Regional Geology of the Roebuck Basin

The Roebuck Basin is largely underexplored; however, it has experienced recent exploration success with the discovery of oil at Phoenix South 1 and gas at Roc 1 in the Bedout Sub-basin. Exploration is continuing within the basin, and the existence of an active petroleum system has been proven in the Bedout Sub-basin and future production is likely. Current 3C contingent resources for the Bedout Sub-basin, based only on the resource assessments for Phoenix 1, Phoenix South 1 and Roc 1, are estimated at 78 MMbbls of crude oil, 18 MMbbls of condensate, and 372 Bcf of gas (DeGolyer and MacNaughton, 2015; Carnarvon Petroleum Ltd, 2016a).

Basin outlineThe Roebuck Basin forms the central part of the Westralian Superbasin, and covers approximately 93 000 km2 on the North West Shelf between the Northern Carnarvon Basin to the south and the Browse Basin to the north (Figure 1). The inboard part of the Roebuck Basin abuts a major northwest-trending intracratonic basin (offshore Canning Basin) of Paleozoic age (Colwell and Stagg, 1994). The basin is subdivided into the Bedout Sub-basin in the south and the Rowley Sub-basin in the north which are separated from one another by the Bedout High. The geology of the Roebuck Basin is described by Lipski (1993; 1994), Colwell and Stagg (1994), Smith (1999), Smith et al (1999), and the adjacent onshore Canning Basin is described by Drummond et al (1991), Colwell and Stagg (1994), Hocking et al (1994), Kennard et al (1994), Smith et al (1999), Jones et al (2007), Department of Mines and Petroleum (2014), and Totterdell et al (2014).

The Bedout Sub-basin consists of an east-northeast to west-southwest-trending Mesozoic depocentre (Figure 1) 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 southeast and northeast by the offshore extension of the Willara Sub-basin and Broome Platform (both Canning Basin), respectively (Figure 1).

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 and is associated with a Moho uplift of approximately 7–9 km. Lower to Middle Triassic sediments onlap the feature from all directions and approximately 3 km of Upper Triassic–Holocene sediments are draped over the top. The upper surface of the Bedout High is a peneplain approximately 30 km wide (Smith et al, 1999; Müller et al, 2005).

The Rowley Sub-basin is situated on the outer continental shelf where it covers an area of approximately 66 000 km2 (Figure 1). It contains about 9 km of Permo-Carboniferous or older strata and up to 6 km of Mesozoic–Holocene sediments (Smith et al, 1999). Structurally, the sub-basin is separated from the Beagle Sub-basin and Exmouth Plateau (of the Northern Carnarvon Basin) to the southwest by the North Turtle Hinge Zone and Thouin Graben (Figure 1) and from the Oobagooma Sub-basin (Canning Basin) to the east by the Oobagooma High. To the northeast, the Rowley Sub-basin is separated from the Browse Basin by the northwest-trending Leveque Ridge.

Seismic section locations are shown in Figure 1 and well locations in Figure 2. Gravity trends of the Roebuck Basin are shown in Figure 3, and the regional stratigraphy is given in Figure 4. The location of permits and their operators are given in Figure 5. Regional seismic lines are shown in Figure 6, Figure 7 and Figure 8. Figure 9 shows the location of Commonwealth Marine Reserves and key ecological features.

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Tectonic developmentSeismic interpretation indicates that the Roebuck Basin was initiated in the Ordovician as a result of intraplate extension, as was the adjacent offshore Canning Basin. This was followed by multiple phases of rifting with intervening periods of basin sag during the Paleozoic and Mesozoic, and the formation of a passive margin from the Late Cretaceous onward. Deposition within the offshore Canning and Roebuck basins was punctuated by rift-related uplift and compressional events (Kennard et al, 1994; Smith et al, 1999). The major tectonic events and their resultant unconformities divide the basin-fill into a series of packages, which are listed below. The post-Devonian stratigraphy of the Roebuck Basin and offshore part of the Canning Basin is summarised in Figure 4 (Smith, 1999; Smith et al, 1999; Nicoll et al, 2009; Smith et al, 2013).

The Roebuck Basin developed as a result of a multiphase rift history under four main stress regimes (Smith, 1999):

Cambrian to Silurian northeast-southwest extension resulting in an intra-cratonic fracture sequence associated with the separation of the Chinese blocks from Gondwana.

