Roebuck Basin Regional Geology

The Roebuck Basin forms the central part of the Westralian Superbasin, and covers approximately 90 000 km² on the North West Shelf between the Northern Carnarvon Basin to the south and the Browse Basin to the north, the Canning Basin to the east, and the Argo Abyssal Plain to the northwest (Figure 1). The Roebuck Basin is subdivided into the Bedout Sub-basin in the south and the Rowley Sub-basin in the north, which are separated by a structural trend that includes the Bedout High. The geology of the Roebuck Basin is described by Lipski (1993; 1994), Colwell and Stagg (1994), and Smith et al (1999).

The Bedout Sub-basin is an east-northeast trending depocentre (Figure 1) that is filled with approximately 2.5 km of Paleozoic and 7 km of Mesozoic sediments (Smith et al, 1999). The sub-basin is separated from the Beagle Sub-basin to the west by the North Turtle Hinge Zone. The sub-basin is bounded to the south by the Lambert Shelf of the Northern Carnarvon Basin and to the east by the Willara Sub-basin and Broome Platform elements of the Canning Basin.

The Rowley Sub-basin (Figure 1) contains about 9 km of Permo-Carboniferous or older strata and up to 6 km of Mesozoic–Holocene sediments (Smith et al, 1999). The sub-basin is separated from the Beagle Sub-basin and Exmouth Plateau elements of the Northern Carnarvon Basin to the west by the North Turtle Hinge Zone and Thouin Graben. To the east, the Oobagooma High separates the Rowley Sub-basin from the Oobagooma Sub-basin (Canning Basin). To the northeast, the Rowley Sub-basin passes into the Barcoo sub-basin and Scott Plateau elements of the Browse Basin.

The Bedout High consists of uplifted and eroded Permo-Carboniferous sedimentary rocks above a faulted basement core, and is capped by Permian volcanics (Colwell and Stagg, 1994; Smith et al, 1999). It has a maximum relief of approximately 6 km above the surrounding depocentres. Lower to Middle Triassic sediments onlap the feature from all directions and it is draped by approximately 3 km of Upper Triassic–Holocene sediments. The upper surface of the Bedout High is a peneplain approximately 30 km wide (Smith et al, 1999; Müller et al, 2005).

Locations of seismic sections and wells are shown in Figure 1. Regional stratigraphy is given in Figure 2. The location of permits and their operators are given in Figure 3. Regional seismic lines are shown in Figure 4, Figure 5 and Figure 6. Figure 7 shows the location of Commonwealth Marine Reserves and key ecological features.

The Roebuck Basin is one of a series of extensional basins that together form the Westralian Superbasin (Yeates et al, 1987; AGSO North West Shelf Study Group, 1994; Etheridge and O’Brien, 1994). Structural elements of the Roebuck Basin (Figure 1) are after Hocking et al (1994). Totterdell et al (2014) re-allocated the Oobagooma Sub-basin from the Roebuck Basin to the offshore Canning Basin. The Roebuck Basin is divided into two depocentres, the Rowley Sub-basin, and the Bedout Sub-basin. The stratigraphy and structure of these Westralian (mid-Carboniferous to Holocene) depocentres have been described by Colwell and Stagg (1994), Etheridge and O’Brien (1994), Lipski (1993, 1994) and Smith et al (1999). The Westralian depocentres are underlain by an older Palaeozoic succession that at least in part represents a continuation of the Canning Basin.

Rowley Sub-basin

The Rowley Sub-basin contains up to 9 km of Paleozoic strata and up to 6 km of Mesozoic to Holocene strata, and the sub-basin fill forms a seaward-thickening wedge that extends to the modern, fault-bound, continent-ocean boundary. The main structural orientation is northeast-southwest and is linked to northwest-southeast extension associated with the evolution of the Westralian Superbasin. The Paleozoic part of the sub-basin fill includes several Permo-Carboniferous syn-rift wedges attributed to the initiation of the Westralian succession. The Rowley Sub-basin is regarded as mildly structured compared to other Westralian basins. An older Paleozoic succession has been mapped in seismic profiles across the Rowley Sub-basin.

Bedout Sub-basin

The Bedout Sub-basin contains up to 2.5 km of Paleozoic strata and up to 7 km of Mesozoic strata. In contrast to the Rowley Sub-basin, the Bedout Sub-basin structural architecture is partly characterised by east-southeast trending faults. This structural trend and associated northeast-southwest extension characterises the underlying Canning Basin succession, and persisted during the Permo-Carboniferous evolution of the Bedout Sub-basin. Paleozoic deposition was thickest in the extensions of the Fitzroy Trough (Oobagooma Sub-basin) and Willara Sub-basin, where seismic mapping indicates thick Ordovician to Devonian successions. The Mesozoic fill was deposited during a regional sag phase of basin development.

