Release Areas W17-9 and W17-10 in the northern Houtman Sub-basin, Perth Basin, Western Australia

HighlightsBids close 22 March 2018 First release of acreage in the underexplored northern Houtman Sub-basin Up to 19 km of sediments in the main depocentre Likely presence of Permian, Triassic and Jurassic oil- and gas-prone source rocks Potential for large volumes of hydrocarbons expelled since the Triassic Wide variety of potential traps at different stratigraphic levels Further guidance available, refer to 2017 Special Notices

Release Areas W17-9 and W17-10 are located from 100 km to 300 km west of Carnarvon, on the Western Australian coast, in water depths of 100 m to over 3000 m (Figure 1). This acreage overlies the northern extension of the Houtman Sub-basin, an area that has not been released previously. This part of the sub-basin is adjacent to the Bernier Platform in the east, and the Cuvier Abyssal Plain, Wallaby Saddle and the Perth Abyssal Plain, in the west. Release Area W17-9 consists of 316 graticular blocks (24 445 km2). Release Area W17-10 consists of 320 graticular blocks (24 465 km2). The Release Areas cover an underexplored region, with sparse seismic coverage and no well control. However, results from a recently acquired 2D seismic survey (GA-349) in the northern Houtman Sub-basin, combined with regional mapping of the northern Perth Basin, geophysical modelling and petroleum systems analysis, suggests substantial prospectivity of this region.

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DISCLAIMER: This information has been provided as a guide only. Explorers should not rely solely on this information when making commercial decisions.For more information see - http://petroleum-acreage.gov.au/2017/disclaimer. Image courtesy of Chevron Australia.

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Release Area geology

Tectonic setting and basin architectureThe Houtman Sub-basin is the largest structural element in the Perth Basin, covering an area of 52 900 km2 (Figure 1; Copp, 1994). It is an elongate, northwest–southeast trending depocentre, approximately 700 km long and 160 km at its widest. Sediment thickness is variable, but is interpreted to reach up to 19 km in the Release Areas (Borissova et al, 2017). The sub-basin is extensively faulted and its proximity to the Wallaby–Zenith Transform Margin suggests the basin developed in a transtensional setting (Bradshaw et al, 2003; Hall et al, 2013; Rollet et al, 2013a, 2013b; Borissova et al, 2017).

Regional studies of the northern Perth Basin (Bradshaw et al, 2003; Rollet et al, 2013a, 2013b;) have shown that the basin has undergone two major stages of rifting, the first in the Permian and the second in the Early Jurassic–Early Cretaceous. The latter culminated in continental breakup in the Valanginian. However, the breakup in the far north, where the Release Areas are located, was influenced by the development of a Large Igneous Province over the Wallaby Plateau and intervening Wallaby Saddle (Symonds et al, 1998). The new seismic data clearly show a transition from the non-volcanic margin of the central–southern Houtman Sub-basin to the volcanic margin in the northern part of the sub-basin. Formation of the volcanic margin segment included emplacement of extensive intrusive and extrusive complexes in the Early Cretaceous, which are clearly imaged on the data. Geodynamic reconstructions indicate that final continent–continent separation, on this part of the margin, occurred only at the end of the Barremian–Early Aptian (Gibbons et al, 2012; Hall et al, 2013).

The new GA-349 seismic data, acquired with a 8 km streamer in broadband mode, reveal crustal architecture down to 25–30 km. The data clearly images a large depocentre, controlled by basement-involved faults, with a maximum thickness of 19 km. The most prominent feature of the basin is a series of large half-graben (Figure 3) which extend along the inboard part of the basin and correspond with a well defined gravity low (Figure 2). The Moho and Proterozoic basement are mappable on most seismic lines (Figure 3) and their position indicates that the basin is underpinned by very thin crust (about 5 km). Only in the inboard southern part of the depocentre is the crust thicker (up to 10 km; Figure 3c). This crustal architecture was confirmed by integrated geological and geophysical modelling of four of the seismic lines (Figure 4; Sanchez et al, 2016). Modelled hyperextension is interpreted to be a result of the two phases of rifting, one in the Permian and the other in the Middle Jurassic–Early Cretaceous.

Consistent with the regional geological understanding of the Perth Basin tectonic evolution (Bradshaw et al, 2003; Hall et al, 2013; Rollet et al, 2013a) the main half-graben formed during Permian rifting. The thick succession overlying the late Permian unconformity (up to 6 km) is interpreted to correlate with the thermal subsidence that followed Permian rifting and, therefore, corresponds to the upper Permian to Lower Jurassic. The highly faulted and heavily intruded succession in the outboard part of the basin (Figure 3) was correlated, through regional mapping, to the Lower Jurassic–Lower Cretaceous syn-rift succession to the south. This succession is up to 5 km thick in the south of the study area, but thins to 2 km in the north. The intrusive and extrusive complexes, clearly imaged on the seismic data in the outboard part of the sub-basin adjacent to the Wallaby Saddle, are interpreted to have formed mostly during the Valanginian to Aptian.

Stratigraphy and tectonic evolution Seismic interpretation of the GA-349 data was integrated with the regional seismic interpretation of the northern Perth Basin (Jones et al, 2011), which resulted in the development of a new tectonostratigraphic framework for the northern Houtman Sub-basin (Figure 5; Borissova et al, 2016). It reflects regional geological knowledge from the wells in the Houtman and Abrolhos sub-basins (Figure 2; Jorgensen et al, 2011) and specifics of the study area derived from seismic interpretation (i.e. missing seismic stratigraphy, unconformities, presence of volcanic successions). Stratigraphic control of the Jurassic–Lower Cretaceous syn-rift section was provided by wells within the Houtman Sub-basin (i.e. Houtman 1 and Charon 1); whilst stratigraphic control of older sequences was inferred from wells in the adjacent Abrolhos Sub-basin (i.e. Livet 1, Morangie 1, Fiddich 1, Hadda 1, Flying Foam 1 and Wittecarra 1) as well as seismic facies correlation. The mapped seismic sequences and their interpreted lithostratigraphic equivalents are described below.

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Seismic sequences and interpreted lithostratigraphyBasement is 12–18 km deep in the central part of the basin and shallows to about 9–11 km in the west (Borissova et al, 2017). To the south, basement intersections on the Beagle Ridge (e.g. Cliff Head 1) encountered Precambrian granites of the probable Proterozoic Pinjarra Orogen, which forms the basement to the onshore Perth Basin (Dentith et al, 1994). The overlying pre-Permian sequence is characterised by high amplitude, high continuity reflectors and is confidently mapped throughout the inboard part of the depocentre, where it is at least 2–3 km thick. This pre-rift sequence is tentatively correlated with the Ordovician–Silurian Tumblagooda Sandstone (Hocking, 1991), which has been intersected on the Wittecarra Terrace (Hadda 1 and Livet 1; Jorgensen et al, 2011). In these intersections, the Tumblagooda Sandstone consists predominantly of very light grey to red brown, very fine- to medium-grained, sub-angular to sub-rounded sandstone (quartz arenite) with minor amounts of claystone (Jorgensen et al, 2011). Onshore, Tumblagooda Sandstone outcrops comprise red, very fine- to coarse-grained sandstones deposited in a fluviatile environment (Playford et al, 1976; Hocking, 1991; Mory and Iasky, 1996).