Late Carboniferous to late Permian transitional phase from northeast-southwest to north-northwest–south-southeast extension associated with the separation of the China-Burma-Malaya-Sumatra (SIBUMASU) blocks, with development of both northwest and east-northeast-trending structures.

Post late Permian north-northeast–south-southwest extensional phase resulting in the formation of the Westralian Superbasin succession (separation of Argoland and India).

Middle Miocene to Holocene northeast-southwest compression phase during convergence with South-east Asia.

Rowley Sub-basinDuring the Triassic, the Rowley Sub-basin was part of a broad intracratonic downwarp that also encompassed the Northern Carnarvon and Browse basins. A thick succession of marine claystones (Locker Shale) was deposited and overlain by a thick fluvial to marginal marine succession (Keraudren Formation) (Lipski, 1994). During the Jurassic pre-breakup phase, the Rowley Sub-basin was a subsiding depocentre; however, deep marine troughs similar to the Lewis, Cossigny and Beagle troughs did not develop due to the relatively gentle pre-breakup structuring in the Roebuck Basin.

Bedout Sub-basinThe structural architecture of the Bedout Sub-basin is dominated by east-southeast-trending faults representing the development of a discrete rift depocentre during the Paleozoic. North to north-northeast-trending structures are associated with deformation and volcanism during the Permian–Triassic Bedout Movement (Becker et al, 2004). This established depocentres in which thick Permo-Triassic sediments accumulated (Chowdury, 2012). Interpretation of aeromagnetic data shows variations in sediment thicknesses of up to 5 km over relatively short horizontal distances in the vicinity of the Bedout High, and a significant shallowing of the basement towards the Lambert Shelf in the south (Chowdury, 2012).

Basin evolutionLittle is known about the nature of Paleozoic sedimentation offshore as no wells in the Roebuck Basin have penetrated this succession. However, the Carboniferous to Permian clastic succession of the Oobagooma Sub-basin was encountered at the base of Wamac 1 and Lacepede 1A. A succession of Paleozoic sediments is interpreted to onlap the Lambert Shelf, Broome Platform and Bedout High, and is presumed to be an offshore extension of the onshore Canning Basin (Passmore, 1991; Lipski, 1993; Colwell and Stagg, 1994; Smith, 1999).

The first extensional event recognised in the region is a northeast–southwest extension in the Ordovician, related to the separation of continental blocks from the North West Shelf. This was followed by north–south compression and uplift (Prices Creek Movement) in the Early Devonian. Three northeast–southwest extensional events occurred in the Late Devonian to Mississippian (early Carboniferous). The north-northwest–south-southeast oblique-slip reactivation of pre-existing structures, termed the Meda Transpression, terminated this phase of deposition (Kennard et al, 1994; Smith et al, 1999).

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The latest Devonian to early Carboniferous undifferentiated Fairfield Group is believed to be present in the Roebuck Basin. The Fairfield Group is a mixed carbonate and siliciclastic marine succession with demonstrated source and reservoir potential on the Lennard Shelf of the onshore Canning Basin (Crostella,1998), with commonly associated oil and gas shows. The most prospective Fairfield Group source in the onshore Canning Basin is the Laurel Formation. Onshore, the Fairfield Group is absent south of the terraces that border the southern Fitzroy Trough and Gregory Sub-basin (Haines and Ghori, 2010).

Initiation of the Westralian Superbasin in the Pennsylvanian (late Carboniferous) was characterised by a change from northwest-oriented structures associated with the Canning Basin, to the predominantly northeast-oriented structures of the Roebuck Basin. This was a period of transition from a Paleozoic regime of northeast–southwest intracratonic extension to one of northwest–southeast extension related to the separation of the Sibumasu Terrane from Gondwana (Metcalfe, 1988; Smith et al, 1999). In parts of the Canning Basin, syn-rift sedimentation continued along reactivated intracratonic fractures that formed during the northeast–southwest extension. Pennsylvanian (upper Carboniferous) fluvial deposits are overlain by a thick succession of Permian glacial deposits (Grant Group), in turn overlain by Permian marine and fluvio-deltaic clastic rocks (Poole Sandstone, Noonkanbah Formation, Liveringa Group). The upper Permian Bedout Volcanics unconformably overlie the Liveringa Group and are well developed on the Bedout High. From open file mudlog interpretation of the recently drilled Woodside Energy Ltd well, Anhalt 1, it appears that the Bedout Volcanics may also be present in the outer Rowley Sub-basin. The complex nature of the basin fill in the offshore Canning and Roebuck basins during this initial evolutionary stage is poorly understood. Overlying the Paleozoic sediments is a prominent regional unconformity, possibly related to the formation of the Bedout High (Colwell and Stagg, 1994).

