is centred north of the Ichthys gas field (PGS, 2017b). Schild 3D Phase 1 and Schild 3D Phase 2, are located west of the Ichthys gas field and in the Echuca Shoals area, respectively (CGG, 2017). Cygnus 3D was acquired to enhance Triassic structure and Jurassic reservoirs over an area that includes the Crux gas field (Spectrum, 2016).

Geoscience Australia has recently conducted marine surveys of the Leveque Shelf (Picard et al, 2014) and Caswell Sub-basin (Nicholson et al, 2015; Howard et al., 2016). The surveys collected data in support a CO₂ storage assessment study that was part of the National CO₂  Infrastructure Plan. In addition to acquiring shallow geological, water column, and seabed habitat data, the surveys aimed to identify and sample features indicative of active or extinct natural fluid seepage.

In the Browse Basin there are a number of projects in various stages of planning and development: the Ichthys LNG Project and the Prelude FLNG project are under construction while the Browse FLNG proposal is in planning.

 

• Ichthys LNG Project — This Project (Production Licences WA-50-L and WA-51-L) is operated by INPEX (62.2%) with joint venture partners Total (30%), CPC Corporation (2.6%), Tokyo Gas (1.6%), Osaka Gas (1.2%), Kansai Electric Power Australia (1.2%), JERA (0.7%), and Toho Gas (0.4%). INPEX Browse Ltd (INPEX) announced the final investment decision (FID) in January 2012 for an offshore central processing facility (CFP) and a floating production, storage and offloading (FPSO) facility, connected by an 890 km gas pipeline to an 8.9 million tonnes per annum (mtpa) liquefaction plant and export terminal in Darwin. The project is expected to produce first gas in September 2017, with a life of 40 years (INPEX, 2017a), and will also produce condensate and LPG. Resource estimates indicate 12.8 Tcf (362.5 Bcm) of gas and 527 MMbbl (83.8 Gl) of condensate (Department of Mines and Petroleum, 2011). The pipeline between the Ichthys Field and the Project’s onshore facilities at Bladin Point near Darwin was completed in November 2016 (INPEX, 2017b).

 

• Prelude FLNG — Shell (67.5%) is the operator of Production Licence WA-44L with INPEX (17.5%), KOGAS Prelude Pty Ltd (10%) and CPC (5%) in which the Prelude and Concerto gas accumulations occur. Shell Development Australia Pty Ltd (Shell) announced the FID for a 3.6 mtpa Floating LNG (FLNG) project in May 2011 for production from the Prelude and Concerto gas accumulations. The FLNG vessel is currently in the construction phase, drilling of production wells commenced in 2014, and first gas is due in 2017 (Offshore Energy Today, 2016). The Prelude accumulation is situated in the northeastern area of the Ichthys accumulation (Figure 1). Production is expected to deliver 3.6 mtpa of LNG, 1.3 mtpa of condensate and 0.4 mtpa of LPG, and the Prelude FLNG facility is anticipated to operate for up to 25 years before needing to dock for inspection and overhaul (Shell Australia Ltd, 2017). The Crux field is also likely to become part of the Prelude development (Offshore Energy Today, 2012). The Retention Lease AC/RL9, containing the Crux accumulation, was granted in February 2013. Titleholders include the operator Shell (82%) and its JV partners, SGH Energy (15% - who acquired Nexus in 2014/15) and Osaka Gas (3%). The Crux accumulation contains resources of 2.2 Tcf (62 Bcm) gas and 74 MMbbl (11.8 Gl) liquids.

 

• Browse FLNG— Operated by Woodside (30.6 %) with joint venture partners Shell Australia (27.0 %), BP Developments Australia Pty Ltd (17.3%), Japan Australia LNG (MIMI Browse) Pty Ltd (14.4 %), and PetroChina International Investment (Australia) Pty Ltd (10.7 %). Woodside Petroleum Limited (Woodside) announced in September 2013 the Browse Joint Venture’s intention to commercialise the Torosa, Brecknock and Calliance gas accumulations via a FLNG project in retention leases WA-28-R, WA-29-R, WA-30-R, WA-31-R and WA-32-R. The estimated resources are 16.0 Tcf of dry gas and 46 MMbbl of condensate (Woodside, 2016). Following the completion of front-end engineering and design (FEED) work, the Browse Joint Venture (BJV) participants decided in March 2016 not to progress further with the floating LNG development concept. However, the BJV participants are now preparing a new work program to assess and select concept phase activities (Woodside, 2016).