The succession in the central Permian half-graben is 7–10 km thick and it extends the length of the northern sub-basin (Borissova et al, 2017; Figure 6). Two seismic sequences were mapped: NH-P1 (Permian syn-rift 1) and NH-P2 (Permian syn-rift 2) (Figure 3). The age of the sequences is unconstrained, but the boundary between the two is interpreted to be at the base of the middle Permian, consistent with the onset of a regional marine transgression across the Perth Basin (Ferdinando and Longley, 2015). Based on the stratigraphy of the onshore northern Perth Basin, sequence NH-P1 is interpreted to contain glacial to pro-glacial marine sediments of the Nangetty Formation and Holmwood Shale; glacially influenced paralic to non-marine fluvial and coal-rich sequences equivalent to the High Cliff Sandstone and Irwin River Coal Measures (Mory and Iasky, 1996; Norvick, 2004; Jones et al, 2011). In contrast, sequence NH-P2 is likely to consist of shallow marine shales of the Carynginia Formation.

A prominent unconformity, separating the Permian syn-rift sequences from the younger succession, is correlated to the regional Late Permian unconformity of the northern Perth Basin (Mory and Iasky, 1996). Overlying this unconformity is a 500–1000 m thick, relatively transparent, seismic sequence NH-P3 (Figure 3 and Figure 5). This is only present over the Permian graben and has an onlapping, sag-fill geometry. The sequence is interpreted to comprise early post-rift sands equivalent to the upper Permian Dongara Sandstone and mixed shales, limestones and clastics equivalent to the upper Permian Beekeeper Formation.

A second, thick (800–1800 m), regionally extensive seismic sequence NH-TR1, overlying the Permian unconformity and NH-P3, is interpreted to be equivalent to the upper Permian–Lower Triassic Kockatea Shale (Figure 3 and Figure 5). This interpretation is largely based on its stratigraphic position above the late Permian unconformity and its transparent seismic character, which is typical for this unit in the Abrolhos Sub-basin. Furthermore, the lowermost section of the sequence ties to the Kockatea Shale in Livet 1, some 100 km to the southeast on the Wittecarra Terrace. The sequence extends over the main Permian half-graben, where it reaches a maximum thickness of almost 2000 m; however, to the west, it cannot be resolved confidently on seismic (Figure 3). The Kockatea Shale is widespread across the Perth Basin and comprises shale, claystone and siltstone, with minor sandstone and limestone (Playford et al, 1976). The unit was deposited in a shallow marine environment and records a major marine incursion as well as the onset of a subsequent regression (Jones et al, 2011; Jorgensen et al, 2011).

The seismic sequences overlying NH-TR1 (NH-TR2, NH-TR3, NH-TR/J1; Figure 3) are interpreted to be equivalent to the Lower–Middle Triassic Woodada Formation, Middle–Upper Triassic Lesueur Sandstone and Upper Triassic–Lower Jurassic Eneabba Formation (Figure 5; Borissova et al, 2017). The Triassic sequences are uniformly thick (about 4–5 km) throughout the central parts of the northern Houtman Sub-basin (Figure 6). However, the succession thins outboard and also to the east, where there is some onlap within sequences and partial erosion by the Valanginian unconformity. These sequences are interpreted as a gradual regional regression culminating in a lowstand in the Late Triassic–Early Jurassic (Jones et al, 2011; Jorgensen et al, 2011), with deltaic to fluvial siltstones and sandstones of the Woodada Formation transitioning to fine- to medium- and coarse-grained fluvial sandstones of the Lesueur Sandstone and fine- to very coarse-grained lowstand sands of the Eneabba Formation (Jorgensen et al, 2011). Further south, in the offshore parts of the basin, the fluvio-deltaic system responsible for the deposition of the Woodada and Lesueur sequences had a northwards flow resulting in finer grained lithologies in the north, particularly for the Woodada Formation (Mory and Iasky, 1996; Jones et al, 2011). Therefore, if these sequences (NH-TR2 and NH-TR3) comprise the same depositional system, then finer lithologies may be expected in the northern Houtman Sub-basin.

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The Lower Jurassic to Lower Cretaceous syn-rift sequences (NH-J2, NH-J3, NH-J4 and NH-K1) are characterised in the seismic data by high frequency, high continuity reflectors. In addition, the base of sequence NH-J2 (equivalent to the Cattamarra Coal Measures) corresponds to a regional unconformity (Rollet et al, 2013a), but this is difficult to discern on the seismic data in this northern area. The Lower Jurassic to Lower Cretaceous syn-rift succession is thickest (about 4 km) in the southern part of the northern Houtman Sub-basin and thins to the north (Figure 6), where only the lower part of the Jurassic succession (NH-J2 and NH-J3) is present and most of the upper part (NH-J4) is missing. The rift-fill succession is predominantly non-marine, comprising fluvial sandstones, carbonaceous shales and coals, with the exception of the Middle Jurassic sequence NH-J3 (Cadda Formation equivalent) which represents a short-lived marine incursion (Mory and Iasky, 1996; Jorgensen et al, 2011). In the western part of the depocentre, adjacent to the Wallaby Saddle, the Jurassic–Lower Cretaceous section is heavily intruded and may underlie the Seaward Dipping Reflector Sequences (SDRSs) defining the volcanic margin (Figure 3a).

The Valanginian breakup unconformity marks the base of the Cretaceous–Cenozoic post-rift succession (Figure 3 and Figure 5). This clear angular unconformity truncates older Triassic–Cretaceous sequences. The post-rift succession is 500–1500 m thick (Borissova et al, 2017; Figure 6). The oldest post-breakup sequence (NH-K2) is interpreted to have developed as the volcanic margin evolved and contains SDRSs extending onto the Wallaby Saddle. Lava flows and individual volcanoes are clearly imaged by the seismic data within this sequence, unique to the northern Houtman Sub-basin. The distribution of the SDRSs and major sill and dyke complexes has been confirmed by magnetic modelling of one of the seismic lines of the GA-349 survey (Sanchez et al, 2016).The lower Albian age of the upper boundary of this sequence is inferred from interpretation of the two regional seismic lines from survey GA-310 which extend from the Houtman Sub-basin across the Wallaby Saddle to the Wallaby plateau (Figure 2). On these lines, the angular unconformity on the Wallaby Plateau corresponds to the top of the SDRSs, while in the northern Houtman Sub-basin, the Valanginian breakup unconformity is interpreted to underlie the SDRS. As the Wallaby Plateau did not separate from Greater India until about 120 Ma, the angular unconformity on the Wallaby Plateau is considered to be Lower Aptian. The SDRS wedge onlaps the Valanginian unconformity along the western margin of the northern Houtman Sub-basin (Figure 3a), so both unconformities are imaged.

The overlying Cretaceous (NH-K3) and Cenozoic successions (NH-K/T1, NH-T2 and NH-T3) were mapped by tying to exploration well Herdsman 1 (Woodside Energy Ltd, 2003) and to Livet 1 (Seafield Resources Plc, 1997). The lower sequence (NH-K3) is interpreted to comprise siliciclastics equivalent to the Lower Cretaceous Winning Group while the upper sequences (NH-K/T1, NH-T2 and NH-T3) comprise largely carbonate units prevalent across the Carnarvon Basin. NH-K/T1 is interpreted to mark the onset of widespread carbonate deposition and potentially includes Upper Cretaceous to Lower Paleogene equivalents to the Haycock Marl, Toolonga and Korojon calcilutites, Miria Formation and Cardabia Calcarenite. The base of the NH-T2 sequence is interpreted to correspond to a mid-Eocene regional unconformity (Hocking et al, 1987; Haig and Mory, 2003) and comprises equivalents of the Giralia Calcarenite, Mandu Formation and Trealla Limestone. NH-T3 is interpreted as comprising upper Miocene to Holocene sediments, with the base of this sequence interpreted as a Late Miocene regional unconformity related to the convergence of the Australian and Eurasian plates to the north.

Tectonic evolution and basin phasesThe Palaeozoic to Mesozoic development of the Perth Basin was controlled by multiple phases of tectonic activity culminating in the Early Cretaceous breakup of Australia and Greater India (Bradshaw et al, 2003; Norvick, 2004). The latter includes the Indian sub-continent and its postulated northern extension (Matte et al, 1997; Ali and Aitchison, 2005; Gibbons et al, 2012).