The Triassic–Early Jurassic period was dominated by thermal sag with transgressive marine and fluvio-deltaic sedimentation (Locker Shale, Keraudren and Bedout formations). Extrapolating from open file data obtained from the recent Anhalt 1 well, it is possible that the Locker Shale is absent in the outer Rowley Sub-basin. Separating the lower and upper Keraudren Formation is the Middle Triassic Cossigny Member, a regionally widespread limestone unit seismically expressed as a high-amplitude reflector. Triassic to Early Jurassic deposition was punctuated by a series of northwest–southeast transpressional events (Fitzroy Movement) that were focused along the margins of the sub-basins. Smith et al (1999) identified three phases of the Fitzroy Movement in the Roebuck Basin:

Fitzroy Movement I (Ladinian) is responsible for large transpressional “flower structures” along the North Turtle Hinge Zone;

Fitzroy Movement II (Norian) is responsible for major en-echelon anticlines in the Fitzroy Trough, and a subtle unconformity in the Phoenix 1 and Phoenix 2 wells; and

Fitzroy Movement III (Sinemurian) marked a major change in gross stratal geometries within the Roebuck Basin from predominantly back-stepping to prograding and aggrading.

Before the drilling of Anhalt 1, Hannover South 1 and Steel Dragon 1, the only well to intersected the oldest part of the succession in the Rowley Sub-basin was Huntsman 1 when it drilled into the Rhaetian (uppermost Triassic) Bedout Formation (Brigadier Formation equivalent). Huntsman 1 is located in the western part of the Rowley Sub-basin and reached total depth within the mudstone-dominated, marine to marginal marine Bedout Formation (Figure 4; Woodside Energy Ltd, 2007). This result, and correlation with wells drilled in the adjacent Bedout and Beagle sub-basins, provides an age-constraint for the post-rift (rapid thermal sag) succession unconformably overlying the basal half graben.

Following Early Jurassic uplift and erosion, a broad prograding wedge of fluvio-deltaic sediments (Depuch Formation) was deposited during thermal subsidence across the shelf. Continental breakup of northwestern Australia and Argo Land during the Callovian (Müller et al, 1998; Veevers et al, 1991) resulted in a second phase of prominent uplift and erosion that marked the end of active rifting in areas adjacent to the Roebuck Basin. Subsequent thermal subsidence drove a rapid transgression and the accumulation of condensed marine mudstones (Baleine and Egret formations) until the Early Cretaceous. An influx of siliciclastic material (Broome Sandstone) occurred with further uplift of the sediment source in the Valanginian, when Greater India moved away from the western margin of Australia (Smith, 1999; Smith et al, 1999).

Thermal relaxation of the crust soon after the Valanginian break-up, led to the development of a passive margin succession of marine mudstones and marls. Full oceanic circulation was established by the end of the Aptian. Reactivation of some Paleozoic structural features, possibly related to the separation of Antarctica and Australia and northward drift of the Australian Plate, resulted in inversion and oblique slip movement, especially in the adjacent Oobagooma Sub-basin (Smith et al, 1999). A major progradational carbonate wedge developed across the entire North West Shelf during the Cenozoic. Convergence of the Australian and Eurasian plates in the middle Miocene led to transpressional inversion of north-northwest-trending Paleozoic faults in the northeast Oobagooma Sub-basin.

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Regional hydrocarbon potentialRegional petroleum systemsSeveral potential petroleum systems may operate in the Roebuck and offshore Canning basins (Bradshaw, 1993; Bradshaw et al, 1994; Kennard et al, 1994; Smith 1999; Smith et al, 1999). These are:

Ordovician Larapintine 2: Within the Ordovician section of the onshore Canning Basin, organic-rich marine shales are well documented (Taylor, 1992; Kennard et al, 1994; Ghori, 2013). These oil-prone Ordovician source rocks are characterised by the occurrence of the organism Gloeocapsomorpha prisca, and it is suspected that similar successions occur in the offshore Canning Basin and eastern Roebuck Basin

Devonian Larapintine 3: Source rocks of the Larapintine 3 Petroleum System are probably absent in the Roebuck Basin, but may be present in the offshore Canning Basin

Carboniferous Larapintine 4: Source rocks of the Larapintine 4 Petroleum System are probably absent in the Roebuck Basin, but may be present in the offshore Canning Basin

Pennsylvanian–Permian Gondwanan 1: 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, Rowley Sub-basin, Oobagooma Sub-basin and offshore Willara Sub-basin.