Bedout High and Oobagooma High

The Bedout and Oobagooma highs are prominent features that lie on a structural trend that separates the Rowley sub-basin from the Bedout Sub-basin and the Oobagooma Sub-basin. This trend represents a hinge line between Westralian northwest-southeast extension (Rowley Sub-basin) and northeast-southwest extension (Oobagoooma and Bedout sub-basins). The Bedout High forms part of an anticlinal structure that separates the Rowley and Bedout sub-basins. Development of the Bedout High is linked to the late Permian Bedout Movement (Forman and Wales, 1991). Paleozoic strata within the Bedout High are unconformably overlain by volcanics that have been radioisotopically dated at about 253 Ma (i.e. near the Permian-Triassic boundary). The structure is onlapped from all sides by Triassic strata, and overlain by 3 km of Mesozoic strata. The Oobagooma High lies on the boundary between the Rowley Sub-basin and the Oobagooma Sub-basin. This feature is fault bounded along its eastern side and a magnetic anomaly on its western side indicates volcanic lithologies.

The origin of the Oobagooma and Bedout highs remain poorly understood. Smith (1999) proposed that these structures formed to accommodate differential northeast-oriented extension during the Paleozoic, with the Bedout High subsequently reactivated during Westralian extension. Colwell and Stagg (1994) regarded the Bedout High as an intruded basement high underlain by magmatic upwelling, with a degree of thrust control. Others (Gorter, 1997; Becker et al, 2004) have invoked an extraterrestrial impact origin for the Bedout High.

The Roebuck Basin succession comprises six basin phases according to the current understanding (after Colwell and Stagg, 1994; Smith et al, 1999):

• Extension 1: northeast-southwest extension and deposition in pre-Westralian intracratonic basins.
• Extension 2: Early Carboniferous to late Permian northwest-southeast extension linked to initiation of the Westralian Superbasin. Regarded as a transitional phase in the Roebuck Basin because of the persistence of east-northeast-trending structures in the Bedout Sub-basin.
• Sag 1: Thermal subsidence terminated by a tectonic event attributed to the Fitzroy Movement.
• Sag 2: Thermal subsidence during a period of Jurassic extension in the Northern Carnarvon and Bonaparte sectors of the Westralian Superbasin, terminated by a Callovian unconformity formed by separation of Argoland from the outboard Roebuck and Browse basins.
• Passive Margin: Post Callovian thermal subsidence.
• Inversion: Middle Miocene to Holocene northeast-southwest compression in response to convergence of the Australian and Eurasian tectonic plates.

Pre-Westralian strata beneath the Roebuck Basin are at least in part contiguous with the Canning Basin. In particular, the extensional depocentre beneath the Bedout Sub-basin is inferred from seismic mapping to be a continuation of the Willara Sub-basin that contains Ordovician strata including the Willara, Goldwyer and Nita formation equivalents from the onshore Canning Basin (Lipski, 1993). The Paleozoic succession beneath the Westralia Superbasin is regarded as part of a system of intracratonic rift basins formed under northeast-southwest extension (Lipski, 1993, AGSO North West Shelf Study Group, 1994). Veevers (1988) regarded these early Paleozoic basins, including the Canning and Bonaparte basins, as aulacogens, initiated during the breakup of the ‘ancestral’ northwest margin. According to Metcalfe (2013), rifting culminated in the separation of North China and associated crustal blocks, and the opening of Palaeo-Tethys, occurred in the Middle Devonian.

The Roebuck Basin was initiated, as part of the broader Westralian Superbasin, by a phase of early Carboniferous to late Permian, northwest-southeast extension (Etheridge and O’Brien, 1994; AGSO North West Shelf Study Group, 1994). Extension was related to the Early Permian separation of the Cimmerian continent from the northwest margin of Gondwana and the opening of Meso-Tethys (Metcalfe, 1988). Smith et al (1999) recognised the presence of half-graben up to 120 km long in the Rowley Sub-basin formed during this extension phase but also observed that in the Bedout Sub-basin (and offshore Canning Basin), northeast-southwest extension persisted.

Apart from the Bedout Volcanics, Permo-Carboniferous syn-rift strata in the Roebuck Basin remain undrilled. The early Carboniferous unconformity (G. maculosa spore-pollen zone, late Visean) in the neighboring Browse Basin (Blevin et al, 1998b) provides the best indication of the initiation of Westralian syn-rift wedges in the Roebuck Basin. In the Bedout Sub-basin, the Westralian syn-rift succesion is inferred to include the Grant and Liveringa groups from the onshore Canning Basin (Lipski, 1994). The upper Permian Bedout Volcanics unconformably overlie the inferred Liveringa Group on the Bedout High. The Westralian syn-rift basin phase was terminated by basin-wide uplift and erosion related to the Bedout Movement (Smith et al, 1999).

During Triassic time, the Rowley Sub-basin was part of a broad intracratonic sag that also encompassed the Northern Carnarvon and Browse basins. A thick succession of marine claystone (Locker Shale) was deposited and overlain by a thick fluvial to marginal marine succession (Keraudren Formation) (Lipski, 1994). Separating the lower and upper Keraudren Formation is the Middle Triassic Cossigny Member, a regionally widespread limestone unit expressed in reflection seismic profiles as a high-amplitude reflection. Triassic and Early Jurassic deposition was punctuated by three transpressional events, attributed to the Fitzroy Movement, the effects of which were focused along the margins of the sub-basins (Smith et al, 1999):

• Fitzroy Movement I (Ladinian) generated transpressional “flower structures” along the North Turtle Hinge Zone;
• Fitzroy Movement II (Norian) caused major en-echelon anticlines in the Fitzroy Trough (Canning Basin), and a subtle unconformity in the Phoenix 1 and Phoenix 2 wells; and
• Fitzroy Movement III (Sinemurian) marked basin-wide uplift and erosion, and a change in gross stratal geometry within the Roebuck Basin from predominantly back-stepping and aggrading to prograding and aggrading.