The Browse Basin is located on the northwest margin of Australia and primarily includes continental shelf and slope with isolated reefs located offshore from the Kimberley region.

Climate of the region

The northwest shelf region experiences a dry (arid tropical) climate with two distinct seasons: the north west (summer) monsoon (October-March) and the north east to south east (winter) monsoon (April-September), with a rapid transitional period between each season. On average, approximately 90% of the annual mean rainfall (779.2 mm) occurs in the period between December and March and is associated with storm activity. Winds during the summer monsoon are typically westerly/northwesterly and humid, while during the winter monsoon winds are typically drier southeasterlies, which originate over the Australian mainland. The annual mean maximum and minimum temperatures recorded at Browse Island were 30.7°C and 26.6°C for the period 1969–2016 (www.bom.gov.au). Offshore, water temperatures in reef lagoons are commonly 28-30°C, while during strong La Nina events, sea surface temperatures may reach 34°C. The region is subject to cyclone activity, and the cyclone season officially runs from November to May, with approximately five tropical cyclones occurring per year on average. Highest cyclone activity in the region typically occurs during March and April. Between 1996/97 and 2006/07, 17 tropical cyclones passed within 200 km of Browse Island (www.bom.gov.au).

Oceanic regime

Oceanic circulation in the Browse Basin is strongly influenced by the southward flowing, offshore, Indonesian Throughflow, and by the Leeuwin Current along the northwest shelf edge (Condie and Andrewartha, 2008). The surface water above the Caswell and Barcoo Sub-basins, together with Scott Plateau and the Seringapatam Sub-basin, form part of the zone over which the Indonesian Throughflow passes. The warm, low salinity, southwesterly directed currents of the Indonesian Throughflow joins the westward directed South Equatorial Current (Adamo et al, 2009; Kuhnt et al, 2004). This surface current transports warm equatorial water south-eastwards towards the Wharton Basin. At shelf break on the Yampi and Leveque shelves, the seasonal (Autumn) Holloway Current transports warm water, heated on the northwest shelf, southwards (Adamo et al, 2009). The Holloway Current begins to flow at the termination of the northwest monsoon season. Between the Indonesian Throughflow and the Holloway Current, an anticlockwise surface gyre exists (Adamo et al, 2009).

The northwest shelf experiences small ocean-swell waves (< 1.5 m significant wave height), but is affected by tropical cyclones during the monsoon season (November–March; Porter-Smith et al, 2001). While the mean annual wave power is low (~0-2 W/m2; Porter-Smith et al, 2001), cyclone-generated currents can erode and transport sediment over large spatial extents (Porter-Smith et al, 2001).

The northwest shelf and the Browse Basin region is macrotidal, and inshore is ebb-tide dominated. The Kimberley Coast experiences some of the largest tides in the Southern Hemisphere. Tidal ranges generally exceed 4 m on the Leveque Shelf; tidal ranges of up to 10 m occur at Broome, and up to 12.5 m in King Sound. Tides at Broome are semi-diurnal (two high tides, and two low tides per day). On the outer shelf, the ebb and flood tidal currents are orthogonal to the coast (James et al, 2004) whereas on the inner shelf, flood tidal currents run parallel to the coast. Tidal currents are strongest around the shelf break (James et al, 2004). However, tidal currents play a significant role in nearshore and coastal sediment transport; e.g. in King Sound, tides mobilise significant areas of medium-grained sand (250–500 µm diameter) 100% of the time (Porter-Smith et al, 2001).

