GEOLOGICAL INFORMATION RELEASE AREAS W11-7, W11-8 AND W11-9, NORTHEASTERN EXMOUTH PLATEAU, NORTHERN CARNARVON BASIN WESTERN AUSTRALIA Bids Close – 12 April 2012 Deepwater frontier of Australia’s premier Northern Carnarvon Basin hydrocarbon province. Large untested structures on trend with large gas fields to the south. Water depths range from 500 m to 4500 m. Play types include horsts, reefs and basin-floor fan sands. Infrastructure in place over 100 km to the south. Active petroleum system present at Nebo 1 to the south, but may indicate a local, rather than a regional, origin. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 1 of 24 LOCATION Release Areas W11-7, W11-8 and W11-9 lie 250-450 km north of Dampier, Western Australia, across the northeastern Exmouth Plateau and Beagle Subbasin of the Northern Carnarvon Basin (Figure 1). Water depth across the Release Areas deepens from the south to the north over the range 5004500 m. The northern half of the Release Areas includes several large canyons that dissect the margin down to the Argo Abyssal Plain. The graticular block map and listings defining the Release Areas are shown in Figure 2. A total of 488 blocks is divided across the Release Areas: 167 blocks (approximately 13655 km2) in W11-7, 154 blocks (approximately 12585 km2) in W11-8 and 167 blocks (13555 km2) in W11-9. The Northern Carnarvon Basin is Australia’s premier hydrocarbon province, with the Exmouth Plateau component hosting world-class giant gas fields (e.g. Io-Jansz, Gorgon and Scarborough), while the Barrow and Dampier subbasins host more moderately sized oil/gas accumulations (e.g. Perseus/Goodwyn, Angel and Stag). The northeastern Exmouth Plateau and Beagle Sub-basin components, however, are relatively under-explored. Only Wigmore 1 well has been drilled in the Release Areas, and it was unsuccessful. In the immediate vicinity, however, Nebo 1 discovered a subcommercial oil accumulation and this suggests, at least, a localised active petroleum system. In addition, Phoenix 1, further to the southeast, discovered gas. The nearest production facilities, within the northern Dampier Sub-basin, are the Santos Ltd-operated Mutineer-Pitcairn and Exeter oil fields, about 100 km to the south, and the large Woodside Energy Ltd-operated Angel gas field, a further 25 km to the south. Production commenced at Mutineer-Pitcairn and Exeter in 2005 as a floating production, storage and offload facility (FPSO). Production at Angel began in 2008, and is carried out via a subsea pipeline link to a trunkline connecting the North Rankin A platform to an onshore facility. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 2 of 24 RELEASE AREA GEOLOGY Local Tectonic Setting The Release Areas are located on the North West Shelf, over the northeastern Exmouth Plateau – an area sometimes termed the Dixon Subbasin (Hocking et al, 1994) – where the Northern Carnarvon Basin abuts the Roebuck Basin at the southeastern end of the Argo Abyssal Plain. In terms of the broad structural elements, the Release Areas overlie the Exmouth Plateau and Beagle Sub-basin (Northern Carnarvon Basin), to the west and south, and the western flank of the Rowley Sub-basin (Roebuck Basin), to the east (Figure 3). To the north lies the oceanic crust of the Argo Abyssal Plain. The general distinction between these elements reflects the partitioning of a broad intracratonic downwarp, during the Triassic, into various structural grains related to - and modified later by - Mid-Late Jurassic and Early Cretaceous oblique rifting episodes which led, respectively, to Oxfordian and Valanginian breakups. The Exmouth Plateau is a broad, sunken continental block, which is underlain by 10-15 km of a generally flat-lying, or block-faulted and tilted, Lower Cretaceous, Jurassic, Triassic and Paleozoic section. These sediments were deposited during periods of extension that preceded the breakup of Australia and Argo Land in the Middle Jurassic and then Greater India in the Early Cretaceous. The plateau’s northern margin is defined by a series of broad northeast- to north-trending canyons and a large, high-standing detached continental block (i.e. Wombat Plateau). Exon et al (1982) considered this margin’s underlying Mesozoic geology as distinct from the Exmouth Plateau proper to the south, with its complex morphology being related to a more dominant component of shearing leading to breakup. The Beagle Sub-basin is dominated by a horst-and-graben morphology modified by transcurrent motion which reflected oblique extension prior to the separation of Argo Land. It