Permian rifting

Permian rifting resulted in the development of NNW-oriented half graben across the northern Perth Basin (Iasky et al, 2003; Norvick, 2004; Jones et al, 2011; Rollet et al, 2013a, 2013b; Mory, 2017). Within the northern Houtman Sub-basin, the Permian graben are primarily controlled by three large en-echelon faults. These exhibit more than 10 km throw and separate the northern Houtman Sub-basin from the Bernier Platform (Figure 3; Borissova et al, 2017).

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The outboard part of the basin (adjacent to the Wallaby Saddle) features antithetic easterly dipping faults, shallower basement and the Permian section is largely absent. The major angular unconformity marking the top of the lower to middle Permian syn-rift succession is interpreted to be the result of regional uplift and erosion (Figure 5). Northwest to southeast extension has been proposed for Permian rifting in the offshore northern Perth Basin (Pryer et al, 2002; Borissova et al, 2017), although the stress regime remains poorly understood.

Late Permian – Early Jurassic subsidence

Initial extension was followed by late Permian to Early Jurassic thermal subsidence resulting in the formation of a broad sag basin across the region (Figure 5; Norvick, 2004). In the Houtman Sub-basin this resulted in the formation of a westward-thickening sag basin (Enterprise Oil Exploration & Nippon Oil Exploration, 1994a, 1994b; Bradshaw et al, 2003; Jones et al, 2011; Pfahl, 2011; Rollet et al, 2013a, 2013b) that reaches thicknesses in excess of 5 km in the northern sub-basin (Figure 6). Upper Permian–Lower Triassic volcanics and some intrusives are recorded in the northern Perth Basin (Gorter and Deighton, 2002).

Middle Jurassic – Early Cretaceous rifting

Late Jurassic–Early Cretaceous rifting affected the entire western margin and culminated in the final separation of Australia and Greater India in the Early Cretaceous (Figure 5; Larson et al, 1979; Veevers et al, 1991; Bradshaw et al, 2003; Norvick, 2004; Gibbons et al, 2012; Rollet et al, 2013a: Hall et al, 2013). The northwest–southeast extension controlled much of the final structural architecture of the entire Perth Basin, including the Houtman Sub-basin (Bradshaw et al, 2003).

The regional unconformity marking the base of NH-J2 (Cattamarra Coal Measures) is interpreted to correspond to the onset of Late Jurassic rifting (Rollet et al, 2013a). The Jurassic sequences are thickest in the outer part of the basin, showing that the main depocentre during this period was located outboard of the Permian graben (Figure 6; Jones et al, 2011; Rollet et al, 2013a; Borissova et al, 2017).

Late Jurassic extension resulted in the development of a large number of closely spaced faults (Borissova et al, 2017). Some of these are listric while others are interpreted to extend to, and link with, the older basement-involved faults that control the geometry of the Permian syn-rift.

Early Cretaceous breakup

Continental breakup of Australia and Greater India occurred during the Valanginian. Separation was accommodated by the development of the Wallaby–Zenith Transform Margin, which is 1500 km in length (Figure 4a; Veevers et al, 1985, 1991; Norvick, 2004; Gibbons et al, 2012: Hall et al, 2013). Active transform motion occurred outboard of the Zeewyck Sub-basin from approximately the Valanginian to early Aptian (137–124 Ma), and may have continued outboard of the northern Houtman Sub-basin until the late Aptian (115 Ma; Hall et al, 2013). During this time, the Houtman Sub-basin margin experienced uplift and erosion, while the western flank of the northern Houtman Sub-basin, adjacent to the Wallaby Saddle, was dominated by voluminous volcanism. The volcanic successions include SDRS, abundant sills, cones and dykes (Gorter and Deighton, 2002). SDRS are typically found on volcanic margins close to the continent-ocean boundary (Symonds et al, 1998; Jackson et al, 2000; Quirk et al, 2014), however, rifting on the Wallaby Saddle did not proceed to breakup, instead, a spreading ridge formed to the north-west of the Wallaby and Zenith plateaus, which subsequently connected with the Perth Abyssal Plain spreading centre in the Aptian.

Late Cretaceous – Cenozoic subsidence

Following breakup, the southern part of the Houtman Sub-basin experienced rapid passive margin subsidence and widespread westward regional tilting in the Valanginian–Aptian, while the northern part of the sub-basin remained largely exposed subaerially as the Wallaby Zenith Transform Margin continued to develop (Hall et al, 2013; Rollet et al, 2013a).

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Evidence of late-stage fault reactivation and inversion, with minor folding of Cretaceous and younger strata extending to just below the seafloor, is evident along some of the major basin-bounding fault systems in the offshore northern Perth Basin (Gorter et al, 2004; Rollet et al, 2013a). This inversion event is interpreted to be the result of Miocene convergence between the Australian and Eurasian plates (Rollet et al, 2013a). However, within the northern Houtman Sub-basin the post-rift sequences remain largely unfaulted and there are only a few indications of recent fault reactivation along the major basin bounding faults (Borissova et al, 2017).

Petroleum systems and hydrocarbon potentialThe northern Perth Basin is a proven hydrocarbon province and includes the Cliff Head oil field in the Abrolhos Sub-basin and the recent onshore Waitsia gas discovery (Tupper et al, 2016). While the Houtman Sub-basin is an under-explored region, the regional seismic stratigraphic correlation with wells across the offshore northern Perth Basin indicates that this depocentre has the potential to contain multiple petroleum systems equivalent to those identified in the adjacent producing depocentres (Borissova et al, 2017; Table 1).

Table 1. Potential petroleum systems elements

Sources Permian gas prone shales and coals Marine oil-prone Hovea Member of the upper Permian-Lower Triassic Kockatea Shale Triassic oil and gas prone marine shales Jurassic marine and non-marine oil and gas prone shales and coals

Reservoirs Permian fluvio-deltaic and marine sandstones Triassic shallow marine sandstone facies Jurassic fluvial and deltaic sandstones

Seals Regional seals associated with marine shales of the Triassic Kockatea Shale Regional seals associated with marine shales of the Jurassic Cadda Formation Intraformational seals throughout the Triassic-Jurassic section

Traps Large stratigraphic pinch-outs Variety of fault block plays and rollover anticlines Sub-unconformity plays

Source rocksRegional stratigraphic correlation with well and seismic data across the offshore Perth Basin suggest that the northern Houtman Sub-basin potentially contains a range of Permian to Jurassic source rocks (Figure 7) consisting of marine and non-marine carbonaceous shale and coal (Borissova et al, 2017; Hall et al, 2017).

Permian source rocks

Permian rocks are the source of gas for multiple Perth Basin onshore fields, including Elegans (Boreham et al, 2011) and Waitsia (Tupper et al, 2016). Within the northern Houtman Sub-basin, the thick Permian syn-rift sequences NH-P1 and NH-P2 have the potential to contain source rocks equivalent to those within the Cisuralian Irwin River Coal Measures (IRCM) and the Cisuralian-Guadalupian Carynginia Formation.

The IRCM is intersected in 13 wells on the Beagle Ridge and in the central Abrolhos Sub-basin (Jorgensen et al, 2011). Source intervals comprise non-marine shales and coals and evidence for a contribution from these source rocks has been observed in gases retrieved from Dunsborough 1, Frankland 1 and Perseverance 1 (Rollet et al, 2013a). The IRCM has excellent organic richness with mean Total Organic Carbon (TOC) of 5.9%. While its mean Hydrogen Index (HI) of 120 mgHC/gTOC is indicative of a predominantly type III gas-prone kerogen, derived from terrigenous organic matter, some IRCM HI values reach 250 mgHC/gTOC, suggesting the potential for some liquids generation (Figure 7).