Following Fitzroy Movement III, a broad prograding wedge of fluvio-deltaic sediments (Depuch Formation) was deposited during thermal subsidence. A breakup unconformity generated by separation of Argo Land from the outboard Roebuck and Browse basins in the Callovian (Müller et al, 1998; Veevers et al, 1991) marked the end of a Jurassic phase of rifting elsewhere in the Westralian Superbasin (Marshall and Lang, 2013).

With the formation of oceanic crust in the Argo Abyssal Plain, a passive margin basin phase commenced with transgression and accumulation of condensed marine mudstone (Baleine and Egret formations) until the end of the Jurassic. Early Cretaceous (Berriasian-Valanginian) rifting and separation of Greater India from Australia is linked in the Roebuck Basin to minor fault reactivation and an influx of sand (Broome Sandstone). This unconformity-bounded succession is the equivalent of the Barrow Delta succession in the Northern Carnarvon Basin. Subsequent deposition in the Roebuck Basin during the Early Cretaceous was dominated by marine mudstone (Mermaid Formation, Nelson Rocks Formation), with a change to mainly fine-grained carbonate and calcareous lithologies in the Late Cretaceous (Toolonga and Miria formations). Deposition of fine-grained carbonate-rich lithologies continued into the Cenozoic. The post‑Oligocene succession comprises a carbonate clinoform wedge that extends across the inner Rowley Sub-basin (Smith, 1999). The effects of late Miocene collision of the Australian continent with Eurasia were limited in the Roebuck Basin to transtensional wrenching along footwalls of northeast-southwest-oriented Paleozoic half-graben in the Rowley Sub-basin (Smith, 1999).

Regional petroleum systems

At least one, and possibly two, petroleum systems operate in the Roebuck Basin (Bradshaw, 1993; Smith et al, 1999; Edwards and Zumberge, 2005). These are:

• Gondwanan 1(Pennsylvanian–Permian): This is part of a globally extensive succession rich in organic matter (Warris, 1993; Petersen, 2006) and may be present, and possibly mature, in the Bedout Sub-basin and the Rowley Sub-basin.
• Westralian 1 (Triassic to Lower–Middle Jurassic): Triassic sediments are potential sources of both oil and gas in the Roebuck Basin, particularly in the Bedout Sub-basin where they contain algal-rich layers (Molyneux et al, 2015, 2016). The Phoenix South 1 oil discovery demonstrates a pre-Jurassic (probably Lower Triassic) petroleum system in the Roebuck Basin (Thompson et al, 2015; Wehr et al, 2015). The fluvio-deltaic Lower–Middle Jurassic sediments of the Roebuck Basin contain source rocks which are considered to be mostly gas-prone (Bradshaw, 1993; Bradshaw et al, 1994).

Source rocks

Source rocks of the Gondwanan 1 Petroleum System comprise the Cisuralian (lower Permian) transgressive marine shales of the Poole Sandstone and Noonkanbah Formation, which are known to be organic-rich in the Fitzroy Trough of the onshore Canning Basin (Kennard et al, 1994). Marine shales of the Grant Group, which underlie the Poole Sandstone and Noonkanbah Formation, are also locally organic-rich, but generally have poor hydrocarbon generation potential.

The Lower to Middle Triassic transgressive marine shales of the Locker Shale and the overlying Middle Triassic lower Keraudren Formation, are Westralian 1 source rocks that are potentially widespread in the Roebuck Basin. In the Bedout Sub-basin, the recently interpreted oil column in the lower Keraudren Formation at Phoenix 1, the oil discovery at Phoenix South 1, and the gas discovery at Roc 1, have been charged from source rocks inferred to occur within the Lower Triassic lower Keraudren Formation and possibly the Locker Shale (Pedley et al, 2015; Molyneux et al, 2016; Thomson et al, 2015). Modelling by Smith et al (1999), O’Brien et al (2003) and Woodside Energy Ltd (2003, 2007) also indicates that Lower Triassic source rocks should be mature in certain parts of the Rowley Sub-basin.

The Locker Shale is in part coeval with the Kockatea Shale in the northern Perth Basin. However, an equivalent of the oil-prone Hovea Member of the Kockatea Shale, which is the source of many of the onshore Perth Basin gas and oil discoveries, and the offshore oil at Cliff Head (Summons et al, 1995; Thomas and Barber, 2004; Grice et al, 2005; Edwards et al, 2013) has not yet been shown to occur in the Roebuck Basin (Molyneux et al, 2016).