contains over 10 km of Upper Paleozoic to Cenozoic sediments. Tectonically, it is defined by two regions: one dominated by fault-controlled basement flanks to the south and the other a broad, heavily faulted depocentre in the north (Figure 3; Blevin et al, 1993a, 1993b, 1994). The Rowley Sub-basin represents a major westward-thickening upper Paleozoic-Cenozoic depocentre containing about 9 km of PermoCarboniferous, or older, strata overlain by up to 6 km of Mesozoic-Cenozoic sediments (Smith et al, 1999). It is structurally separated from the Beagle Sub-basin and northeastern Exmouth Plateau, to its west, by a long, sinuous zone of uplift and faulting termed the North Turtle Hinge Zone. Unlike the dissected form of the northern margin to the Exmouth Plateau, the outer margin of the Rowley Sub-basin is characterised by margin-parallel largethrow rift faults descending to the Argo Abyssal Plain. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 3 of 24 Although direct lithostratigraphic information from wells is limited, the general chronostratigraphic setting within the Release Areas is expected to reflect Northern Carnarvon Basin (i.e. Dampier and Beagle sub-basins) depositional cycles. However, the large distances from siliciclastic sediment sources to the south and east, as well as the effects of widespread structuring and igneous activity closer to the locus of breakup to the north, is likely to have substantially affected the sedimentary facies. Structural Evolution and Depositional History of the area The dominant fault trend across the Exmouth Plateau is north-south to northeast-southwest, reflecting an early widespread extensional structural grain recognised in the Paleozoic of the onshore Carnarvon Basin (Stagg and Colwell, 1994; Stagg et al, 2004) and offshore, over a ductile lithosphere (Gartrell, 2000). These trends are broadly reflected inboard by the later development of thick Mesozoic sequences across a series of broad troughs and highs (e.g. Exmouth, Barrow and Dampier sub-basins and Rankin Platform). The late Paleozoic rift cycle was succeeded across the Northern Carnarvon Basin by an Early Triassic sag phase resulting in an initial regional marine transgression. This transgression is marked by the marine Locker Shale, which unconformably overlies the Permian section and grades upwards into the thick fluvio-deltaic Mungaroo Formation (Figure 4; i.e. equivalent to the Keraudren Formation in the Roebuck Basin). The Locker Shale was deposited in shallow shelf and shoreline environments, while the Mungaroo Formation was deposited on a broad, low-relief, rapidly subsiding coastal plain as northward-prograding deltaic sequences that extended across the Exmouth Plateau. The Mungaroo Formation consists of numerous fining-upward cycles of thick fluvio-deltaic sandstone with minor interbeds of siltstone, claystone and coal (e.g. Delambre 1). Along the northern parts of the plateau it is expected to be influenced by more marine sedimentation pulses. For example, the Middle Triassic Cossigny Member comprises paralic to marine siltstone, claystone and limestone/dolomite, and where the limestone/dolomite is well developed it forms an important transgressive seismic marker horizon (Figure 5 and Figure 6). The Late Triassic was marked by an episode of faulting across the Northern Carnarvon Basin prior to a limited marine transgression from the north. This tectonic pulse was manifest along the northern margin of the Exmouth Plateau by faulting within the Swan Graben and Kangaroo Syncline, concomitant with the development of volcanics on the northwestern margin of the Wombat Plateau (Exon et al, 1982). The Wombat Plateau ODP wells show that this period was characterised along the northern part of the plateau by Carnian prodelta mudstones and carbonate banks grading into Norian shallow marine to paralic sedimentation of siliciclastics, shelfal carbonates and coal (von Rad et al, 1992a). This was followed in the Rhaetian by a marine transgression marked by marls (Brigadier Formation equivalent; Figure 4) and shelf and reefal limestones (Williamson et al, 1989; Haq et al, 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 4 of 24 1990; Exon and von Rad, 1994). Further inboard, where intersected (Delambre 1), the Brigadier Formation comprises numerous fining- and coarsening-upward cycles of thinly bedded sandstone, siltstone and claystone deposited in a shallow to marginal marine setting. The Jurassic is characterised by continued structuring across the region, as rifting between Australia and Greater India proceeded to