The Browse Basin and Kimberley coastal region are characterised by large internal waves, internal tides and strong seasonality in the near-surface ocean circulation (McKinnon et al, 2015). Internal waves occur around shelf break (Holloway, 1987), with resulting internal tides present across the continental slope from approximately 100 m to 1000 m depths (Holloway, 2001). Internal waves with 60 m amplitudes have been observed to form around Scott Reef, located in water depths of ~500 m on the continental slope of the northwest shelf (Wolanski and Deleersnijder, 1998).

Important oceanographic drivers of biological productivity in the Northwest region include 1) the weakening of the Indonesian Throughflow and Leeuwin Current during the northwest monsoon (December to March) which allows for increased upwelling and productivity; 2) the seasonal reversal in wind direction in which the Leeuwin Current is opposed during summer and upwelling is facilitated; and 3) cyclones which result in physical disturbance, nutrient input via run-off, and changes in plankton distributions (DEWHA, 2008; McKinnon et al., 2015).

Seabed environments: regional overview

The Browse Basin extends from shallow shelf waters adjacent to the Kimberley coast of Australia, to near-abyssal depths on the western margin of Scott Plateau. At the basin scale, the Yampi and Leveque Shelves form terraced and plateaued seabed on the eastern margin of the main Browse Basin depocentres of the Caswell and Barcoo sub-basins (Harris et al, 2003). The width of the shelf is greatest (~250 km) across the Yampi Shelf-Prudhoe Terrace to Scott Reef transect.

The edge of the Yampi Shelf and the transition to the Caswell Sub-basin occurs in water depths of 100-130 m on the Prudhoe Terrace, with most of the Caswell Sub-basin lying in water depths of 200-500 m. The western margin of the Caswell Sub-basin marks a transition from shelf to slope settings. Where there are shelf-edge reefs, the transition to deeper water is sharp. However, the transition to deep water across inter-reefal areas from shelf to slope, is significantly more gradual. The majority of the Scott Plateau is composed of terrace and plateau, in water depths of 2000-3000 m, while its western slope occurs between 3000 m and 5000 m depth. The plateaued Seringapatam Sub-basin mostly lies in water depths of 1700-2300 m. Several submarine canyons are present along the Caswell to Seringapatam Sub-basin transition, and on the western slope of the Scott Plateau where it abuts the Argo Abyssal Plain.

Islands and Reefs

The Kimberley-Browse coastal and offshore region is a poorly studied area of high conservation value, consisting of emergent oceanic reefs, submerged oceanic shoals, fringing coastal coral reefs, mangroves, tidal mudflats and a turbulent water column (McKinnon et al, 2015). The principal islands and reefs in the Browse region include Browse Island, Echuca Shoal, Heywood Shoal, Scott Reef, Adele Island, Seringapatam Reef and Lynher Reef. Additional shoals include, Vulcan Shoal, Goeree Shoal, Eugene McDermott Shoal. Compared with the generally high tidal energy environment of the northwest shelf, reefs provide shallow water environment with comparatively low energy regimes. At North Scott Reef and elsewhere, coral reef has developed since the early Holocene over previously exposed reefal lithologies from the last interglacial, following the post-glacial rise in sea-level (Collins, 2011, 2014). Shallow-water reefs such as Scott Reef are prone to damage during tropical cyclones (e.g. Cyclone Fay of 2004). Surficial sediment on the northwest shelf is predominantly carbonate. The northwest shelf (<300 m depth) is an oceanic carbonate ramp, similar in scale to the Great Bahama Bank (James et al, 2004; Belde et al, 2014), and hosts a large number of coral reefs. These include isolated oceanic reefs (Ashmore, Seringapatam and Scott reefs, Rowley Shoals), and the island-associated shelf reefs of the Kimberley coast and Dampier archipelago; Pilbara reefs (Barrow and Montebello Islands); and Ningaloo Reef (Collins, 2011; Collins and Testa, 2010; Wilson, 2013). The seabed is predominantly composed of unconsolidated sediment on the continental shelf. This consists of medium- and coarse-grained CaCO3-rich (~90%) sands and gravels derived from relict fossiliferous carbonate material, with minor authigenic phosphate and glauconite components (Collins, 2011). Between 200 m and 500 m water depths, hard seabed and sandy carbonate sediments are dominant. In deeper water, particularly in areas distal to reefs, the seabed is dominated by fine and fine-medium grained unconsolidated sediments (Kuhnt et al, 2006). There are, however, also areas of seabed in these deeper settings where hard rock is present. In the western Caswell Sub-basin, large-scale sediment bedforms indicate the presence of strong southeast – northwest oriented bottom-flow currents. These are potentially related to internal tides (Belde et al, 2014).