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The shallow-marine Cisuralian-Guadalupian Carynginia Formation is intersected in the Abrolhos Sub-basin, Beagle Ridge and Wittecarra Terrace. Despite good organic richness (TOC up to 7.9% and mean of 2.4%), the offshore wells indicate that the Carynginia Formation has limited potential for generating gas at present day (HI <100 mgHC/gTOC; Figure 7).

Triassic source rocks

Sequence NH-TR1, equivalent to the Kockatea Shale, has the potential to contain multiple regionally extensive source rock intervals (Borissova et al, 2017). The most prolific potential source rock for oil generation is a sapropelic interval within the Lopingian to Lower Triassic Hovea Member. This is the source rock of oil and condensate accumulations in the greater Dongara area onshore and Cliff Head offshore (Summons et al, 1995; Thomas and Barber, 2004) and has been recognised in most offshore northern Perth Basin wells (Jorgensen et al, 2011; Rollet et al, 2013a).

Extensive oil-charge from the Hovea Member source has been recorded in the offshore Perth Basin to the south, including in the central Houtman Sub-basin based on the geochemical composition of reservoired and migrated hydrocarbons, as well as fluid inclusions (Geotech, 2005; Kempton et al, 2011; Rollet et al, 2013b). Offshore, samples of the Hovea Member are typically organic-rich with an average TOC of 2% (Figure 7), consisting predominantly of liptinite-rich type II kerogen derived from marine algae (Rollet et al, 2013a).

The sapropelic interval of the Hovea Member was formed during a period of rapid marine transgression as a condensed section and deposited under anoxic marine conditions (Thomas et al, 2004). In the northern Houtman Sub-basin, deposition of the Hovea Member equivalent would be most likely within localised depocentres, where post-rift thermal subsidence was the greatest (Figure 6).

Younger Triassic oil-prone source rocks may also be present within the upper parts of NH-TR1 and NH-TR2, equivalent to source rocks within the upper Kockatea Shale and the Woodada Formation, respectively. Across the northern Perth Basin the middle and upper Kockatea Shale source intervals are leaner than those in the Hovea Member, with only a fair generative potential (mean TOC 0.5%; mean HI 280 mgHC/gTOC; Figure 7). Fair to good oil potential has also been recorded in the Woodada Formation (TOC >0.5%; HI >200 mgHC/gTOC; Figure 7), particularly at Wittecarra 1.

Jurassic source rocks

The Houtman Sub-basin also contains potential source rocks within the Jurassic succession (Gorter et al, 2004; Jones et al, 2011; Rollet et al, 2013a, 2013b). Sequences NH-TR/J1, NH-J2, NH-J3 and NH-J4 contain shales, both marine and non-marine, and coals, equivalent to source rocks found within the Eneabba, Cattamarra, Cadda and Yarragadee formations (Borissova et al, 2017). The presence of a working Jurassic petroleum system in the Houtman Sub-basin is suggested by fluid inclusion data, which shows that a palaeo-oil column in the Cadda Formation in Houtman 1 was most likely sourced from Jurassic strata (Volk et al, 2004).

Overall, the organic content of the Eneabba Formation is low (mean TOC <1%), although predominantly gas-prone organic-rich source rocks were intersected in Gun Island 1. In contrast, where intersected by wells, the overlying Cattamarra Coal Measures contains terrestrial organic matter with good to excellent potential for generating gas (mean TOC 3.2%; mean HI 126 mgHC/gTOC; Figure 7). However, marine shales may also be present in the northern part of the basin (Robertson et al, 2011).

Marine shales of the Cadda Formation typically contain type III kerogens, and are predominantly gas-prone (mean TOC >1%; mean HI 50–200 mgHC/gTOC), although a few organic-rich samples show this source rock may have some liquids potential (>250 mgHC/gTOC) (Figure 7).

Coals and coaly shales within the Yarragadee Formation (TOC >4%) are variable in quality, although some samples show excellent potential to generate oil (HI >300 mgHC/gTOC; Figure 7).

ReservoirsThe reservoir intervals are interpreted using regional correlations with stratigraphic units from further south in the northern Perth Basin (Figure 5).

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Permian reservoirs

The thick succession in the Permian half-graben as interpreted in the northern Houtman Sub-basin (Figure 6) include several potential reservoir units.

Seismic sequence NH-P1 is interpreted to include equivalents to the Cisuralian Irwin River Coal Measures (IRCM) and High Cliff Sandstone (Figure 5), both of which have good reservoir potential. The IRCM is a succession of fluvial sands, shales and coal. The unit hosts several discoveries and comprises the primary reservoir for the Cliff Head oil field. In offshore intersections the High Cliff Sandstone is a broadly upward-coarsening sequence of silty sandstone interpreted to record a major regression (Jorgensen et al, 2011), while onshore occurrences indicate deposition in glacially influenced shallow marine, beach and lower deltaic environments (Mory and Iasky, 1996).

Seismic sequence NH-P3 is interpreted to be equivalent to the Lopingian Dongara Sandstone (time equivalent to Wagina Sandstone and Beekeeper Formation in other parts of the Perth Basin; Figure 5), one of the primary reservoirs in the offshore northern Perth Basin (Jones et al, 2011). This unit is thought to have been deposited in a shallow marine environment (Ferdinando and Longley, 2015) and is interpreted to represent nearshore to dunal facies along the rims of exposed land and over paleo-highs. It hosts several sub-commercial hydrocarbon discoveries including Frankland 1, Perseverance 1 and Dunsborough 1. In the Release Areas, the distribution and thickness of NH-P3 broadly follows the geometry of the Permian depocentres, attaining thicknesses of more than 1 km near the eastern border fault.

Although the prospectivity of these Permian reservoirs is thought to decrease significantly at depths greater than 2.5 km, due to porosity degradation by silica cementation (Roc Oil, 2006), their viability may be maintained at depth (>3 km), where cementation has been inhibited by grain-coating clay, preserving intergranular porosity (Ferdinando et al, 2007). This process has resulted in preservation of excellent primary porosity and permeability at significant depths for the Wagina Sandstone in the Beharra Springs Field (Tupper et al, 1994), and for the Kingia and High Cliff sandstones in the recently discovered Waitsia Field (Tupper et al, 2016).

Triassic reservoirs

The Triassic succession in the offshore northern Perth Basin includes the Lower to Middle Triassic interbedded deltaic sandstones and siltstones of the Woodada Formation and the fluvial sandstones of the Middle to Upper Triassic Lesueur Sandstone. In wells the Lesueur Sandstone reaches thicknesses over 1 km (Wittecarra 1), and, typically, has relatively good porosity (e.g. 18–21% in Batavia 1), whereas the interbedded sandstones and siltstones of the underlying Woodada Formation have variable porosity.

In the northern Houtman Sub-basin, seismic sequence NH-TR2, interpreted as a Woodada Formation equivalent, has a relatively constant thickness of about 1 km, with some thinning to the east, near the main border fault, and to the west of the main Permian depocentre. Similarly, the NH-TR3 sequence, interpreted as a Lesueur Sandstone equivalent, is widespread in the Release Areas, with a maximum thickness of 3–4 km.

Jurassic reservoirs

Upper Triassic to Upper Jurassic intervals of reservoir sandstones are potentially present throughout seismic sequences NH-TR/J1, NH-J2 and NH-J4, interpreted as equivalents to the Eneabba Formation, Cattamarra Coal Measures and Yarragadee Formation, respectively.