eventual breakup. Jurassic sedimentation is absent on the Wombat Plateau (von Rad et al, 1992a, 1992b) where the full effects of post-breakup erosion are expected to have occurred. The Early Jurassic was marked by deposition of the North Rankin Formation and Murat Siltstone (“Picard Formation” in Wigmore 1). For the Release Areas, information on these units is limited to that obtained from Wigmore 1. The North Rankin Formation comprises thick, clean sandstone units, interbedded with siltstone and claystone, deposited in a high-energy shallow marine environment. It is directly overlain by the “Pliensbachian Limestone” unit/member of the Murat Siltstone, which is widespread across the northern part of the Exmouth Plateau. In the Pliensbachian, rifting began to be focused more inboard in the Exmouth, Barrow, Dampier and Beagle sub-basin depocentres. In these troughs, several kilometres of marine Jurassic sediments were deposited; these are the equivalent of condensed sections a few metres thick preserved in some locations of the central Exmouth Plateau. After a minor hiatus in the Toarcian, the widespread Athol and Legendre formations were deposited in the Middle Jurassic. Athol Formation claystones were deposited in a restricted marine environment and represent the marine facies distal to the widespread, major prograding sandstones of the Legendre Formation, which reached its maximum extent in the Bathonian, just prior to Argo breakup. The Legendre Formation is characterised by a series of stacked coarsening-upward fluvio-deltaic cycles of sandstone, siltstone, shale and minor coal (e.g. Wigmore 1 and Nebo 1). Rift-related faulting across the northern Exmouth Plateau reached its peak with Argo breakup in the Callovian. The Callovian marks the major movement on the rift faults along the northern Exmouth Plateau, while oceanic crust did not develop in the Argo Abyssal Plain until the Oxfordian (Norvick, 2002). The Wombat Plateau, and emergent syncline flanks in the adjoining Exmouth Plateau, were uplifted and eroded . The resulting erosion is particularly marked on the Wombat Plateau as the Triassic section is directly overlain by the Cretaceous, with thin Berriasian shallow marine sands/silts deposited directly on Rhaetian reefal systems (von Rad et al, 1992a). Further inboard, this period is marked by deposition of the transgressive Callovian-Oxfordian Calypso Formation (e.g. Manaslu 1 and Nebo 1), which consists of glauconitic claystone and thinly interbedded sandstone and siltstone. Wells across the region indicate that the Upper Jurassic succession is very thin or absent (Figure 4; ODP Leg 122, Wigmore 1, Nebo 1, Whitetail 1 and Manaslu 1). This interval is largely represented in the inboard sub-basins, to 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 5 of 24 the south, by the extensive Dingo Claystone. Given that the wells in the Exmouth Plateau are generally located on structural highs, where Callovian and Valanginian erosional events are expected to be pronounced, this unit or its equivalent, may be present within intervening lows. A very thin interval of Dingo Claystone/Angel Formation is interpreted in Darwin 1, and this most likely represents a more condensed development of the much thicker unit found in the Barrow/Dampier/Exmouth sub-basins to the south. The Early Cretaceous in the Northern Carnarvon Basin was a more quiescent phase of structuring. The Berriasian-Valanginian is characterised by some late syn-rift style sedimentation, as breakup proceeded far to the west and south in the Gascoyne and Cuvier abyssal plains respectively. Breakup was followed by the establishment of an early passive margin setting with the onset of rifted margin thermal relaxation. This continued until the Aptian when deposition of the Windalia Radiolarite marked the onset of largely carbonate deposition in an open marine setting. The Berriasian to Valanginian Forestier Claystone is a marine claystone widespread across the northeastern Exmouth Plateau, and is the late syn-rift equivalent of the massive Barrow Delta developed elsewhere. The unit probably thickens into the troughs and forms a regional seal (e.g. Nebo 1), but it is absent (Wigmore 1) or thin (Whitetail 1 and Manaslu 1) further north. The Valanginian to Barremian Muderong Shale is a thick marine claystone that is widespread across the Northern Carnarvon Basin, and acts as a secondary seal to the Forestier Claystone. A basal sandstone unit may be considered equivalent to the Mardie Greensand Member, present in the southern sub-basins. The upper Lower Cretaceous to Holocene section comprises claystone, marl and calcilutite deposited on a prograding, increasingly carbonate-dominated, open marine, passive ramp margin. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 6 of 24 EXPLORATION HISTORY Exploration of the Beagle Sub-basin area began in 1965 with regional seismic, gravity and magnetic surveys. Subsequent exploration led to the drilling of 13 wildcat wells between 1971 and 1983. The wells tested a range of plays, including uplifted Triassic–Jurassic fault blocks, anticlines and faultcontrolled structures down-dip of the main basin-margin fault. Drilling confirmed the presence of a thick Upper Paleozoic–Cenozoic sedimentary succession which contained prospective reservoir, source and seal units. Despite several minor shows, no significant hydrocarbon discoveries resulted. Only two wells were drilled in the sub-basin between 1983 and 1992 (Trafalgar 1 in 1988 and Aurora 1 in 1990), together with two wells along the northern margin of the adjacent Dampier Sub-basin (Bounty 1 in 1983 and Calypso 1 in 1985). However, no hydrocarbon shows were recorded in these wells. In the early 1990s, a third wave of exploration activity was initiated in the subbasin. Nebo 1 (1993) encountered thin oil-bearing sands in the Callovian Calypso Formation (Figure 4), and was the first well to confirm the presence of an active petroleum system in the Beagle Sub-basin (Osborne, 1994). In 1994–1995 the same joint-venture consortium drilled three unsuccessful wells (Cimba 1, Darwin 1 and Halo 1). To the south, within the northernmost Dampier Basin, Mutineer 1B discovered oil in the Upper Jurassic Angel Formation, which encouraged further exploration in the adjacent Beagle Sub-basin. Woodside subsequently drilled four unsuccessful wells (Serval 1, Ermine 1, Grey Rabbit 1 and Tayra 1) in the western-central sub-basin in 1999–2001, and IB Resources drilled two unsuccessful wells (Manaslu 1 and Huascaran 1) in the central sub-basin in 2001–2002. Wigmore 1 (2002) was drilled by Kerr-McGee in the outer, deepwater, northwest portion of the sub-basin, and was also unsuccessful. To the east of the Release Areas, within the Roebuck Basin, Whitetail 1 (2003) and Huntsman 1 (2006) (Figure 1) were drilled by Woodside Energy Ltd to test Jurassic targets within structural highs. The wells were unsuccessful and, at the time of writing, interpretive results for Huntsman 1 had not been released. For the deepwater Exmouth Plateau, there have been two major exploration campaigns: the first in 1979–1980 for oil targets; and the second, currently underway, for gas. The initial exploration programs undertaken by Esso and Phillips (Barber, 1988) began when no proven technology existed to develop a deepwater oil field. Eleven deepwater (740–1375 m) wells were drilled by the early 1980s (Walker, 2007) with the anticipation of oil charge from the Upper Jurassic Dingo Claystone. A result of this campaign was the discovery of a giant gas accumulation in a Lower Cretaceous Barrow Group basin floor fan by the Scarborough 1 well. The Scarborough domal anticline, which was generated by structural inversion in the Campanian, is approximately 350 km2 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 7 of 24 in area and contains reserves of about 8 Tcf of gas (BHP Billiton, 2008). At the time of this discovery (i.e. 1979), the available technology and the undeveloped LNG market made the remote, deepwater gas accumulation uneconomic to develop. In this initial phase of exploration on the Exmouth Plateau, gas was also discovered in Upper Triassic Mungaroo Formation sandstones by the Jupiter 1 well, in a tilted fault block trap (approximately 0.15 Tcf; Walker, 2007). After the 1979-1980 drilling campaign no further exploration on the deepwater Exmouth Plateau was undertaken for over a decade. The second phase of activity commenced in the mid-1990s with a focus on the established Triassic fault block play along the eastern margin of the Exmouth Plateau, outboard of the Rankin Trend. Activity returned to the central Exmouth Plateau in 1996 with the appraisal drilling of Scarborough 2 in Retention Lease WA-1-R, jointly held by ExxonMobil and BHP Billiton. The largest discovery yet made on the Exmouth Plateau is Io-Jansz, a super-giant gas field in a new play type drilled in 2000 (Jenkins et al, 2003). In 2003, BHP Billiton was awarded petroleum exploration permit WA-346-P, adjoining the Scarborough retention lease to the north, and in 2004 it undertook a 3D seismic survey over both areas. Thebe 1 (2007) was drilled and is interpreted