Petroleum Geological SUMMARY

Release Areas W11-4, W11-5 AND W11-6,

RowleySub-basin, RoebuckBasin,

Western Australia

Bids Close – 12 April 2012

  • Under-explored sub-basin.
  • Offshore extension of the proven OnshoreCanningBasin hydrocarbon province.
  • Phoenix1 gas column suggests active petroleum system in area.
  • Water depths of approximately 80m to less than 600m.

Location

The Rowley Sub-basin Release Areas are located approximately 250km north-northeast of Port Hedland,150km west of Broome, and 150km northwest of the proposed Great Northern Pipeline (Figure1). The Release Areas cover the westernmost inboard region of the RowleySub-basinand extend into the adjacentBedoutSub-basin, as well as the Broome Platform and OobagoomaSub-basin of the offshore CanningBasin. Water depths in this region range from 80m in the southeast to 590m in the west-northwest.

The three Release Areas over the RowleySub-basin are, anticlockwise starting in the northeast, W11-4, W11-5 and W11-6 and comprise approximately 76, 106 and 100 graticular blocks respectively(Figure2.). While all the Release Areas cover part of the RowleySub-basin, a small portion of W11-4 extends into the westernOobagoomaSub-basin, a small portion of W11-5 lies within the northern BedoutSub-basin, and W11-6 also covers part of the BedoutSub-basin, OobagoomaSub-basin and Broome Platform(Figure3). All Release Areas have an interpreted Mesozoic section with a thickness of 2.2s (TWT) to greater than 2.8s (TWT) and contain a variety of potential structural and stratigraphic plays.

Currently, the petroleum potential of the RowleySub-basin is considered poor due to the perceived absence of a prolific source rock (Smith et al, 1999). The limited exploration of the sub-basin has made no economic discoveries, although a tight gas column has been interpreted in Phoenix1 in the BedoutSub-basin(Figure1). Evidence of hydrocarbon systems in adjacent sub-basins includes the Perindi1 oil indication in the OobagoomaSub-basin, and the Nebo1 oil discovery in the BeagleSub-basin.

Release Area Geology

Local Tectonic Setting

The Rowley Sub-basin is a major westward-thickening upper Paleozoic–Mesozoic depocentre containing about 9km of Permo-Carboniferous or older strata and up to 6km of Mesozoic–Holocene sediments terminated at the present continental boundary by large rift faults (Smith et al, 1999). The sub-basin is separated from the BeagleSub-basinand Exmouth Plateau of the NorthernCarnarvonBasin to the southwest by the North Turtle Hinge Zone and Thouin Graben, and from the Bedout and Oobagooma sub-basins by the Bedout High and Oobagooma High, respectively. The Paleozoic and lower Mesozoic successions within the RowleySub-basin onlap the Broome Platform, Bedout High and Oobagooma High(Figure3).

Structural Evolution and Depositional History of the Sub-basin

The development of the RowleySub-basin is largely inferred from what is known of the geological evolution of the adjacent BedoutSub-basin and CanningBasin, including alower Ordovician–lower Carboniferous section postulated to underlie the RowleySub-basin (Lipski, 1993; Kennard et al, 1994; Smith et al, 1999). The Ordovician–Cenozoic structural and depositional history of the RowleySub-basin can be divided into several phases ((Figure4)

  • Ordovician–middle Carboniferous
  • Late Carboniferous–Permian
  • Triassic–Early Jurassic
  • Early Jurassic–Early Cretaceous
  • Early Cretaceous–Holocene

The earliest Ordovician to Carboniferous phase began with the first extensional event recognised in the region,a northeast–southwest extension in the Ordovician. 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). Ordovician to middle Carboniferous sedimentary rocks, as observed in the onshore CanningBasin, primarily consist of alternating sequences of marine clastic and carbonate rocks, including the primarily transgressive Devonian Reef Complex and Fairfield Group (Kennard et al, 1994).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).

The next phase began in the Pennsylvanian (late Carboniferous) with a change to the predominantly northeast-oriented structures of the RoebuckBasin. This was a period of transition from intracratonic extension to extension related to terrane separation from Gondwana (Metcalfe, 1988; Smith et al, 1999). In parts of the CanningBasin, syn-rift sedimentation continued along reactivated intracratonic fractures. Pennsylvanian 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 basin fill in the offshore Canning and Roebuck basins during this initial stage is poorly understood. Overlying the Paleozoic sequence is a prominent regional unconformity possibly related to the formation of the Bedout High (Colwell and Stagg, 1994).