The fluvio-deltaic Depuch Formation (Jurassic) includes coaly and algal-rich mudstones and pro-delta marine shales  that may have source potential. These Westralian 1 rocks may be early to late mature beneath the Oligocene carbonate wedge (Smith et al, 1999). At Huntsman 1, Woodside Energy Ltd (2007) identified the Middle Jurassic organic-rich rocks within the middle Depuch Formation (equivalent to the Athol Formation of the Northern Carnarvon Basin) as a potential gas-prone source rock with liquids potential. However this source rock was found to be immature. At Whitetail 1, Woodside Energy Ltd (2003) found that potential source rocks in the upper Depuch formation (equivalent to the Legendre Formation of the Northern Carnarvon Basin) were marginally mature for oil and immature for gas. The lack of significant hydrocarbon shows in either Jurassic or Cretaceous reservoirs suggests that Jurassic source rocks may not be effective in the Roebuck Basin. In the Bonaparte and Northern Carnarvon basins, the Westralian 2 Petroleum System is characterised by Upper Jurassic, rift-related, restricted marine source rocks (Edwards and Zumberge, 2005). Equivalent rift structures and facies were apparently not developed in the Roebuck Basin.

Potential Lower Cretaceous source rocks documented in the Browse Basin and parts of the Bonaparte Basin (Westralian 3 Petroleum System; Blevin et al, 1998a; Edwards and Zumberge, 2005), are absent or immature in the Roebuck Basin. However, Lower Cretaceous marine mudstones might be mature beneath the thickest parts of the Cenozoic carbonate wedge (Smith et al, 1999). Potential Cretaceous source rocks were found to be immature at Whitetail 1 (Woodside Energy Ltd, 2003).

A detailed fluid inclusion investigation of potential reservoirs within the offshore Canning and Roebuck basins indicates that widespread oil migration has occurred at multiple Mesozoic and Paleozoic levels (Lisk et al, 2000). Samples from multiple horizons in Bedout 1, East Mermaid 1, Keraudren 1, Lagrange 1 and Phoenix 1 were tested. Grains with oil inclusions (GOI^TM) were discovered in each well but GOI values were below 0.6%, except in Phoenix 1 where it reached 3.3%. The widespread distribution of oil inclusions led Lisk et al (2000) to propose that a lack of valid traps, rather than a lack of oil charge, was the principal reason for the discouraging results experienced prior to the oil discovery at Phoenix South 1.

Reservoirs and seals

Potential reservoirs in the Roebuck Basin occur at several stratigraphic levels, including, shoreward facies of the Triassic Keraudren Formation and Locker Shale, fluvio-deltaic sandstones of the Jurassic Depuch Formation, and sandy deltaic facies in the Lower Cretaceous (Lipski, 1993; Smith et al, 1999).

In the greater Phoenix area in the Bedout Sub-basin, which includes the Phoenix, Phoenix South, Roc and Dorado prospects, the Lower to Middle Triassic lower Keraudren Formation has been subdivided into several informal lithostratigraphic units (Carnarvon Petroleum Ltd, 2017a). The reservoir units are the Huxley Sandstone, Barret Sandstone (Phoenix South 1 oil discovery), sandstone within the Hove Shale (Phoenix South 2 oil discovery), Caley Sandstone (Roc 1, Roc 2 and Phoenix 2 South gas-condensate discoveries) and the Milne Sandstone (Roc 1 and Rock 2 oil shows). These sandstone-prone intervals are separated by mudstone-prone (Hove Shale, Baxter Shale) that act as seals. The uppermost seal is a thin mudstone interval that overlies the Barret Sandstone and defines the Palma Flooding Surface. At the yet to be drilled Dorado prospect, approximately 15 km south of the Roc area, seal to the Caley and Milne reservoirs is provided by the inferred fine-grained fill of an incision originating from the top of the Caley Formation (Carnarvon Petroleum Ltd, 2017b).

The Jurassic Depuch Formation comprises several fluvio-deltaic progradational wedges (Smith et al, 1999). Potential reservoir units that have been the subject of exploration interest (e.g. Woodside Energy Ltd, 2003, 2007) include the Legendre Formation (upper Depuch Formation), a regressive package at the base of the Athol Formation (middle Depuch Formation) and  sandstone intervals within the North Rankin Formation (basal Depuch Formation). The mudstone-prone Athol Formation equivalent provides the seal for the basal Athol and North Rankin reservoirs, while Lower Cretaceous shales (e.g. Baleine Formation) provide seal for the Legendre Reservoir.

Cretaceous deposition in the Roebuck Basin is dominated by marine mudstone, marl, and fine-grained carbonate. Smith et al (1999) inferred that these fine-grained lithologies dominated as the result of a relatively lower sediment supply to the basin during prolonged thermal subsidence. The main potential reservoir unit is the Lower Cretaceous Broome Sandstone, which is linked by Smith et al (1999) to the separation of Greater India in the Valanginian, which punctuated the otherwise thermal subsidence phase.

Timing of generation

Thermal maturity over most of the Roebuck Basin did not increase until the deposition of the thick carbonate wedge that commenced in the Oligocene; this carbonate wedge is clearly visible on seismic line s120-01 (Figure 4) and is well developed in the inboard Rowley Sub-basin. Increased source rock maturities are expected as the sediments have been exposed to increased temperatures since the onset of the carbonate wedge development, which would push source rock units into the dry gas window and possibly cause gas flushing of prospects in the area (Smith et al, 1999). The lower Keraudren Formation currently lies within the peak-oil maturity window, as indicated by vitrinite reflectance values of 1.1% Ro from East Mermaid 1, with subsequent expulsion occurring between the Oligocene and the Quaternary due to loading from the regional carbonate wedge (Smith et al, 1999; Pathfinder Energy Pty Ltd, 2015).