as a successful test of a Triassic fault block that may contain 2-3 Tcf of gas (Journal of Petroleum Technology, 2007). Market conditions have changed markedly since the first phase of exploration on the Exmouth Plateau in the 1970s, with major gas contracts secured to supply LNG to China, in addition to the established trade with Japan. An indication of the viability of the deepwater frontier plays on the Exmouth Plateau was the competitive bidding for the 2006 Release Areas, and the entry of a number of new explorers, including Hess Corporation and OMV Australia. For example, in 2007, Hess Corporation was awarded the deepwater petroleum exploration permit WA-390-P, located southwest of the super-giant Io-Jansz field, with an aggressive bid, including a 16 well drilling commitment. Three of the four wells drilled in 2008 were gas discoveries in the post-Callovian interval: Glencoe 1 intersected 28 m of net gas pay, Briseis 1, 46 m and Nimblefoot 1, 28 m (Hess Corporation, 2008a, 2008b, 2008c). Briseis 1 had an additional pay zone in the Triassic Mungaroo Formation (Smallwood et al, 2010). This initial success was followed by the drilling of seven wells (Toporoa 1, Dunlop 1, Lightfinger 1, Bravo 1, Rimfire 1, Mentorc 1 and Hijinx 1), in 2009, of which six were gas discoveries (Jonasson, 2010). A twelfth well, Glenloth 1, was completed in 2010, and was also a gas discovery (Jonasson and Mack, 2010). Other recent giant gas finds have been made at Wheatstone and Pluto to the east, Chandon to the northwest, and Martell 1 and Larsen Deep 1 (Woodside Petroleum Ltd, 2009, 2010a) to the north, of the Io-Jansz field. Of further significance is the discovery of a 185 m gross gas column within the Triassic at Alaric 1 (Woodside Petroleum Ltd, 2010b), in the outermost southwestern 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 8 of 24 part of the Exmouth Plateau and, further inboard, a 273 m net gas column at Acme 1 (Chevron Australia, 2010). Well Control Only Wigmore 1 has been drilled within the Release Areas (Figure 1), and no accumulation of hydrocarbons was encountered. In the vicinity, however, Nebo 1 is the only significant oil discovery within about 50 km with Phoenix 1, a significant gas discovery, lying further to the southeast. Other wells have been drilled near the Release Areas and provide significant stratigraphic control. Along with Wigmore 1 and Nebo 1, Delambre 1, Ermine 1, Manaslu 1, Grey Rabbit 1 and Darwin 1, all located to the south, indicate a strong correlation to the stratigraphy of the remainder of the Northern Carnarvon Basin. Whitetail 1, just to the east, also indicates the presence of Northern Carnarvon Basin type Upper Jurassic (i.e. Legendre Formation) to Holocene sedimentary sequences. ODP Leg 122 wells (Haq et al, 1990; von Rad et al, 1992a), on the Wombat Plateau to the west, confirm the northernmost extent of the Triassic Mungaroo Formation and Brigadier Formation type sediments. Delambre 1 (1980) Delambre 1 was drilled by Woodside Petroleum Development Pty Ltd in water depths of 884 m in permit WA-90-P. It was designed to test both Middle Jurassic sandstones, sealed by Middle Jurassic and Lower Cretaceous claystones, and Lower Jurassic to Triassic sandstones, sealed by Lower Jurassic claystones, over a large north-trending horst (i.e. the northeasttrending “Brigadier Trend”). The well reached a total depth of 5495 mRT within a thick Upper Triassic section of claystones, sandstones and siltstones. The well was a valid test of structure with both reservoir and sealing objectives confirmed, but no significant hydrocarbons were encountered. However, solvent fluorescence was widespread in ditch cuttings and sidewall cores taken throughout the Lower Jurassic to Triassic section. Nebo 1 (1993) Nebo 1 was drilled by Kufpec Australia Pty Ltd in water depths of 171 m in permit WA-225-P. The well was designed to test sandstones of the Upper Jurassic Angel and Middle Jurassic Legendre formations on the “Nebo High”, a northwest-trending faulted anticline traversing the north- to northeasttrending Thouin Graben. The well reached a total depth of 3132 mRT within Legendre Formation massive sandstones characterised by interbedded claystones and thin coal seams. The well was a valid test of structural closure. The trap structure was developed through a combination of faulting during the Callovian breakup event and arching associated with transcurrent motion during the Early Cretaceous (Osborne, 1994). Intersected sandstones of the Legendre Formation were permeable and porous, but the lack of shows was