Shelf

Along the Yampi and Leveque shelves, sediments are palimpsest, and of mixed age, with little modern material being deposited. Sediment here is either sourced from the post-mortem deposition of skeletal carbonate or in places sourced from the coast and transported across the shelf. However, at the coast, the transport of modern terrigenous sediment, where it exists, is generally in a shore-parallel direction.

The continental shelves of the Yampi Shelf, Prudhoe Terrace and Leveque Shelf, located in a high-energy, shallow water, tropical setting, have a much thinner sediment cover than the centrally located Caswell Sub-basin (Stephenson and Cadman, 1994). Hardgrounds are present at several points along this sediment-starved margin (Rollet et al, 2006). Vibracores recovered on the Leveque Shelf, which are up to 2.5 m long, bottom out on hard substrates (Picard et al, 2014). These cores were recovered from an area with elongate, shore-normal banks and ridges, carbonate terraces, channels, and large bedforms up to 8 m high dominated by coarse sediment.

Ecology

A diverse range of benthic marine habitats have been identified within the Browse Basin, including habitat found in shallow water (seagrass beds, coral reefs and atolls, shoals and banks), moderate depth (methane seeps, shelf and slope), and deep water (canyons and marginal plateaus). The majority of the Browse Basin, however, comprises shelf and marginal plateau habitats. Many shallow habitats are comparatively well studied (e.g. Scott Reef and Seringapatam Reef), but in contrast, little is known about the biology and physical environments of deeper habitats (e.g. Scott Plateau and canyons). Field studies within the region have identified highly diverse marine assemblages, with many species endemic to this region. Shelf habitats are dominated by crustaceans and polychaetes, with mixed patches of sponges and octocorals (Picard et al, 2014; Howard et al, 2016). Biogeographical comparisons of biodiversity across the broader NW-Indonesian region have found that the Browse Basin may lie mid-way along a biodiversity gradient from extremely high diversity in the north to much lower diversity in the south (Berry, 1986; Veron and Marsh, 1988). However, due to low sampling, it is unclear how general these biodiversity patterns may be among habitats or taxa.

Biodiversity in the Browse Basin may be higher than surrounding basins, as suggested by bioregionalisation and seascape maps. The Browse Basin is composed of two Provincial Bioregions, which by definition are centres of demersal fish endemism (Heap et al, 2005). In comparison, transition areas located to the northeast and southwest of Browse Basin are regions of overlap between Provincial Bioregions. Thus, Browse Basin is predicted to have a higher level of endemism than surrounding regions.

Rich assemblages of invertebrates (particularly corals and sponges) and fish (especially sharks) have been recorded from a variety of benthic habitats within the Browse Basin. The offshore coral reef assemblages, such as those recorded from Scott and Seringapatam reefs, are unique to the northwest region, and represent some of the few offshore coral atolls in Western Australia (Skewes et al, 1999; Veron and Marsh, 1988; Wells and Allen, 2005). Diverse communities of corals have also been found in deeper waters surrounding Scott Reef (up to 60 m depth; Berry, 1986), suggesting that these unique coral habitats may extend beyond the normal shallow water zones. A variety of reefs, shoals and banks have also been identified in the Browse Basin, and are known to support often large aggregations of pelagic and benthic fishes, making these areas both ecologically and commercially important (CSIRO, 1980; Stehouwer, 1981). Hydrothermal vents and methane seeps have also been reported from several locations within the Browse Basin region (O’Brien and Glenn, 2005; CSIRO, 2005; Rollet et al, 2006). Although, seeps and vents are known to provide a unique environment for chemosynthetic organisms, the vents and seeps examined within the Browse Basin to date have not had chemosynthetic organisms (CSIRO, 2005). Rather, these vent and seep habitats seem to provide a novel hard substratum that supports a range of filter-feeding organisms, such as sponges and corals, that are commonly found on other hard substrata (e.g. shoals and reefs) within the area.