Triassic to Early–Middle Jurassic Westralian 1: 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). 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 rocksIn the onshore Canning Basin, Ordovician source rocks are particularly well developed on the terraces along the northern flank of the onshore Broome Platform, but generally have poor source rock quality within the offshore Willara Sub-basin (Kennard et al, 1994). Sub-salt organic-rich shales of the Ordovician Bongabinni Formation, on the northern margin of the offshore Willara Sub-basin, locally have excellent source rock quality but their offshore extent is unknown (Kennard et al, 1994; McCracken, 1994). These potential Ordovician source facies are likely to be mature in the offshore Willara Sub-basin/Samphire Embayment where the system lies just beneath the inboard Bedout Sub-basin. The Larapintine 3 and Larapintine 4 petroleum systems are considered absent in the Roebuck Basin; however, both systems may exist within the offshore Canning Basin. The marine shales at the base of the Devonian Pillara Sequence (Larapintine 3) and the lower Laurel Formation (Larapintine 4) have fair to good generative potential and are inferred to be present throughout the Oobagooma Sub-basin (Kennard et al, 1994).

Source rocks of the Gondwana 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. Hydrocarbon accumulations and shows within the Grant Group are believed to be sourced from the underlying Laurel Formation within the Fairfield Group (Goldstein, 1989; Kennard et al, 1994; Edwards et al, 2013).

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The Lower to Middle Triassic transgressive marine shales of the Locker Shale and the overlying Middle Triassic lower Keraudren Formation, are potential source rocks in the Roebuck Basin, in particular in the Bedout Sub-basin. The Locker Shale in Phoenix 1, Phoenix 2 and Keraudren 1 exhibit oil- and gas-prone source rock facies (Smith et al, 1999), whereas Anhalt 1, recently drilled in the outer Rowley Sub-basin, did not confirm the presence of Lower Triassic source rocks. The Locker Shale is in part age equivalent to the Kockatea Shale in the northern Perth Basin. However, 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 proven to occur in the Bedout Sub-basin. The newly 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 presumed to occur within the Lower Triassic lower Keraudren Formation and possibly the Locker Shale (Pedley et al, 2015; Molyneux et al, 2015; Thomson et al, 2015). These source rocks are early mature to marginally mature in both Phoenix 1 and Keraudren 1, and maturity analyses are pending for other wells. In addition, basin modelling indicates that these postulated Lower Triassic source rocks may be presently expelling hydrocarbon liquids in the outer Rowley Sub-basin and on the flanks of the Bedout High (O’Brien et al, 2003). However, the lack of hydrocarbon shows in wells on the Bedout High (Bedout 1 and Lagrange 1) and within the Rowley Sub-basin (Anhalt 1, Steel Dragon 1, Hannover South 1, East Mermaid 1, Whitetail 1 and Huntsman 1) suggests that these source rocks are either not generating hydrocarbons or are not present in these areas.

The fluvio-deltaic Lower–Middle Jurassic sediments of the Roebuck Basin contain source rocks placed within the Westralian 1 Petroleum System (Bradshaw, 1993; Bradshaw et al, 1994). The Nebo 1 oil discovery in the adjacent Beagle Sub-basin is presumed to be from an equivalent Lower–Middle Jurassic deltaic coaly or lacustrine mudstone source rock (Edwards and Zumberge, 2005), and is reservoired in the Callovian Calypso Formation (equivalent to upper Depuch Formation in the Roebuck Basin). However, the lack of significant hydrocarbon shows in either Jurassic or Cretaceous reservoirs suggests that this petroleum system 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); however, equivalent rift structures and facies were apparently not developed in the Roebuck Basin. Likewise, potential Lower Cretaceous source rocks documented in the Browse Basin and parts of the Bonaparte Basin (Westralian 3 Petroleum System; Blevin et al, 1998; Edwards and Zumberge, 2005), are absent or immature in the Roebuck Basin.