Petroleum systems modelling by Woodside Energy Ltd (2007) for Triassic and Middle Jurassic source rocks in the outer Rowley Sub-basin demonstrated that the main phase of hydrocarbon generation took place during the Jurassic, with expulsion occurring at a slower rate during the Cretaceous, and with no significant hydrocarbon generation during the Cenozoic.

The Roebuck Basin is one of the least explored regions of the North West Shelf. The initial phase of seismic exploration in the Roebuck Basin was between the late 1960s to 1982. Approximately 15 surveys were run during this time, focusing on the Bedout Sub-basin and inboard Rowley Sub-basin. During the period from 1986 to 1994, predecessor organisations of Geoscience Australia ran several regional seismic lines across the Roebuck Basin. Over this same period, six industry seismic surveys were conducted, including tightly spaced seismic grids in the Bedout and Rowley sub-basins. Later activity included several surveys conducted between 1998 and 2001.

2D seismic surveys acquired in recent years are NWS Australia Geostreamer (PGS, 2017) and WestraliaSPAN 2D (ION, 2017). These are deep regional surveys that extend across the North West Shelf and each includes several lines within the Roebuck Basin. As of January 2017 the entire 14 000 km Bilby non-exclusive 2D seismic survey over the central Bedout Sub-basin and offshore Canning Basin was made available (Searcher Seismic, 2017).

In the Rowley Sub-basin, 3D seismic survey acquisition commenced in 2001 with Small 3D grids that delineated drilling prospects for Huntsman 1 and Whitetail 1. Between 2012 and 2014, Polarcus Limited, on behalf of Woodside Petroleum Ltd, completed the Lord 3D, Admiral 3D and Curt 3D surveys across the outer Rowley Sub-basin.

In the Bedout Sub-basin, acquisition of the 1100 km² Phoenix MC3D multiclient survey was completed over the greater Phoenix Area in 2011 (Spectrum, 2017a). The 3854 km² Zeester MC3D 3D multiclient seismic survey, located north of Phoenix, was acquired in 2011/2012 (Spectrum, 2017b). In 2015, Polarcus Seismic Limited acquired the 22 385 km² Capreolus high-resolution broadband 3D multi-client survey that straddles the boundary between the Northern Carnarvon and Roebuck Basins (Pedley et al, 2015). In the Roebuck Basin, the Capreolus survey covers the greater Phoenix area in the Bedout sub-basin, and extends into the south-western Rowley Sub-Basin.

In 2010 Fugro Airborne Surveys acquired a 21 622 line km aeromagnetic survey in the Bedout Sub-basin on behalf of joint venture partners Carnarvon Petroleum Ltd and Finder Exploration. The survey was designed to provide information about the tectono-structural evolution of the sub-basin, depth to magnetic basement, and an estimation of total sediment thickness (Chowdhury et al, 2012).

Geoscience Australia conducted a hydrocarbon seepage survey of the Roebuck and offshore Canning basins in June 2006 (Jones et al, 2007). No definitive evidence of natural hydrocarbon seepage was detected. Head-space gas analyses of gravity core sub-samples, and biomarker screening of sediments and carbonate concretions, revealed no thermogenic hydrocarbons or biomarkers diagnostic of methane oxidation. Re-evaluation by Logan et al (2010) of vertical seismic features distributed throughout the region, with a strong clustering over the Bedout High, suggests that they more likely represent zones of either small-scale faulting or fracture with little or no displacement, while velocity pull-ups are associated with late Miocene channels, rather than hydrocarbon related diagenetic zones and gas chimneys as previously interpreted by O'Brien et al (2003).

Only sixteen wells have been drilled in the Roebuck Basin. An initial phase of drilling occurred between 1970 and 1974, with drilling in both the Bedout Sub-basin (Bedout 1, Keraudren 1 and Minilya 1) and Rowley Sub-basin (East Mermaid 1). Three further wells were drilled in the Bedout Sub-basin (Phoenix 1, Phoenix 2, and Lagrange 1) between 1980 and 1983. Subsequent drilling in the Rowley Sub-basin has tested the deep-water potential with the following wells: Whitetail 1 (2003), Huntsman 1 (2006), Hannover South 1 (2014), Steel Dragon 1 (2014) and Anhalt 1 (2015). Recently, the Bedout Sub-basin has received renewed interest because of the oil discovery at Phoenix South 1 (2014), and gas-condensate discoveries at Roc 1 (2015), Roc 2 (2016) and Phoenix South 2 (2017).

In Phoenix South 1 at least four oil columns, ranging in thickness from 26 m to 46 m, were discovered (Finder Exploration Ltd, 2016). This discovery has opened up the first new oil play on the North West Shelf in 25 years. In light of this discovery it is now believed that the Phoenix 1 well had intersected an unrecognised significant gross oil pay, and was incorrectly classified as a gas well, and that Phoenix 2 was drilled outside of closure (Finder Exploration Ltd, 2016).