attributed 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 9 of 24 to the poor sealing qualities of the overlying Calypso Formation. The predicted Angel Formation sandstones were not intersected. The well is an oil discovery. Oil was recovered from thin sands within the Callovian Calypso Formation. A total of 5.9 m of potential net oil pay was interpreted in six discrete thin sands (17–26% average log porosity) over a gross section of 33 m, below 2663 mRT. A drill stem test, perforated over the best reservoir zone in the depth range 2664.5–2668 mRT, was carried out, and the well flowed 42° API oil at a maximum flow rate of 1840 bopd (293 m3/d) on a ½ inch (12 mm) choke. Wigmore 1 (2002) Wigmore 1 was drilled by Kerr-McGee NW Shelf Energy Australia Pty Ltd in water depths of 1247 m in permit WA-295-P, in the northern Beagle Subbasin. The well was designed to test a fault-bounded structural closure for sands within the primary objective, the Lower Jurassic North Rankin Formation and the secondary objective, the Middle Jurassic Legendre Formation. Structural closure was offset at both target levels necessitating a deviated well trajectory. The well reached a total depth of 5395 mRT within North Rankin Formation sandstones with minor interbedded claystone. The well was a valid test of structural closure at both objective formations. However, the Muderong Shale was found to be thinner than predicted, suggesting a less than effective seal for the Legendre Formation target. In addition, widespread porous sandy facies in the lower part of the Athol Formation may have provided cross-fault seal failure for the North Rankin Formation target sands within the drilled structure. No significant hydrocarbons were encountered, nor was there any fluorescence. No hydrocarbon charge into the structure was confirmed and the absence of an effective petroleum system for the area was concluded. For further details regarding wells and available data follow this link: http://www.ret.gov.au/Documents/par/data/documents/Data%20list/data%20li st_ne exmouth plateau_AR11.xls 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 10 of 24 Data Coverage The Release Areas are well covered by grids of regional 2D and more focused 3D seismic surveys of varying orientations and vintages. Amongst the earliest surveys in the broader region is the more inboard Montebello– Mermaid Shoals M.S.S regional 2D grid acquired by B.O.C. Australia in 1965. In contrast, the most recent seismic coverage is provided by PGS’s longoffset, widely spaced 2007 MC2D New Dawn survey (includes gravity and magnetics; Petroleum Geo-Services Data Library, 2007), which covers a broad swathe of the North West Shelf, including the Release Areas. To highlight good quality 2D seismic coverage across the Release Areas, the following major seismic surveys are noted: the Deep Water North West Shelf Spec M.S.S. (acquired in 1998 by GHD-Gardline Surveys Pty Ltd) grid with approximately 20 km line spacing; the Beagle Deep M.S.S (acquired in 1997 by Nopec International Pty Ltd) grid with approximately 10 km line spacing across Release Area W11-9 and the southern halves of W11-7 and -8; and the Tarantula 2D M.S.S. (acquired in 1999 by Woodside Energy Ltd) grid with a variable 1-10 km line spacing across the eastern half of Release Areas W11-8 and -9. Apart from the PGS MC2D New Dawn survey, deep 2D seismic coverage across the broader region is also provided by Geoscience Australia surveys 95, 110, 120, 128 and 136. Two major 3D surveys were acquired across two of the Release Areas (Figure 3): the Canning TQ3D M.S.S. (WesternGeco Multiclient Services, 1999) covers 4365 km2 over a large portion of the western half of Release Area W11-9; and Woodside Energy’s 2001 Whitetail 3D M.S.S. covers 555 km2 along a portion of the eastern boundary of Release Areas W11-8 and W11-9. These data sets are available through Geoscience Australia’s Acreage Release Seismic Workstation Packages. Various company open file seismic interpretation, well and destructive analysis reports are available from Geoscience Australia or from Exploration Data & Scanning Services. In addition to commercial petroleum exploration efforts on the Exmouth Plateau, there have also been scientific investigations by the Ocean Drilling Program (ODP). In 1988, ODP Leg 122 (Haq et al, 1990) drilled four locations on the Wombat Plateau (i.e. sites 759, 760, 761 and 764). Detailed descriptions of the fully cored holes and the interpretation of the results are given by von Rad et al (1992a). Related investigations include Williamson et al (1989) and Exon and von Rad (1994). A large amount of satellite-based Synthetic Aperture Radar (SAR) and Airborne Laser Fluorosensor (ALF) data have been acquired across the broader region. This data has been interpreted by O’Brien et al (2003). 