The Triassic to the Early Jurassic phase was dominated by thermal sag with transgressive marine and fluvio-deltaic sedimentation (Locker Shale, Keraudren and Bedout formations). Separating the lower and upper Keraudren Formation is the Middle Triassic Cossigny Member, a widespread limestone unit seismically expressed as a high-amplitude reflector. Triassic to Early Jurassic deposition was punctuated by multiple northwest–southeast transpressional events, the Fitzroy Movement, focused along the margins of the sub-basins in the Ladinian, Norian and Sinemurian (Smith et al, 1999).

The next phase from the Early Jurassic to the Early Cretaceous began after Sinemurian uplift and erosion withaccumulation of a broad prograding wedge of fluvio-deltaic sediments (Depuch Formation). The deposition occurred during thermal subsidence across the shelf. Continental breakup in the Callovian resulted in a second phase of prominent uplift and erosion. Subsequent thermal subsidence allowed accumulation of condensed marine mudstones (Baleine and Egret formations) until the Early Cretaceous. An influx of siliciclastic material (Broome Sandstone) occurred in the Valanginian with further uplift of the sediment source (Smith 1999, Smith et al, 1999).

The most recent phase began with thermal relaxation of the crust soon after the Valanginian break-up. This led to the development of a passive-margin succession of marine mudstones and marls. A major progradational carbonate wedge developed across the entire North West Shelf in the Cenozoic. Collision of the Australian and Eurasian plates in the mid-Miocene led to transpressional inversion of north-northwest-trending Paleozoic faults, especially in the northeast of the adjacent OobagoomaSub-basin (Smith et al, 1999).

Exploration History

Few wells have been drilled in the Roebuck and offshore Canning basins due in part to the historical perception that these Basins are not highly prospective.Almost all of the drilling (12 of the total of 15 wells) in these offshore basins was undertaken in the Oobagooma and Bedout sub-basins in the early 70sand early 80s, the other three wells were drilled in the outboard Rowley Sub-basin; East Mermaid1 (1973),Whitetail1 (2003) and Huntsman1 (2006).

Drilling in the early 70s was underpinned by several small 2D seismic grids and a few regional 2D grids. In the late 80s to early 90s, more regional 2D seismic was acquired and the North West Shelf tie lines were shot by AGSO. An additional episode of seismic acquisition occurred between 1998 and 2002, and included the first 3D grids used to determine objectives drilled in 2003 and 2006 by Whitetail1 and Huntsman1respectively. Currently,two seismic grids are being acquired by Petroleum Geo-Services (PGS) across the Release Areas,and other 2D and 3D seismic has recently been acquired to thesouth of W11-5. Recent data acquisition in the area conducted by Geoscience Australiaincludes a hydrocarbon seepage survey in 2006 over the Roebuck and offshore CanningBasins and an aeromagnetic survey in the OobagoomaSub-basin in 2007.

Within the RoebuckBasin, results to date have been limited to gas shows and discoveries in the Triassic Sandstones of Phoenix1 and2. In the OobagoomaSub-basin, Perindi1 intersected several intervals in Paleozoic clastics and carbonates with oil indication. In the more intensively explored Beagle Sub-basin to the southwest, oil was discovered in the Callovian Calypso Formation sandstones at Nebo1 (Osborne, 1994), but no economic discoveries in any of these offshore basins have been made.

From the 2007 and 2008 acreage release permits immediately to the east and south of the Release Areas have been awarded(Figure1). Woodside Energy Ltd has WA-415-P and WA-416-P abutting W11-4 and W11-6 respectively.In 2010 Woodside Energy completed a 4,100 km 2D seismic survey (Koolama 2D) mostly over WA-415-P and WA-416-P (Woodside Petroleum Ltd, 2010, p.7). The blocks adjacent to W11 and W11-6 are operated/held by Finder Exploration (WA-435-P and WA-436-P) and Carnarvon Petroleum Ltd (WA-443-P). Finder Exploration conducted a 15,856 km2aeromagnetic survey overthe entirety of WA-436-P and a portion of WA-443-P (Carnarvon Petroleum Ltd, 2010, p. 6), with further plans for 2D and 3D seismic surveys acrossWA-435-P and WA-436-P,including a significant portion ofthe 1,100km2Phoenix 3D survey that should lie within WA-435-P.