Roc 1 (2015) was designed to target the same lower Keraudren Formation reservoir (i.e. Barret Sandstone) as the oil discovery at Phoenix South 1 (Carnarvon Petroleum Ltd, 2016b). The Barret Sandstone was water wet but liquids rich gas pay was discovered in the deeper secondary target in the Caley Sandstone. In addition, the Roc 1 well exhibited excellent oil shows in the upper Keraudren Formation reservoir, and reservoir quality sandstones that extended to TD.

The Roc 2 (2016) well was designed to appraise the Roc 1 discovery in the Caley section, and to test the Milne and Huxley intervals where oil shows were recorded at Roc 1 (Carnarvon Petroleum Ltd, 2016c). The well flow tested at equipment-constrained rates up to 51.2 MMscf (1.45x10⁶ m³) per day of gas and 2943 bbl (467.9 m³) of condensate per day (Carnarvon Petroleum Ltd, 2016d).

Phoenix South 2 was drilled by Quadrant Energy in late 2016 to appraise the Phoenix South 1 oil discovery and evaluate the Roc 1 and Roc 2 gas-condensate discoveries (Carnarvon Petroleum Ltd, 2016e). The well intersected an estimated 39 m gas-condensate zone between 5176 m and 5215 m within the Caley reservoir. Drilling ceased at 5215 m because of high formation pressure, and as much as 185 m of Caley reservoir below this depth could not be assessed (Carnarvon Petroleum Ltd, 2016f).

In March 2017, Carnarvon Petroleum Ltd (2017b) announced that planning had commenced for Dorado 1, to be drilled approximately 15 km to the south of the Roc 1 and Roc 2 discoveries. The well will target Barret Sandstone, Caley Sandstone and Milne Sandstone reservoirs all within the lower Keraudren Formation. The spud date is expected be in late 2017. The Dorado structure comprises lower Keraudren Formation reservoir facies sealed by fine-grained submarine canyon fill (Carnarvon Petroleum Ltd, 2017c).

In March 2017, 3D Oil Ltd was awarded WA-527-P (formerly W16-8; Figure 1), located to the east of the greater Phoenix area and to the immediate southeast of Release Area W17-4 (3D Oil Ltd, 2017). The company plans to attract a farm-in partner to the permit.

The Phoenix and Roc discoveries are proximal to Western Australia’s main gas pipeline at Port Hedland, the Fortescue River gas pipeline that links to Pilbara iron ore mines, and to LNG export facilities (Carnarvon Petroleum Ltd, 2017c).

Introduction

This section provides an overview of the environmental characteristics of the Release Area and the general setting of the region overlying the Roebuck Basin (Figure 7), together with information focused on the marine and environmental characteristics of the Release Area.

The Roebuck Basin is located on the northwest margin of Australia and primarily includes continental slope with terrace and ridge geomorphological features and is located offshore from the Kimberley region.

Port Hedland is the closest industrial port and Broome is the closest major town.

Climate of the region

Climate statistics for Port Hedland indicate a mean annual rainfall of 315.5 mm and mean annual high and low temperatures of 33.3°C and 19.5°C, respectively (average of 69 years, www.bom.gov.au). Mean maximum and minimum temperatures and precipitation for Barrow Island are similar; 30.2°C and 21.5°C, and 318.2 mm of rainfall per annum (average over 22 years). Approximately half of the precipitation in this coastal region is associated with tropical cyclones, and most precipitation occurs between December and June (www.bom.gov.au). Tropical cyclones often occur, most commonly in January and February. The region between Broome and Exmouth is the most cyclone-prone region of the Australian coastline, and has the highest frequency of cyclone crossings each year (~2 per annum) (www.bom.gov.au). Approximately 22 cyclones have affected Broome since 1910, approximately one every five years. The impacts of tropical cyclones include extreme waves, and higher than normal wind-driven currents.

Oceanic regime

The offshore marine environment of the Roebuck Basin is strongly influenced by the seasonal (May to September), largely wind-driven, Holloway current (Wilson, 2013). This current comes from the Indonesian Throughflow Current (ITC), and combines with the Eastern Gyral Current in the vicinity of the Rowley Shoals to form the headwaters of the Leeuwin Current (Wilson, 2013). Rowley Shoals Islands form the southern portion of the marginal Scott Reef–Rowley Shoals platform that extends southwesterly in water depths of 200–600 m (McLoughlin et al, 1988). Water temperature is approximately 24°C adjacent to Rowley Shoals, with little annual variation.

Sea surface temperatures in this region are comparatively warm for Australian waters (around 27°C) and are predicted to increase by 0.2°C per decade in the immediate future (Foster et al, 2014). Internal tides affect water depths of 50–150 m, while shore-normal saline bottom flow, that likely varies seasonally in intensity, affects shallower coastal seabed (Collins et al, 2014). Conductivity, Temperature and Depth (CTD) data indicates there may be four different water masses in this region, with bottom water temperatures of 7°C at 670 m depth (Jones et al, 2007).