Of the Jurassic sequences, the Lower to Middle Jurassic Cattamarra Coal Measures is considered to have the best potential in the offshore northern Perth Basin (Jones et al, 2011), comprising a series of interbedded sandstones, siltstones and coals deposited in a deltaic environment. Houtman 1 and Leander Reef 1 contain oil and gas shows in the upper part of the Cattamarra Coal Measures, while a palaeo-oil column has been detected over at least 15 m of this interval in Houtman 1 (Kempton et al, 2011). Intersections of a relatively coarse-grained sandstone interval, underlying the thick shales of the Middle Jurassic Cadda Formation, have been shown to have good reservoir potential (Crostella, 2001). In the offshore northern Perth Basin, porosities in Jurassic sandstone successions range up to 30% at depths of 1–1.5 km, but decrease with depth to about 10–15% at 3–3.5 km (Gorter et al, 2004).

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In the Release Areas, NH-J2, interpreted as the Cattamarra Coal Measures equivalent, is mapped as a 1000–1800 m thick package at depths of 1.5-7 km and is mostly confined to the Jurassic depocentre outboard of the Permian depocentre (Figure 3). The NH-TR/J1 (Eneabba equivalent) is mapped at depths of 2-7.5 km as a relatively constant 600–800 m thick package over the central to outer portion of the sub-basin, and is mostly absent inboard. NH-J4 (Yarragadee equivalent) is largely restricted to the southwestern portion of the Release Areas and is mapped as a 400–1400 m (locally >1800 m) thick sequence at depths of 2–6 km.

Lower Cretaceous reservoirs

The post-rift succession in the Abrolhos and southern Houtman sub-basins is thin, ranging from about 400 m in the inboard part of the basin to 800 m outboard, and no reservoir units have been intersected by wells. The post-rift succession is dominated by prograding carbonate shelf units (Toolonga Calcilulite, Korojon Calcarenite and undifferentiated Cenozoic to Holocene shelfal carbonates). In most wells, the siliciclastic Winning Group, overlying the Valanginian unconformity, is very thin (<100 m) or absent, and no hydrocarbon indications have been recorded in this succession (Jorgensen et al, 2011).

In the Release Areas, a much thicker post-rift succession is mapped, ranging from about 1000 m in the inboard part to 1500 m over the central part of the sub-basin. The Lower Cretaceous NH-K3 sequence (Winning Group equivalent) is 500–600 m thick and is likely to contain siliciclastics; however, the lithologies are completely unknown. Seismic mapping indicates a possible regionally extensive thin (30–80 m) transgressive sand interval overlying the Valanginian unconformity (Figure 5), which has been tied back to Livet 1, where it has been identified as a clean sandstone interval overlying the breakup unconformity (Jorgensen et al, 2011). This unit may be analogous to, though not a direct time equivalent of, the Lower Cretaceous transgressive sandstones found across the Carnarvon Basin (e.g. Mardie Greensand Member of the Muderong Shale, and the underlying Birdrong Sandstone) which form reservoirs for many petroleum accumulations in the Northern Carnarvon Basin (Hocking et al, 1987).

SealsThe Kockatea Shale is proven to be an effective regional seal across the northern Perth Basin (Jones et al, 2011). In the Release Areas, sequence NH-TR1, equivalent to the Kockatea Shale, is regionally extensive across the Permian depocentre of the northern Houtman Sub-basin, and its mapped thickness (up to 1800 m) is sufficient to provide robust vertical and cross-fault sealing, unless breached by subsequent reactivation.

Although constraints on the lithologies within the Release Areas are limited, a general south-north fining of the Woodada Formation in the offshore northern Perth Basin (Jorgensen et al, 2011) suggests that the NH-TR2 sequence (Woodada Formation equivalent) may be clayey or contain clayey intervals in the northern Houtman Sub-basin, thus providing a potential regional or intraformational seal. Intraformational seals are also potentially present within Jurassic sequences NH-TR/J1, NH-J2 and NH-J4, equivalent to the Eneabba, Cattamarra and Yarragadee formations, respectively (Figure 5; Jones et al, 2011; Robertson et al, 2011; Borissova et al, 2017). Fluid inclusion data from the Abrolhos and southern Houtman Sub-basin indicate that these can provide impedance barriers to oil migration, such as in Leander Reef 1 (Kempton et al, 2011), but their potential to seal hydrocarbon accumulations remains speculative.

Marine shales associated with sequence NH-J3 (Cadda Formation equivalent) have the potential to form a regional seal covering the outboard part of the sub-basin (Figure 3). Cadda Formation marine shales are demonstrated to be an effective local seal at Houtman 1, where they overly the 15 m palaeo-oil column detected in the sandy basal Cadda Sequence (Jones et al, 2011; Kempton et al, 2011). The Cadda Formation is 400 m thick in well intersections in the Houtman Sub-basin (Jones et al, 2011), but within the Release Areas, its thickness may exceed 500 m (Borissova et al, 2017). Thin sandstone intervals in the Cadda Formation were also intersected by Houtman 1 and Gun Island 1, potentially compromising fault juxtaposition seals (Gorter et al, 2004).

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No regional seals have been described in the post-breakup section in the offshore Perth Basin, apart from the Lower Cretaceous South Perth Shale in the Vlaming Sub-basin of the southern Perth Basin (Nicholson et al, 2008). In the Carnarvon Basin, the regionally extensive Muderong Shale, overlying the Birdrong Sandstone, is known as a good quality seal (Baillie and Jacobson, 1997). In the Release Areas, age-equivalent units are likely absent inboard, due to non-deposition or erosion, while outboard, the post-breakup sequence (NH-K2) contains Seaward Dipping Reflector Sequences which developed as the volcanic margin evolved. However, considering the substantial thickness (500–600 m) of the siliciclastic NH-K3 sequence (Winning Group equivalent), the potential for regional or intraformational seals cannot be excluded.

Maturation, generation and expulsionResults of petroleum systems modelling for the Release Area indicate that potential source rocks within both the upper and lower Permian syn-rift sequences (NH-P2 and NH-P1) may have generated large amounts of gas, along with some liquids (Hall et al, 2017). However, peak generation occurred in the Triassic, and the majority of the Permian source kitchen is now overmature (Figure 8). This is in contrast to the onshore Perth Basin where generation from Permian source rocks occurred much later in the Late Jurassic–Early Cretaceous (Thomas and Barber, 2004).

If the oil-prone Hovea Member of the Kockatea Shale is present at the base of the NH-TR1 sequence in the northern Houtman Sub-basin, it has the potential to have generated large volumes of oil and gas across the depocentre (Figure 9; Hall et al, 2017). This source kitchen is modelled as overmature in the central basin, but remains within the oil window along the basin margin (Figure 9). Modelling suggests that peak oil expulsion occurred in the Triassic (Hall et al, 2017), earlier than predicted by Gorter et al (2004) for the southern part of the Houtman Sub-basin. Deposition of an additional 500–1500 m of post-breakup overburden (Figure 6) resulted in some additional Late Cretaceous and Cenozoic oil and gas expulsion along the basin margin (Gorter et al, 2004; Hall et al, 2017).

The younger Triassic oil-prone source rocks within NH-TR2 and the upper section of NH-TR1 (equivalent to source rocks within the Woodada Formation and upper Kockatea Shale, respectively), if present, are of a more favourable maturity than the Hovea Member, at the base of NH-TR1, with a larger area of source kitchen remaining in the late oil to early gas window (Hall et al, 2017). However, the total generative potential of these sources is expected to be much lower than the Hovea Member. The main risk is that the presence of significant thicknesses of higher quality source rock within these sequences remains speculative.

Within the Release Areas, Lower Jurassic source rocks (Eneabba and lower Cattamarra formations) reach the oil window in the outer part of the basin, with peak generation occurring in the Early Cretaceous. Middle Jurassic and younger source rocks (upper Cattamarra, Cadda and Yarragadee formations) remain predominantly immature as the thickness of these sediments within the basin thins to the north (Hall et al, 2017). However, increased heat-flow associated with Lower Cretaceous volcanism (Borissova et al, 2017) may have resulted in localised hydrocarbon generation of potential source intervals (Gorter and Deighton, 2002; Hall et al, 2017).