A detailed fluid inclusion investigation of potential reservoirs within the offshore Canning and Roebuck basins suggested that widespread oil migration has occurred at multiple Mesozoic and Paleozoic levels (Lisk et al, 2000). Samples from multiple horizons in key wells in the region (Bedout 1, East Mermaid 1, Keraudren 1, Lagrange 1 and Phoenix 1) were tested, and grains with oil inclusions (GOITM) 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 sealsPotential reservoirs in the Roebuck Basin occur at several stratigraphic levels, including sandstones within the Permian Grant Group, shoreward facies of the Triassic Keraudren Formation and Locker Shale, fluvio-deltaic channel sandstones and shoreline sandstones of the Jurassic Depuch Formation, and sandy deltaic facies in the Lower Cretaceous that developed during the final continental breakup in inboard areas of the basin (Lipski, 1993; Kennard et al, 1994; Smith et al, 1999). The Keraudren Formation and Locker Shale are likely to have higher porosity and permeability in the more shallowly buried areas where there is less potential for secondary carbonate and silica precipitation (Lipski, 1993). Regionally, reservoirs at less than 2000 m display high net-to-gross sand with up to 1000 mD permeability and an average porosity of 20% (Molyneux et al, 2015).

In Huntsman 1, average porosities ranged from 20.8% in Lower–Middle Jurassic marine facies (Athol Formation equivalent) to 29.9% in Middle Jurassic deltaic sandstones (Depuch or Legendre Formation) with net-to-gross values around 75% (Woodside Energy Ltd, 2007).

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The oil at Phoenix South 1 was encountered in the Lower to Middle Triassic Keraudren Formation over a 315 m gross interval, with a combined 151 m oil column (59 m net oil pay) in predominantly fine to medium grained sandstones (Carnarvon Petroleum Ltd, 2015). The depositional environment for the lower Keraudren Formation reservoir was interpreted as fluvial channel sands to nearshore marine and pro-deltaic sequences. Multidisciplinary analyses (e.g. palynology, organic and inorganic geochemistry, sedimentology, log interpretation) were conducted by Quadrant Energy and Joint Venture partners, which confirmed the presence of a thick succession of non-marine to marginal marine sediments. The hydrocarbon charged reservoir appears to be sealed by a highly cemented, fine grained clastic zone related to a marine incursion. The low primary porosity of the reservoir is a potential risk to commercial deliverability; however, several faults and fractures, good permeability (tens to hundreds of millidarcies), and an uncharacteristically low fracture gradient, indicates a geomechanically weak reservoir with good permeability, coupled with an oil that has a high formation volume factor and is significantly undersaturated, possibly aiding well deliverability (Carnarvon Petroleum Ltd, 2016a; Thompson et al, 2015; Wehr et al, 2015; Finder Exploration Ltd, 2016).

Pre-drill, Roc 1 potentially contained better reservoir quality than Phoenix South 1, indicated by interpreted improving reservoir quality towards Keraudren 1. Initial pressure testing in Roc 1 indicates good reservoir quality between 4384 m to 4424 m, good mobility of reservoir fluids (CGR of 20–40 bbl/MMCF) and a strong water drive (Carnarvon Petroleum Ltd, 2016a).

Potential seals within the Triassic succession include shales in the Keraudren Formation (effective at Phoenix 1 and probably Phoenix South 1 and Roc 1), as well as the Cossigny Member of the Keraudren Formation, and the Locker Shale (Lipski, 1993). Potential seal facies are also present within the Lower–Middle Jurassic succession (e.g. Athol Formation equivalent), and Lower Cretaceous shales (e.g. Muderong/Baleine Formation) could provide seal for reservoirs in the upper part of the Legendre/middle Depuch Formation (Woodside Energy Ltd, 2003, 2007).

Timing of generationThermal maturity over most of the Roebuck Basin did not increase until the deposition of the thick carbonate wedge during the Oligocene and Miocene; this carbonate wedge is clearly visible on seismic line 120-01 (Figure 6) 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 in the early Oligocene, 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% 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.