Seabed environments: regional overview

The surface of the Roebuck Basin comprises gently sloping to steep continental slope and shelf, with several terraces aligned sub-parallel to the modern coastline (Heap and Harris, 2008). Biogenic reef growth, initiated in the Miocene (Jones, 1973; Ryan et al, 2009) is ongoing on three topographic highs in the Rowley Shoals on the outer shelf ramp (Mermaid, Clerke and Imperieuse: Ryan et al, 2009; Collins, 2010), where growth has kept pace with subsidence. These shelf edge atolls exhibit central lagoons and steep to vertical perimeter walls (Wilson, 2013).

Water depths are 230 m at Imperieuse Reef, 390 m at Clerke Reef and 440 m at Mermaid Reef. Tidal range is reported as 4.5 m at the offshore shelf reef edges (Wilson, 2013)

Sediment grain size distribution across the north-west shelf region has been modelled by the National Environmental Research Program Biodiversity Hub (2017a, 2017b and 2017c). Sediment size across the Roebuck Basin is distinctly different between the areas seaward and shoreward of the Rowley Shoals (McLoughlin et al, 1988). This division essentially coincides with the boundary of two internal sub-basins: the more landward Bedout Sub-basin and more seaward Rowley Sub-basin. The seafloor of the Bedout Sub-Basin is characterised by gravelly sand (>50% sand, with up to 30% gravel and only minor mud). In contrast, the seabed of Rowley Sub-Basin, which includes the Rowley Shoals, is typically sandy mud (>60% mud and <30% sand), although the bed material coarsens markedly towards the shoreward limit of the sub-basin, in the vicinity of the Shoals (which are fringed by gravelly, muddy sand; Passlow et al, 2005). Away from the reefs, sediment in this area is likely to be dominated by pelagic facies, commonly as green planktonic foraminifera–pteropod-rich carbonate sand or muddy carbonate sand (James et al, 2004; Jones et al, 2007). Riverine-derived sediments are restricted to the inner ramp (James et al, 2004).

Ryan et al (2009) examined the triggers for reef development in this area and were unable to find any direct evidence that active or ancient hydrocarbon seepage has triggered or controlled growth of the Rowley Shoals reefs (or their buried Miocene predecessors). Sand waves, with crests oriented roughly perpendicular to the local tidal currents, and fluid expulsion features (pockmarks) have been identified immediately to the east of the Roebuck Basin, on the Offshore Canning Basin (Jones et al, 2007, 2009). Jones et al (2009) defined two types of sand waves in the region: a low-amplitude composite form comprising dunes and ripples, and a more laterally restricted form with high bedforms and high surface slopes. The latter were associated with seabed fluid flow. Pockmarks have also been identified ~50 km to the north east on the Leveque Shelf and their formation at both of these locations has been attributed to the expulsion of seawater at the seabed, driven by tidal pumping through the shallow sub-surface (Nicholas et al, 2016).

Ecology

The Roebuck Basin is part of the North-West Marine Bioregion which extends from the Western Australia-Northern territory border to Kalbarri, south of Shark Bay. This region has comparatively low rates of endemism and high biodiversity and is dominated by Indo-west Pacific tropical flora and fauna; it is also home to globally significant populations of internationally threatened species (Commonwealth of Australia, 2008). At the smaller scale of provincial bioregions (IMCRA 4.0), the Roebuck Basin is included in the Northwest Transition, which is an area of overlap (i.e. it does not have distinct fauna, flora and ocean conditions compared to adjacent areas; Commonwealth of Australia, 2005). The deep ocean seafloor of the Roebuck 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). The Roebuck Basin includes part of the north-west slope which has been shown to support the second richest demersal fish assemblage in Australia (Last et al, 2005).

Recent geological history

The Roebuck Basin was previously (prior to 1994) known as part of the Offshore Canning Basin, and is located on the central North West Shelf between the Northern Carnarvon and Browse Basins and outboard of the offshore portion of the predominantly Paleozoic Canning Basin. It has recently been proposed that a seismically active fault system deforms Pliocene to Holocene strata across the Browse, Roebuck and Carnarvon basins (Hengesh and Whitney, 2015). Buried Miocene reefs provide a major stratigraphic marker in this region over which more recent stratified sediment has been deposited. During the Miocene, the Rowley Shoals area formed a narrow continental shelf. Post-Miocene subsidence has occurred in this area, and since the last interglacial interval of higher than present sea level, differential subsidence of the Rowley Shoals reefs has occurred (Collins, 2002). Apart from reef areas, the seabed would not have been exposed during intervals of lower sea-level during the Quaternary, and the presence of stratified sediment layers overlying the Miocene reefs appear to support this. However, unconformities are present in these young strata, most likely the result of changing oceanographic conditions (Jones et al, 2007). Additionally, shallow sub-bottom seismic data indicates that in places, recent, possibly Quaternary, fault movement has occurred (Jones et al, 2007).