Plays and critical risks

Play typesInterpretation of the GA-349 seismic data in the northern Houtman Sub-basin reveals a structurally complex basin containing a wide range of structural and stratigraphic traps at several stratigraphic levels. Petroleum systems modelling (Hall et al, 2017) indicates that many of these traps would have been charged since the Triassic. However, the preservation potential of the older accumulations is uncertain.

Potential plays have been identified in the Upper Permian, Triassic and Jurassic successions. They include large stratigraphic plays in the Upper Permian/Lower Triassic, rollover anticlines within the Lower Triassic and Jurassic, and fault propagation folds and fault block plays in the Jurassic. The presence of shallow plays immediately above (stratigraphic) or below (sub-cropping) the Valanginian unconformity is uncertain due to the lack of knowledge on reservoir and seal lithologies in the post-breakup succession. However, the potential for these plays being present in the sub-cropping Triassic and Jurassic sandstone reservoirs should be considered. Play types are illustrated in Figure 10 and described below.

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Upper Permian and Triassic plays1. Stratigraphic pinch-out of the upper Permian/Lower Triassic sandstones (NH-P3, Dongara Sandstone equivalent) against the major basin bounding fault. The overlying marine shales of NH-TR1 (Kockatea Shale equivalent) acting both as a source and the top seal.

2. Triassic Fault block plays and rollover anticlines bounded by listric faults in the fluvio-deltaic sandstones of the NH-TR2 sequence (Woodada Formation equivalent). These plays could be charged by the underlying source rock intervals in NH-TR1 (Kockatea Shale equivalent) and sealed by intraformational shales, providing both top and cross fault closure.

3. Sub-cropping plays below the Valanginian unconformity. Provided a viable seal is present directly above the unconformity, sub-cropping fluvio-deltaic sandstones of NH-TR3 (Lesueur Formation equivalent) may contain hydrocarbons expelled by source rocks of NH-TR1 (Kockatea Shale equivalent) in the Early Cretaceous.

Jurassic plays4. Fault block plays, including fault propagation fold anticlines within NH-TR/J1 (Eneabba Formation equivalent). The mixed lithologies of this sequence, including both sandy and shaly facies, suggests the potential for viable intraformational source, reservoir and seal combinations.

5. Fault block plays within fluvio-deltaic sandstones of the NH-J2 (Cattamarra Coal Measures equivalent) with top seal provided by NH-J3 marine shale (Cadda Formation equivalent). Potential source rocks include shales and coals of NH-TR/J1 and NH-J2, as well as from the overlying NH-J3 shales.

6. Sub-cropping plays below the Valanginian unconformity reservoired within NH-TR/J1 (Eneabba Formation equivalent) or fluvio-deltaic sandstones of the NH-J2 and NH-J4 sequences (Cattamarra Coal Measures and Yarragadee Formation equivalents, respectively). Potential source rocks for this play include shales and coals of NH-TR/J1 and NH-J2 or NH-J3 marine shales.

Critical risks The stratigraphic pinch-out play of the upper Permian reservoir is considered the most prospective play in the Houtman Sub-basin (Jones et al, 2011). Petroleum systems modelling suggests that if the Hovea Member is present in the NH-TR1 sequence, it would now be in the oil window at the eastern margin of the sub-basin. Modelling also suggests that, although peak oil generation occurred during the Triassic, hydrocarbon generation triggered by sediment loading after breakup in the Early Cretaceous may have been sufficient to fill this trap type. The major risks for this play are the presence/absence of the source rich Hovea Member equivalent, preservation of reservoir quality, and the sealing capacity of the major basin bounding fault.

Triassic plays in the NH-TR2 and NH-TR3 sequences have a good probability of being charged by the underlying NH-TR1 marine shales; although the presence of intraformational shales to provide cross-fault and top seals is unproven.

The primary risk for the Jurassic plays is charge. Petroleum systems modelling suggests that potential Lower Jurassic source rocks have reached the oil window, with peak generation occurring in the Early Cretaceous. However, the maturity and extent of generation is highly dependent on the amount of uplift and erosion associated with Valanginian breakup. Potential Middle–Upper Jurassic source rocks remain mostly immature, although additional heat from volcanic intrusions may have been sufficient for localised hydrocarbon generation. The marine shales of NH-J3 may provide a regional seal for the Lower Jurassic reservoirs, however, this sequence is highly faulted and fault reactivation post-breakup may have resulted in trap breach. Sub-unconformity plays would require the presence of shales in the Winning Group equivalent directly overlying the Valanginian Unconformity, and it is unknown if any are present. Faulting and structuring during the Jurassic–Cretaceous rifting is a major risk to trap preservation.

In summary, despite different trap types being present at multiple stratigraphic levels within the Release Areas, there are several overarching risks to the validity of the potential plays, including: i) the absence of well data to provide lithological control on characteristics of the identified reservoirs and seals; ii) uncertainties in the relative timing of hydrocarbon generation and trap formation; and iii) reactivation of some major basement-involved faults in the Jurassic and in the Valanginian potentially resulting in a trap breach.

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Infrastructure and marketsThere is no existing infrastructure in this part of the northern Perth Basin, however, in close proximity are the towns of Carnarvon, Denham and Kalbarri, the port of Geraldton, and the Dampier to Bunbury gas pipeline.

NOPIMS dataData relating to the Release Areas can be accessed through the National Offshore Petroleum Information Management System (NOPIMS), an online data discovery and delivery system for all Australian offshore petroleum wells and seismic surveys, at www.ga.gov.au/nopims.

Marine and environmental informationRelease Area W17-9

Characteristics Release Area W17-9 is 40 km from the coast where Carrarang and Shark Bay are located, and 26 km west of

Dirk Hartog Island. The area overlies continental slope and terrace geomorphic features in water depths of approximately 110–3200 m, and overlaps the Shark Bay Commonwealth Marine Reserve.

On the shelf adjacent to the release area, sediment is composed predominantly of carbonates (80–100%), which slightly decrease in percentage in an offshore direction (continental slope and terrace carbonates: 60–100%) (Potter et al, 2008). Potter et al (2008) described pelagic sand and silt in ~150 m water depth, well-washed pelagic sand at slightly greater depths (150–200 m), and carbonate silt on the continental slope at depths up to ~600 m. Holocene skeletal shelf sediment components have attributes of both warm and cool water carbonates. Sediment texture is described as gravelly sand on the continental shelf, with slightly gravelly sandy mud and sandy mud on the continental slope and terrace (Richardson et al., 2005; Potter et al., 2008; MARS: http://www.ga.gov.au/oracle/mars/index.jspw).

Potential hazards may include cyclones that originate in the tropics but occasionally persist into the area of interest and as far south as Perth (http://www.bom.gov.au/cyclone/history/wa/perth.shtml). Tidal sand waves/sandbanks are present in Shark Bay (Baker et al., 2008), approximately 40 km to the east, and updrift of the release area (James et al., 1999).

Commonwealth marine reservesRelease Area W17-9 overlaps the Shark Bay Commonwealth Marine Reserve (CMR). It is also 35 km to the west and south-west of the Abrolhos CMR, and 9 km south of Carnarvon Canyon CMR.

Key ecological featuresRelease Area W17-9 overlaps the western demersal slope and associated fish communities key ecological feature and is 45 km east of the Abrolhos Wallaby Saddle key ecological feature (National Conservation Values Atlas, 2017).

Biologically important areasRelease Area W17-9 overlaps or is close to the following biologically important areas (National Conservation Values Atlas, 2017). The Area:

Lies to the west of foraging areas for the Lesser Crested Tern, Fairy Tern and Wedge-tailed Shearwater, with breeding sites for these species located in Shark Bay. Bridled Tern and Sooty Tern forage in the Release Area and to the east.