Well Control

Nine wells have been drilled in the RoebuckBasin and six in the offshore CanningBasin; none with commercial success (Figure1). Three wells (East Mermaid1, Huntsman1 and Whitetail1) have unsuccessfully tested the potential of the RowleySub-basin, including deep-water prospects. EastMermaid1 lies within Release AreaW11-5, only 10s of meters from Release Area W11-4 and approximately 1 km westof Release Area W11-6. Six wells have been drilled in the adjacent BedoutSub-basin (Bedout1, Keraudren1, Lagrange1, Minilya1 and Phoenix1 and2), with the only significant hydrocarbon indications being the tight gas column discovery in Phoenix1 and gas shows in Phoenix2. Of the six wells drilled in the offshore CanningBasin (Kambara1, Lacepede1, Minjin1, Pearl1, Perindi1 and Wamac1) all are within the OobagoomaSub-basin. Ten wells have been drilled in the eastern part of the BeagleSub-basin, southwest of the Release Areas (Bruce1, Cimba1,Darwin1, Depuch1, Halo1, Huascaran1, Nebo1, NorthTurtle1, Picard1 and Poissonnier1). The only significant discovery in the BeagleSub-basin is a small oil pool at Nebo1. The following summaries of key wells are based on the relevant well completion reports.

Lacepede1 and1A (1970)

Lacepede1 and1A were drilled by BOC of AustraliaLtd approximately 129 km northwest of Broome in the central OobagoomaSub-basin. Lacepede1 was abandoned at 224m due to technical difficulties and Lacepede1A reached a total depth (TD) of 2,286mRT. The wells were drilled to test an elongate east–west-trending anticlinal structure. Lacepede1A penetrated Miocene sediments unconformably overlying an Early Cretaceous to Pennsylvanian (upperCarboniferous)sedimentary succession(originally interpreted to be upperPermian; Helby and Partridge, 1982). Lacepede1A penetrated the most complete Jurassic section in the CanningBasin at that time, with rocks of Early Jurassic age being recorded for the first time. The entire Triassic and most of the Permian section is absent, such that Jurassic (Tithonian–Toarcian) sediments unconformably overlie lower Permian (Asselian) to upper Carboniferous sediments. No significant hydrocarbons shows were encountered in the wells. Slight increases in the background gas readings occurred in the Jurassic and Permian sections and gas prone Middle to Upper Jurassic shales were identified with TOC of up to 2.3% (Conoco, 1992). However, no fluorescence was observed and wireline logs indicated 100% water saturation throughout the well. Lacepede1A confirmed the presence of thick, porous and permeable reservoirs in the Lower Cretaceous and Jurassic sections and is thought to have adequately tested the Cretaceous, Jurassic and uppermost Permian sections of the Lacepede structure.

Bedout1 (1971)

Bedout1 was drilled by BOC of Australia Ltd to test the seismically-delineated, unfaulted, anticlinal dome of the Bedout High. The well reached a total depth of 3,073mRT, and penetrated 465m of Upper Cretaceous claystone and carbonate, 450m of Lower Cretaceous clastic sediments, and a 1,113m thick Jurassic succession of deltaic sandstone with interbedded claystone and coal, including potential source rocks. A thin section (52m) of Triassic sandstone and claystone overlies volcaniclastic conglomerate in the bottom of the well. The latter were not anticipated and were interpreted as basement. Good reservoirs were confirmed in the Upper Cretaceous, Jurassic and Upper Triassic intervals, but no hydrocarbon shows were encountered in the well.

Wamac1 (1973)

Wamac1 was drilled by Amax Petroleum (Australia) Inc. to a TD of 2,764mRT and is located 110 km northwest of Broome in the central OobagoomaSub-basin (Figure5). The well tested thePaleozoic sedimentary succession in an east–west elongated structural feature. Seismicmappingindicated an areal closure of >250 km2 in the Devonian and also possible closure in the Lower Permian. Wamac 1 penetrated a Quaternary to Carboniferous sedimentary succession similar to that in Lacepede1A. The Paleozoic sediments are intruded by Permian to Lower Triassic dolerite. Difficulties encountered while drilling one of these sills led to termination of the well ~900m above planned TD. Absence of hydrocarbon shows may have been due to lack of structural closure in the Mesozoic andupper Paleozoic or low porosity and negligible permeability throughout the Paleozoic, although logs and lithologic analysis indicate good reservoir quality through the Lower Cretaceous and Jurassic. Immature source rock intervals in the Jurassic indicate potential for oil and gas in deeper parts of the basin.

East Mermaid1 (1973)

East Mermaid1 lies within Acreage Release Block W11-5 in the southeastern part of the RowleySub-basin, and was drilled by Shell Development Australia Pty Ltd. It was drilled to test a series of northeast–southwest oriented, low relief, anticlinal highs mapped and displaying closure in the Middle Jurassic and throughout the Cretaceous. East Mermaid1ST1 reached a total depth of 4,068mRT, intersecting a predominantly fine-grained Cretaceous succession and a thick (1,184m) Jurassic succession of sandstone, siltstone and coal. The well confirmed the predicted structural configuration and identified the primary target Jurassic sandstones as terrestrial in nature. No hydrocarbons were encountered in the well, which was attributed to the lack of a proximal source rock. Recent research suggests that the tested anticlinal structures are associated with the 'Mermaid Fault Zone', in which faults extend to or near to the seabed, and are currently active (Jones et al, 2007). Therefore, trap integrity is a major risk at this site(Figure6).