Recent geological history

The modern geomorphic characteristics of the Browse Basin are inherited from pre-existing structures. During the Quaternary these were further developed, particularly through reef growth and differential sedimentation during sea-level oscillations of up to 130 m. Modern reefs are generally present on pre-existing palaeo-reefal structures. Since the onset of the middle Pleistocene (Ionian Age) (Lisiecki and Raymo, 2005; Gibbard et al, 2009; Head et al, 2008), glacially low sea levels would have exposed the Yampi and Leveque shelves and Prudhoe Terrace to a level approximately 130 m below that of the present. Associated remnant glacial coastlines were situated in the vicinity of Browse Island, in the Caswell Sub-basin. Areas including the Cornea site would have been sub-aerially exposed for moderate amounts of time since the middle Pleistocene. A prominent terrace-edge scarp at a depth of 125 m on the Leveque shelf has been attributed to formation during glacial low sea levels, the most recent being 23-19 ka BP (Collins, 2011; James et al, 2004). It has been suggested that subsidence has been occurring on the northwest shelf since at least the last interglacial, ~125 ka BP (Collins and Testa, 2010).

Seabed seepage

In the Browse Basin, hydrocarbon seeps have been observed at Cornea and Heywood Shoals on the northern Yampi Shelf (Logan et al, 2010; Jones et al, 2005; Rollet et al, 2006). Pockmarks have also been noted in the Oobagooma Sub-basin and Broome Platform of the offshore Canning Basin, southwest of the Leveque Shelf (Jones et al, 2007, 2009). Pockmark formation at these latter locations has been attributed to the expulsion of seawater at the seabed, driven by tidal pumping through the shallow sub-surface. Recently, small pockmarks have been observed at the seabed in the northeast of the Caswell Sub-basin (Howard et al, 2016). Pockmarks similar to those observed by Jones et al (2007) are present on the Leveque Shelf (Picard et al, 2014). Active expulsion of gas bubbles imaged by side scan sonar and single beam echo-sounder systems and suggesting seep activity has been observed along the southern flank of the Ashmore Platform (Stalvies at al, 2017).

Commonwealth Marine Reserves

Kimberley Commonwealth Marine Reserve

Major Conservation Values:

https://www.environment.gov.au/topics/marine/marine-reserves/north-west/kimberley

• Important foraging areas for migratory seabirds, migratory Dugongs, dolphins and threatened and migratory marine turtles
• Important migration pathway and nursery areas for the protected Humpback Whale
• Adjacent to important foraging and pupping areas for Sawfish and important nesting sites for Green Turtles.
• The reserve provides protection for the communities and habitats of waters offshore of the Kimberley coastline ranging in depth from less than 15 m to 800 m
• Continental shelf, slope, plateau, pinnacle, terrace, banks and shoals and deep hole/valley seafloor features are all represented in this reserve
• Examples of the communities and seafloor habitats of the Northwest Shelf Transition, Northwest Shelf Province and Timor Province provincial bioregions along with the Kimberley, Canning, Northwest Shelf and Oceanic Shoals meso-scale bioregions

The Kimberly Commonwealth Marine Reserve is mainly zoned Multiple Use IUCN Category VI (Multiple Use Zone), and also includes areas of IUCN II (Marine National Park Zone), and IUCN IV (Conservation Park Zone). Management plans for the North-west Commonwealth marine reserves network are not yet in place.