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Overlaps the Pygmy Blue Whale migration corridor along the shelf edge at depths 500–1000 m and its foraging area off Houtman and Abrolhos Islands.

Overlaps the Humpback Whale migration corridor.

Lies to the west of the Loggerhead Turtle internesting buffer, offshore of Dirk Hartog Island.

Table 1 Breeding periods (b) for key seabirds whose foraging areas are adjacent to or overlap the Release Area.

Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec

Bridled Tern b b b b

Lesser Crested Tern

b b b b

Fairy Tern b b b

Sooty Tern b b b b b b b b b b

Wedge-tailed Shearwater

b b b b b b b b b

The Atlas of Living Australia (www.ala.org.au) provides further information and visualisations on animals and plants recorded from the release area, including Threatened or Listed Species.

HeritageShark Bay is World and National Heritage listed and is located 43 km to the east.

The wreck of the World War II vessel Kormoran is located within the Release Area at -26.09705555, 111.22430556.

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

Atlas of Living Australia http://www.ala.org.au/

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

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

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

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

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

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

Shipwrecks http://www.environment.gov.au/heritage/historic-shipwrecks/australian-national-shipwreck-database

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Release Area W17-10

Characteristics Release Area W17-10 is located over the continental slope and terrace geomorphic features with canyons

located in the south, in water depths of approximately 130-3600 m, 41 km west of the coast where Useless Loop and Shark Bay are located. The Houtman and Abrolhos islands are located 125 km to the south east. The Abrolhos Commonwealth Marine Reserve is located 2 km south of the area.

On the shelf adjacent to the release area, sediment is composed predominantly of carbonates (80–100%), which slightly decreases in percentage in an offshore direction (continental slope and terrace carbonates: 60–100%) (Potter et al, 2008). Potter et al (2008) described pelagic sand and silt in ~150 m water depth, well-washed pelagic sand at slightly greater depths (150–200 m), and carbonate silt on the continental slope at depths up to ~600 m. Holocene skeletal shelf sediment components have attributes of both warm and cool water carbonates. Sediment texture is described as gravelly sand on the continental shelf, with slightly gravelly sandy mud and sandy mud on the continental slope and terrace (Richardson et al, 2005; Potter et al, 2008; MARS: http://www.ga.gov.au/oracle/mars/index.jspw).

Potential hazards may include cyclones that originate in the tropics but occasionally persist into the area of interest and as far south as Perth (http://www.bom.gov.au/cyclone/history/wa/perth.shtml). Tidal sand waves/sandbanks are present in Shark Bay (Baker et al, 2008), approximately 90 km to the northeast, and updrift of the release area (James et al, 1999).

Commonwealth marine reservesRelease Area W17-10 is surrounded by the Abrolhos Commonwealth Marine Reserve (CMR), 26 km to the west and 70 km east and 2 km south. The area is also 73 km south of Shark Bay CMR.

Key ecological featuresRelease Area W17-10 overlaps the western demersal slope and associated fish communities, as well as Perth Canyon and adjacent shelf break, and other coast canyons which are key ecological features. The Wallaby Saddle key ecological feature is located 37 km to the northwest. Located more than 90 km to the east are the following key ecological features: the western rock lobster; the Commonwealth marine environment surrounding of the Houtman Abrolhos Islands, and; the ancient coastline at 90 – 120 m depth (National Conservation Values Atlas, 2017).

Biologically important areasRelease Area W17-10 overlaps or is close to the following biologically important areas (National Conservation Values Atlas, 2017). The Area:

Overlaps the Pygmy Blue Whale migration corridor along the shelf edge at depths of 500–1000 m and foraging areas off the Houtman and Abrolhos Islands.

Overlaps the Humpback Whale migration corridor.

Lies to the west of the Loggerhead Turtle internesting buffer, offshore of Dirk Hartog Island.

The foraging and breeding areas for the following seabirds are located to the east and also overlap the area, extending out from the coastline and Houtman and Abrolhous Islands; Bridled Tern, Caspian Tern, Lesser Crested Tern, Common Noddy, Fairy Tern, Little Shearwater, Roseate Tern, Sooty Tern and Wedge-tailed Shearwater.

Australian Sea Lions forage around, and have breeding and haul out sites on the Houtman and Abrolhos Islands to the east.

White Shark foraging areas are related to the Australian Sea Lion habitat to the east.

Table 2 Breeding periods (b) for key seabirds whose foraging areas are adjacent to or overlap the Release Area.

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Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec

Bridled Tern b b b b

Lesser Crested Tern

b b b b

Common Noddy

b b b b b b b b b

Fairy Tern b b b

Little Shearwater

b b b b b b b b b b b b

Roseate Tern b b b b b

Sooty Tern b b b b b b b b b b

Wedge-tailed Shearwater

b b b b b b b b b

The Atlas of Living Australia (www.ala.org.au) provides further information and visualisations on animals and plants recorded from the release area, including Threatened or Listed Species.

HeritageShark Bay is World and National Heritage listed, located 43 km to the east.

The wreck of the World War II vessel HMAS Sydney is located at -26.24361111, 111.2175.

A number of vessels (Luggers) wrecked in 1883, are located at -26.66666667, 112.0.

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

Atlas of Living Australia http://www.ala.org.au/

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

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

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

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

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

Shipwrecks http://www.environment.gov.au/heritage/historic-shipwrecks/australian-national-shipwreck-database

Geoscience Australia products

Regional geology and seismic Houtman Sub-basin Geophysical Modelling Study. Report by Sanchez et al, 2016.

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Houtman Sub-basin Seismic Survey GA349: High resolution bathymetry grids; Seismic Field Data; Processed Seismic Data, available from NOPIMS or on request from ausgeodata@ga.gov.au

New Exploration Opportunities in the Offshore Houtman and Abrolhos sub-basins, Northern Perth Basin, WA. Publication in the APPEA Journal by Rollet et al, 2013.

Northern Extension of Active Petroleum Systems in the Offshore Perth Basin—an Integrated Stratigraphic, Geochemical, Geomechanical and Seepage Study. Proceedings of the Petroleum Exploration Society of Australia Symposium The Sedimentary Basins of Western Australia IV by Rollet et al, 2013.

Velocity Analysis and Depth Conversion in the Offshore Northern Perth Basin. Geoscience Australia Record 2012/33 by Johnston and Goncharov, 2012.

New Exploration Opportunities in the Offshore Northern Perth Basin. Publication in the APPEA Journal by Jones et al, 2011.

Offshore Northern Perth Basin Well Folio. Geoscience Australia Record 2011/09 by Jorgensen et al, 2011.

Tectonic and Stratigraphic History of the Perth Basin. Geoscience Australia Record 2004/16 by Norvick, 2004.

Geological Cross-section of the north Perth Basin. Bureau of Mineral Resources, Geology and Geophysics & Australian Petroleum Industry Research Association Record 1990/65 by Seggie, 1990

Stratigraphy Offshore Northern Perth Basin Biozonation and Stratigraphy Chart 38. Chart by Kelman et al, 2013.

Onshore Perth Basin Biozonation and Stratigraphy Chart 39. Chart by Kelman et al, 2013.

Petroleum systems and accumulations Oils of Western Australia II: Regional petroleum geochemistry and correlation of crude oils and condensates

from Western Australia and Papua New Guinea. Geoscience Australia and GeoMark Research Report by Edwards and Zumberge, 2005.

Sources for gas and oil in the Perth Basin. AGSO Research Newsletter by Boreham et al, 2000.

Australian Petroleum Accumulations, Perth Basin. Bureau of Resource Sciences Report by Cadman et al, 1994.