Minilya1 (1974)

Minilya1 was drilled by BOC of Australia Ltd to test an interpreted fault-controlled positive feature located in the outboard part of the BedoutSub-basin. The well reached a total depth of 2,400mRT and intersected Cretaceous claystone, sandstone and carbonates terminating in a succession of Middle Jurassic sandstone and claystone with minor coal. These sandstones, the primary objective, showed variable but good porosity. Sandstones with effective porosity were also intersected in the Upper Cretaceous succession. No significant hydrocarbon shows were recorded in the well. Interpretation of seismic data acquired over the area in 1992 (C92A) suggests that there is no closure on the Minilya1 structure (Esso, 1994).

Phoenix1 (1980)

Phoenix1 was drilled by BP Petroleum Development Australia Pty Ltd to test an interpreted large northeast–southwest trending, elongate anticline transected by a series of north-northeast to south-southwest trending faults in the BedoutSub-basin. These faults were interpreted to have formed four main culminations, with Phoenix1 located on the most central of these. More recent seismic data suggest, however, that structuring in the area is more complex (Apache, 1995). The primary objective of the well was Middle to Upper Triassic sandstone of the Keraudren Formation, while Middle Jurassic sandstone units directly below the Callovian breakup unconformity were a secondary target. However, seismic data indicated a lack of closure at the upperobjective. The well reached a total depth of 4,880mRT, exceeding the planned total depth of 4,450m after gas-bearing Middle Triassic sandstones were intersected. Reservoirs in the Keraudren Formation provided good hydrocarbon indications. Despite good reservoir characteristics in the upper Keraudren Formation sandstones (average porosity of 14%, ranging from 19–12%, decreasing with depth), detected hydrocarbons were limited to oil staining and cut fluorescence. The lower Keraudren Formation sandstone reservoirs exhibited gas and fluorescence (all sandstones below 4,223mRT are gas-bearing; net gas pay is estimated to be 110.5m over seven zones), but logs and core analysis indicate very low porosity (average porosity of 9%, ranging from 4–15%).

Hydrocarbon types in the lower Keraudren Formation sandstones show fractionation with depth, which suggests intra-formational sourcing of gas and existence of discrete reservoirs that are not in communication due to intercalated sealing shales. Drilling was terminated without testing when it was deemed unsafe to continue with the available pressure-control equipment. The well was abandoned in January 1983.

Phoenix2 (1982)

Phoenix2 was drilled by BP Petroleum Development Australia Pty Ltd to test Triassic sandstones in a fault block forming part of the 'Phoenix structure' in the BedoutSub-basin. The well reached a total depth of 4,967mRT. The sedimentary succession intersected is very similar to that in Phoenix1, but the basal 591m of Lower to Middle Triassic Locker Shale is interpreted as the outboard facies equivalent of the lower Keraudren Formation interpreted in Phoenix1. Gas shows are interpreted at low saturations in thin zones within the Triassic sandstones, which have extremely low permeabilities and generally poor reservoir qualities. Oil indications, such as natural fluorescence and possible staining, were also identified. The targeted reservoirs showed significantly lower permeabilities compared to Phoenix1, and therefore had lower hydrocarbon saturations. Calculations based on the low permeabilities, indicated that commercial hydrocarbon flows could not be obtained from the reservoir sandstones. Therefore, no production testing was carried out.

Modelling by Lisk et al (2000) suggests the Triassic Keraudren Formation is mature for gas/condensate and minor oil generation in more deeply buried sections of the BedoutSub-basin. This unit is mature in Phoenix1 and predominantly gas prone, with fluorescence and oil staining likley sourced from thin shales within the unit (Forth and Jourdan, 1982). The Keraudren Formation at Phoenix2 is early mature for oil and predominantly gas prone, but one thin zone is modelled to be capable of generating light oil.

Lagrange1 (1983)

Lagrange1 was drilled by BP Petroleum Development Australia Pty Ltd to test Upper Triassic sandstones on the irregular anticlinal crest of the Bedout High, with secondary targets in the Lower to Middle Jurassic and, more speculatively, in Paleozoic sediments. The well reached a total depth of 3,260mRT and penetrated a sedimentary succession very similar to that in Bedout1. Paleozoic targets were not reached because an unexpectedly thick succession of volcanics (<391m) was encountered. Lagrange1 was plugged and abandoned as a dry hole.