Commonwealth Marine Reserves

Argo-Rowley Terrace Marine Reserve

https://www.environment.gov.au/topics/marine/marine-reserves/north-west/argo-rowley-terrace

Major Conservation Values:

• Important foraging areas for migratory seabirds and the endangered Loggerhead Turtle
• Important area for sharks, which are found in abundance around the Rowley Shoals relative to other areas in the region
• The reserve provides protection for the communities and habitats of the deeper offshore waters of the region in depth ranges from 220 m to over 5000 m
• The reserve provides protection for many seafloor features including aprons and fans, canyons, continental rise, knolls/abyssal hills and the terrace and continental slope
• Examples of the communities and seafloor habitats of the Northwest Transition and Timor Province provincial bioregions
• The reserve provides connectivity between the existing Mermaid Reef Marine National Nature Reserve and reefs of the Western Australian Rowley Shoals Marine Park and the deeper waters of the region

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

Mermaid Reef Commonwealth Marine Reserve

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

Major Conservation Values:

• Mermaid Reef has national and international significance due to its pristine character, coral formations, geomorphic features and diverse marine life.
• Key area for over 200 species of hard corals and 12 classes of soft corals with coral formations in pristine condition
• Important areas for sharks including the Grey Reef Shark, the Whitetip Reef Shark and the Silvertip Whaler
• Important foraging area for marine turtles
• Important area for toothed whales, dolphins, tuna and billfish
• Important resting and feeding sites for migratory seabirds
• The reserve, along with nearby Rowley Shoals Marine Park, provides the best geological example of shelf atolls in Australia
• Examples of the seafloor habitats and communities of the Northwest Transition.

The reserve is currently zoned as a Marine National Park IUCN Category Ia Strict Nature Reserve. Management plans for the North-west Commonwealth marine reserves network are not yet in place.

Eighty Mile Beach Commonwealth Marine Reserve

https://www.environment.gov.au/topics/marine/marine-reserves/north-west/eighty-mile-beach

Major Conservation Values:

• Foraging areas adjacent to important breeding areas for migratory seabirds
• Foraging areas adjacent to important nesting sites for marine turtles
• Includes part of the migratory pathway of the protected Humpback Whale
• Adjacent to important foraging, nursing and pupping areas for Freshwater, Green and Dwarf sawfish
• The reserve provides protection for the shelf, including terrace, banks and shoal habitats, with depths ranging from 15 m to 70 m
• Examples of the communities and seafloor habitats of the Northwest Shelf Province provincial bioregion and the Canning, Northwest Shelf, Pilbara (nearshore), Pilbara (offshore) and Eighty Mile Beach meso-scale bioregions

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

Kimberley Commonwealth Marine Reserve

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

• Important foraging areas for migratory seabirds, migratory Dugong, 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 the Green Turtle.
• 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
• Two key ecological features are included in the reserve. These are: ancient coastline (an area of enhanced productivity attracting baitfish which, in turn, supply food for migrating species); continental slope demersal fish communities (the second richest area for demersal fish species in Australia)

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

Western Australian Marine Parks

Eighty Mile Beach Marine Park

Eighty Mile Beach Marine Park is one of the world's most important feeding grounds for small wading birds that migrate to the area each summer. The marine park is a major nesting area for the Flatback Turtle. Sawfish, Dugong, dolphins and millions of invertebrates inhabit the sand and mud flats, seagrass meadows, coral reefs and mangroves. A management plan is in place for this park.

Fisheries

The following Commonwealth fisheries occur in the Roebuck Basin area:

• North West 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, starting on the 1 July. The boundary of the Commonwealth Western Deep Trawl Fishery, and North West Shelf Fishery and Western Australian State fisheries has been amended and the boundary runs through the release area.(http://www.afma.gov.au/fisheries/north-west-slope-trawl-fishery/)
• Western Deepwater Trawl Fishery, operated in waters deeper than 200 m, offshore Exmouth to Augusta. The fishing season runs for 12 months, beginning 1 July. The boundary of the Commonwealth Western Deep Trawl Fishery, and North West Shelf Fishery and Western Australian State fisheries has been amended and the boundary runs through the release area. (http://www.afma.gov.au/fisheries/western-deepwater-trawl-fishery/)
• Southern and Blue Fin Tuna Fishery season runs for 12 months, starting 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 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/)

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

Heritage

There are no shipwrecks listed in the release area. However, the Lively, a sailing ship wrecked in 1810, is located near Mermaid Reef at -17.0896833, 119.5915. 

The Dampier Archipelago and Murujuga, also known as the Burrup Peninsula, are National Heritage listed. http://environment.gov.au/heritage/heritage-places

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

Argo-Rowley Terrace Commonwealth Marine Reserve https://www.environment.gov.au/topics/marine/marine-reserves/north-west/argo-rowley-terrace

Mermaid Reef Marine Reserve http://www.environment.gov.au/topics/marine/marine-reserves/north-west/mermaid-overview

Eighty Mile Beach Commonwealth Marine Reserve http://www.environment.gov.au/topics/marine/marine-reserves/north-west/eighty-mile-beach-maps

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

Western Australian Eighty Mile Beach Marine Park https://www.dpaw.wa.gov.au/management/marine/marine-parks-and-reserves/ https://parks.dpaw.wa.gov.au/park/eighty-mile-beach/https://www.dpaw.wa.gov.au/images/documents/parks/management-plans/decarchive/FINAL-WEB_READY_EIGHTY_MILE_BEACH_MGT_PLAN_V12.web_up_loadpdfhi-res-maps2.pdf

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

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

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

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