Contact Geoscience Australia’s Sales Centre for more information or to order these reports or products, phone 61 (0)2 6249 9966, email sales@ga.gov.au

AUSTRALIA 2017 Offshore Petroleum Exploration Acreage Release 16 16

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BAILLIE, P.W. AND JACOBSON, E.P., 1997—Prospectivity and exploration history of the Barrow Sub-basin, Western Australia. The APPEA Journal, 37(1), 117–135.

BAKER, C., POTTER, A., TRAN, M. AND HEAP, A.D., 2008—Geomorphology and Sedimentology of the Northwest Marine Region of Australia. Geoscience Australia, Record 2008/07. Geoscience Australia, Canberra. 220 pp.

BERNARDEL, G. AND NICHOLSON, C.J., 2013—Geoscience Australia Seismic Survey GA 310—revealing stratigraphy and structure of the outer northern Perth Basin margin. APPEA 2013 Poster Abstract.

BOREHAM, C.J., CHEN, J., GROSJEAN, E. AND POREDA, R., 2011—Carbon and Hydrogen Isotope Systematics of Natural Gases from the Perth Basin, Australia. Hedberg Conference Natural Gas Geochemistry, Beijing, China, 9–12 May 2011.

BORISSOVA, I., SOUTHBY, S., HALL., L.S., GROSJEAN, E., BERNARDEL, G., OWENS, R. AND MITCHELL, C., 2017—Geology and hydrocarbon prospectivity of the northern Houtman Sub-basin. Extended abstract. APPEA 2017. Perth, Australia.

BRADSHAW, B.E., ROLLET, N., TOTTERDELL, J.M. AND BORISSOVA, I., 2003—A revised structural framework for frontier basins on the southern and southwestern Australian continental margin. Geoscience Australia Record 2003/03 Canberra, 94 pp.

COPP, I.A., 1994—Depth to base Phanerozoic map of Western Australia: Explanatory Notes. Geological Survey of Western Australia, Record 1994/9.

CROSTELLA, A., 2001—Geology and petroleum potential of the Abrolhos Sub-basin, Western Australia. Western Australia Geological Survey, Report 75, 57 pp.

DENTITH, M.C., LONG, A., SCOTT, J., HARRIS, L.B. AND WILDE, S.A., 1994—The influence of basement on faulting within the Perth Basin, Western Australia. In: Purcell, P.G. AND Purcell, R.R. (Eds), The sedimentary basins of Western Australia. Proceedings of the Western Australian Basins Symposium, 791–799.

ENTERPRISE OIL EXPLORATION LTD AND NIPPON OIL EXPLORATION (PERTH BASIN) LTD, 1994a—Plum MSS final report. Part E. Geophysical interpretation, unpublished.

ENTERPRISE OIL EXPLORATION LTD AND NIPPON OIL EXPLORATION (PERTH BASIN) LTD., 1994b—Scarlet MSS final report. Part E. Geophysical interpretation, unpublished.

FERDINANDO, D. AND LONGLEY, I., 2015—Permian play mapping in the northern Perth Basin. Petroleum in Western Australia (PWA) magazine (17–26). Perth: Department of Mines and Petroleum.

FERDINANDO, D.D., BAKER, J.C., GONGORA, A. AND PIDGEON, B.A., 2007—Illite/smectite clays preserving porosity at depth in Lower Permian reservoirs, northern Perth Basin. The APPEA Journal, 47, 67–87.

GEOTECH, 2005—Hydrocarbon characterisation study; Perth Basin study (WA-286-P) (Cliff Head-4, Mentelle-1, Vindara-1 and Twin Lions-1). Professional Opinion prepared for Roc Oil Pty Ltd, unpublished.

GIBBONS, A.D., BARCKHAUSEN, U., DEN BOGAARD, P., HOERNLE, K., WERNER, R., WHITTAKER, J.M. AND MÜLLER, R.D., 2012—Constraining the Jurassic extent of Greater India: Tectonic evolution of the West Australian margin. Geochemistry, Geophysics, Geosystems, 13, Q05W13.

GORTER, J., HEARTY, D. AND BOND, A., 2004—Jurassic petroleum systems in the Houtman Sub-basin, northwestern offshore Perth Basin, Western Australia: a frontier petroleum province on the doorstep. The APPEA Journal, 44, 13–57.

GORTER, J.D. AND DEIGHTON, I., 2002—Effects of igneous activity in the offshore northern Perth Basin—Evidence from petroleum exploration wells, 2D seismic and magnetic surveys. In M. Keep and S. J. Moss(Eds.) The Sedimentary Basins of Western Australia III (875–899). Perth: Proceedings of the Petroleum Exploration Society of Australia Symposium.

HACKNEY, R., 2012—Combined marine and land potential-field datasets for the southwest margin of Australia. Geoscience Australia Record 2012/37, Canberra, 62 pp.

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FiguresFigure 1 Location of the 2017 Release Areas in the Houtman Sub-basin, showing structural elements, petroleum

fields, pipelines, shows and discoveries.

Figure 2 Location of Geoscience Australia survey GA349. Highlighted lines correspond to seismic sections shown in Figure 3. Background image is Bouguer gravity anomaly from Hackney (2012).

Figure 3 Interpreted seismic lines from survey GA349 in the northern Houtman Sub-basin: a) GA349/1011; b) GA349/1031; and c) GA349/1023. Sequence ages and interpreted Perth Basin lithostratigraphic equivalents are shown in Figure 5. Location of the lines is shown in Figure 2.

Figure 4 Geodynamic setting and crustal architecture of the northern Houtman Sub-basin: a) plate tectonic reconstruction of the southwest margin for 130 Ma (modified from Hall et al, 2013); b) interpreted seismic line GA349/1023, c) gravity modelling results for the GA349/1023 seismic line showing correlation between the measured (blue) and modelled (red) gravity anomalies and modelled densities of all sequences (modified from Sanchez et al, 2016).

Figure 5 Tectonostratigraphic chart for the northern Houtman Sub-basin tied to the 2016 geological timescale (Ogg et al, 2016). Basin phases, seismic sequences, interpreted lithostratigraphy, key regional events and potential petroleum systems elements are based on mapping and interprettation of seismic survey GA349 and regional stratigraphy of the Houtman Sub-basin. Also shown are the NW Shelf Supersequences (updated from Smith et al, 2015) and short-term relative sea-level curve (modified from Haq and Schutter, 2008; and Hardenbol et al,1998).

Figure 6 Mapped gross thicknesses of sequences across seismic survey GA349 in the northern Houtman Sub-basin for: a) Upper Cretaceous and Cenozoic; b) Jurassic; c) Triassic and d) Permian successions.

Figure 7 Source rock characteristics (TOC vs HI) by formation based on wells in the Abrolhos and southern Houtman Sub-basin (after Jones et al, 2011).

Figure 8 Burial history models for two Houtman Sub-basin pseudo-wells, calibrated with corrected temperature, maturity (Ro, FAMM) and AFTA data from wells in adjacent areas. a) Pseudo-well 1030-0 located on the basin margin, calibrated with well data from the Abrolhos Sub-basin. b) Pseudo-well 1030-2 located over the deepest part of the northern Houtman Sub-basin, calibrated with well data from the southern Houtman Sub-basin (modified from Hall et al, 2017).

Figure 9 Petroleum systems modelling results for potential Hovea Member source rock in the NH-TR1 Kockatea Shale equivalent: a) gross NH-TR1 seismic sequence thickness; b) maturity of Hovea Member; c) cumulative oil expelled from the Hovea Member; and d) cumulative gas expelled from the Hovea Member (modified from Hall et al, 2017).

Figure 10 Conceptual play diagram for the northern Houtman sub-basin showing a range of possible structural and stratigraphic plays at different stratigraphic levels.

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