Exploration historyThe Roebuck Basin is one of the least explored regions of the North West Shelf with fourteen wells in the Bedout and Rowley sub-basins. The initial phase of seismic exploration in the Roebuck Basin and offshore Canning Basin was from 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, Geoscience Australia and predecessor organisations gathered several regional seismic lines across the Roebuck Basin, tying in existing wells. Over this same period, six industry seismic surveys were conducted, including tightly spaced seismic grids in the Bedout and Rowley sub-basins, and more loosely spaced surveys across the Oobagooma Sub-basin. Later activity included several surveys conducted between 1998 and 2001. These included 2D surveys filling gaps in seismic coverage of the western Rowley Sub-basin, and 3D grids delineated drilling prospects for Huntsman 1 and Whitetail 1. Also of significance are the 2010 surveys of New Dawn, crossing the Browse, Roebuck and Northern Carnarvon basins, and the Golden Orb supplement within the offshore Canning Basin, all conducted by Petroleum Geo-Services (2014a). In addition, the Bedout Sub-basin was the focus of 2D and 3D surveys in 2009 and 2010 to delineate prospects (Petroleum Geo-Services, 2014b).

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In 2014, Woodside Petroleum Ltd. completed the Lord 3D marine seismic survey (3352 km2) in the Browse Basin, which partially extends into the northern Roebuck Basin, north of Hannover South 1 (Woodside Petroleum Ltd, 2014a, 2014b). Quadrant and Joint Venture partners recently interpreted the 3854 km2 Zeester MC3D 3D seismic survey, which was acquired in 2011/2012. Additionally, the Joint Venture partners have recently acquired a further 22 132 km2 of high resolution broadband 3D seismic (the Capreolus 3D multi-client survey) covering WA-435-P and WA-437-P and a large area of the Roebuck and Northern Carnarvon basins. Once processed and interpreted, this survey will enable better delineation of numerous leads to the west of Phoenix South 1 that have been previously identified on existing 2D data. The Capreolus 3D survey will provide numerous opportunities for reduced risk and play-based exploration targeting Triassic and Jurassic structural and stratigraphic plays (Pedley et al, 2015). In addition to the Capreolus 3D survey, as of March 2016, the Joint Venture partners have also acquired and licensed 85% of the ~10 000 km2 2D seismic survey (Bilby MC2D) to further explore the prospectivity in the south eastern portion of the acreage (WA-438-P; Carnarvon Petroleum Ltd, 2016b).

Two aeromagnetic surveys, covering 47 500 km2, have been conducted in the Roebuck and offshore Canning basins; a Geoscience Australia survey in 2007 focused on the Oobagooma Sub-basin (Foss et al, 2008) and a Carnarvon Petroleum and Finder Exploration survey in 2010 on the Bedout Sub-basin (Carnarvon Petroleum Ltd, 2010). Geoscience Australia also 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 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, and velocity pull-ups associated with late Miocene channels, rather than hydrocarbon related diagenetic zones and gas chimneys as previously interpreted by O'Brien et al (2003). Re-evaluation of these features suggests they are more likely brittle faulting of strata above the rigid basement core during Miocene structural reactivation (Logan et al, 2010).

The initiation of exploration drilling in the Roebuck and offshore Canning basins occurred between 1970 and 1974, with drilling in the Bedout Sub-basin (Bedout 1, Keraudren 1 and Minilya 1), Rowley Sub-basin (East Mermaid 1), and Oobagooma Sub-basin (Lacepede 1 and Wamac 1). Three further wells were drilled in the Bedout Sub-basin (Phoenix 1, Phoenix 2, and Lagrange 1) between 1980 and 1983. Exploration drilling in the Oobagooma Sub-basin was undertaken between 1980 and 1983 and comprised four shallow water inboard dry wells. More recent drilling activity 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 with the oil discovery at Phoenix South 1 (2014) and gas discovery at Roc 1 (2016).

The pre-drill targets for Phoenix South 1 were tight gas reservoirs in the Triassic Keraudren Formation and Locker Shale, (Finder Exploration Ltd, 2014). Instead, approximately 59 m of net oil pay was discovered across four discrete undersaturated oil columns. Further upside potential is predicted in the deeper undrilled reservoir sections, which remain largely untested as the gross hydrocarbon containing reservoir extends to TD (Carnarvon Petroleum Ltd, 2015). This discovery has opened up the first new oil play on the North West Shelf in 20 years and in light of this discovery it is now believed that the Phoenix 1 well was incorrectly classified as a gas well, and may actually contain a significant gross oil pay (Finder Exploration Ltd, 2014). It is now evident from recent 3D seismic interpretation that Phoenix 2 was drilled outside of closure (Finder Exploration Ltd, 2014).