Cartier Island Commonwealth Marine Reserve

Major Conservation Values:

https://www.environment.gov.au/topics/marine/marine-reserves/north-west/cartier

Ecosystems, habitats and communities associated with:

• the North West Shelf
• Timor Province
• emergent oceanic reefs

Cartier Island is an important area for protected species:

• internationally significant for its abundance and diversity of sea snakes
• large and significant feeding populations of Green, Hawksbill and Loggerhead Turtles occur around the reefs
• supports some of the most important seabird rookeries on the North West Shelf including colonies of Bridled Terns, Common Noddies, Brown Boobies, Eastern Reef Egrets, Frigatebirds, Tropicbirds, Red-footed Boobies, Roseate Terns, Crested Terns and Lesser Crested Terns
• important staging points/feeding areas for many migratory seabirds

The reserve is currently zoned Sanctuary zone IUCN Category Ia. Management plans for the Northwest Commonwealth marine reserves network are not yet in place.

Ashmore Reef Commonwealth Marine Reserve

Major Conservation Values:

https://www.environment.gov.au/topics/marine/marine-reserves/north-west/ashmore

Ecosystems, habitats and communities associated with:

• the North West Shelf
• Timor Province
• emergent oceanic reefs

The island and reef is an important area for the following protected species:

• internationally significant for its abundance and diversity of sea snakes.
• critical nesting and interesting habitat for Green Turtles, supporting one of three genetically distinct breeding populations in the North-west Marine Region. Low level nesting activity by Loggerhead Turtles has also been recorded.
• large and significant feeding populations of Green, Hawksbill and Loggerhead Turtles occur around the reefs. It is estimated that approximately 11 000 marine turtles feed in the area throughout the year.
• supports a small Dugong population of less than 50 individuals that breeds and feeds around the reef. This population is thought to be genetically distinct from other Australian populations.
• supports some of the most important seabird rookeries on the North West Shelf including colonies of Bridled Terns, Common Noddies, Brown Boobies, Eastern Reef Egrets, Frigatebirds, Tropicbirds, Red-footed Boobies, Roseate Terns, Crested Terns and Lesser Crested Terns.
• important staging points/feeding areas for many migratory seabirds

The reserve is currently zoned Sanctuary Zone IUCN Category Ia and Recreational Use Zone IUCN Category II. Management plans for the North-west Commonwealth marine reserves network are not yet in place.

Fisheries

The following Commonwealth fisheries occur in the Browse Basin area:

• Northwest slope trawl fishery operates in the area between 200 m water depth to the outer limit of the Australian fishing zone. The fishing season runs for 12 months period, starting on the 1 July.
• Southern Blue Fin Tuna fishery. The fishing season runs for 12 months.
• Western Tuna and Billfish fishery operates in the Australian Fishing Zone and adjacent high seas. The fishing season runs a 12 month period, starting on the 1 February.
• Western Skipjack Tuna fishery is not active.

The Browse Basin is located within the Western Australian North Coast Bioregion. The Western Australian Government manages 15 commercial fisheries within the bioregion (Fletcher and Santoro, 2015). Tropical finfish, Spangled Emperor, Snapper and Cod are targeted in the Northern Demersal Scalefish (trap and line) fishery. The Mackerel Managed Fishery targets Spanish Mackerel (Fletcher and Santoro, 2015).

Information resources

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

Ashmore Reef Commonwealth Marine Reserve https://www.environment.gov.au/topics/marine/marine-reserves/north-west/ashmore

Cartier Island Commonwealth Marine Reserve https://www.environment.gov.au/topics/marine/marine-reserves/north-west/cartier

Kimberley Commonwealth Marine Reserve https://www.environment.gov.au/topics/marine/marine-reserves/north-west/kimberley

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

Western Australian Fisheries http://www.fish.wa.gov.au/Fishing-and-Aquaculture/Commercial-Fishing/

ABBOTT, S., CAUST, D., ROLLET, N., LECH, M., ROMEYN, R., ROMINE, K., KHIDER, K. AND BLEVIN, J., 2015—Seven Cretaceous low-order depositional sequences from the Browse Basin, North West Shelf, Australia: a framework for CO₂ storage studies. AAPG International Conference and Exhibition, 13-16 September 2015, Melbourne, Australia. http://www.searchanddiscovery.com/pdfz/documents/2016/51224abbott/ndx_abbott.pdf.html (last accessed 3 February, 2017).

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