Roc 1 (2015) was drilled on the south-western flank of the structure close to the calculated gas water contact (Carnarvon Petroleum Ltd, 2016a). The primary target in the informally named ‘Barrett Member’ of the lower Keraudren Formation (Thompson et al, 2015; Carnarvon Petroleum Ltd, 2016a) was water wet; however, a net 13 m liquids rich gas pay was discovered in the deeper secondary target in the ‘Caley Member’ of the lower Keraudren Formation (Carnarvon Petroleum Ltd, 2016a). In addition, the Roc 1 well exhibited excellent oil shows in the upper Keraudren Formation reservoir, and in possible reservoir quality sandstones that extend to TD.

The Roc 2 well is planned to be drilled 5–6 km to the east of Roc 1 (Carnarvon Petroleum Ltd, 2016a). It has an estimated 80% chance of geological success with the main risk being the extent to which the ‘Caley Member’ reservoir is present within the Keraudren Formation. Seismic data quality and resolution prohibits geophysical mapping of each of the individual hydrocarbon-bearing sandstones at this depth. Planned well testing at the crest of the structure will be designed for this particular reservoir’s characteristics and will lead to a more representative drill stem test result. In addition, full coring of the ‘Caley Member’ is planned. A further appraisal well, Phoenix South 2, is also planned for late 2016 (Carnarvon Petroleum Ltd, 2016a).

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Development statusAlthough no development plans have been finalised for the basin, for minimum economic field development considerations, the hydrocarbons at Roc 1 and Phoenix South 1 are compatible for a hub-style development. This could include possible tie-backs from either future discoveries, or a Floating Production Storage and Offtake (FPSO) vessel for condensate production and a sales gas pipeline to shore to supply the domestic gas market (Carnarvon Petroleum Ltd, 2016a).

To date, there are no discoveries or development plans in the offshore Willara Sub-basin or the offshore Broome Platform.

Marine and environmental information

IntroductionThis section provides an overview of the environmental characteristic of the Acreage Release areas and the general setting of the region overlying the Roebuck Basin (Figure 9), together with information focused on the environmental geoscience characteristics of individual acreage release blocks.

Climate of the regionThe region is characterised by a tropical climate with hot humid summer months and warm winters. A dry season occurs from April to October and a short wet season between December and March. Although monsoonal in nature, precipitation here is comparatively low, compared to the Northern Territory, and amounts to approximately one third that falling on Darwin (Broome Airport mean precipitation = 607 mm per year, over 76 years; www.bom.gov.au). Mean minimum and maximum temperatures are 21.2°C and 32.2°C for the period 1939–2015 (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). 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 regimeThe offshore marine environment of the offshore Canning Basin is strongly influenced by the Indonesian Throughflow, and inshore by the warm southward flowing shelf-edge Leeuwin Current. 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. 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 overviewThe Rowley Shoals reefs (Mermaid, Clerke and Imperieuse) are located over the Roebuck Basin, rising from the distal ramp of the northwest shelf (Collins, 2011). Water depths are 230 m at Imperieuse Reef, 390 m at Clerke Reef, and 440 m at Mermaid Reef. 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).

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Recent geological historyIt 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 ReservesArgo-Rowley Terrace Commonwealth Marine Reserve The reserve provides protection for the communities and habitats of the deeper offshore waters of the region, in

depth ranges from 220 m to 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.

The reserve protects 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.

Mermaid Reef Commonwealth Marine Reserve The reserve, along with nearby Rowley Shoals Marine Park, provides the best geological example of shelf atolls in

Australia.

The reserve protects examples of the seafloor habitats and communities of the Northwest Transition.

Western Australian Marine ParkRowley Shoals Marine ParkRowley Shoals comprises three reef systems, Mermaid, Clerke and Imperieuse reefs. They have been described as the most perfectly formed shelf atolls in Australian waters. Mermaid Reef is protected by a Commonwealth Marine Reserve. The Western Australian Rowley Shoals Marine Park extends across Clerke and Imperieuse reefs, to the limit of Western Australian coastal waters.

There are three management zones in the park. Assessment by the relevant Western Australia authority is required for petroleum exploration.

FisheriesThe following 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

Western tuna and billfish fishery operates in the Australian Fishing Zone and adjacent high seas

Western Skipjack tuna fishery is not active

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

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

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

Rowley Shoals Marine Park (Western Australia) http://www.dpaw.wa.gov.au/images/documents/parks/management-plans/decarchive/RowleyShoalsMP_MgtPlan56.pdf

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

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