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 11 of 24 Interpretations constructed from earlier vintages of sparse deep seismic data, are available as part of the North West Shelf Digital Atlas (2008). To view image of seismic coverage follow this link: http://www.ga.gov.au/energy/projects/acreage-release-andpromotion/2011.html#data-packages 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 12 of 24 PETROLEUM SYSTEMS AND HYDROCARBON POTENTIAL Given the scarcity of well information within the wider area, Table 2 provides a list of possible elements of petroleum systems within the Release Areas (see lithostratigraphy in Figure 4). Table 2: Petroleum Systems Elements Summary Sources Reservoirs Seals Play Types Lower Cretaceous Muderong Shale (gas-prone). Upper Jurassic to Lower Cretaceous post-breakup Dingo and Forestier claystones (limited analysis – highly condensed to absent). Lower-Middle Jurassic Athol Formation (oil-prone). Triassic Mungaroo Formation (gas-prone). Lower Triassic Locker Shale (gas-prone). Upper Jurassic Calypso Formation (marginal marine to marine). Middle Jurassic Legendre Formation (fluvio-deltaic to more marginal marine in the north). Triassic Mungaroo Formation (fluvio-deltaic). Lower Cretaceous Muderong Shale/Forestier Claystone (regional). Middle Jurassic Calypso Formation (intra-formational). Lower–Middle Jurassic Athol Formation (regional). Triassic Cosssigny member of the Mungaroo Formation (regional). Tilted fault blocks, and associated strike-slip related anticlines. Upper Jurassic (post-breakup) basin-floor fan sands downdip of horst flanks. The giant Scarborough and Io-Jansz gas fields, along with the gas discoveries at Jupiter 1, Chandon 1, Thebe 1 and 2, Martell 1, Larsen Deep 1 and Alaric 1, demonstrate that the deepwater Exmouth Plateau is prospective for large gas discoveries. The extension of this prospectivity further north, to the 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 13 of 24 northern margin of the plateau, is yet to be demonstrated. However, several of the key elements that combine to produce successful petroleum systems further south also occur in the region of the Release Areas. For example, ODP drilling on the Wombat Plateau has intersected a similar sedimentary section to the petroliferous central Exmouth Plateau, such as Upper Triassic fluvio-deltaic sandstones and coals (?Mungaroo Formation), which are stratigraphic equivalents of the reservoir and source facies of the proven deepwater giant gas plays further south. In addition, the wells to the south and east (i.e. Wigmore 1, Delambre 1, Manaslu 1, Nebo 1, Darwin 1, Phoenix 1 and Whitetail 1) of the Release Areas indicate that similar Jurassic sedimentation extends into this region, with the caveat that the region was closer to the locus of structuring associated with Late Jurassic breakup. For the Beagle Sub-basin, the discovery of oil in Calypso Formation sands at Nebo 1 (Figure 4) demonstrated the presence of an active petroleum system within the southern Thouin Graben. The well also confirmed adequate top seal at the Breakup Unconformity, and suggested that down-dip potential exists within the drilled structure. However, the subsequent failure of the two nearby wells (i.e. Huascaran 1 and Halo 1) to encounter hydrocarbons indicates risks associated with vertical and lateral migration, intraformational seals within the Legendre and Calypso formations, reservoir sands, fault seal and fault movement/reactivation timing, or adequate source rock quality and distribution for effective regional charge. Furthermore, a palaeogeographic study of the wider region (Stephenson et al, 1998) highlights the spatial and temporal diversity of potential hydrocarbon targets, and suggests that oil at Nebo 1 is possibly related more to a localised lacustrine source pod than to a regional shale unit. Source Rocks In the wider region, the Lower Triassic Locker Shale was intersected at Poissonnier 1 and Bruce 1, both at shallow depths along the Beagle Subbasin’s southern margin to the south of the Release Areas. Poor source quality at Poissonnier 1 (Surdam and Warme, 1984) may, however, improve in more outboard, more deeply buried areas, with the risk that the unit may be overmature in the more deeply buried depocentres. Nevertheless, the region’s troughs remain an important potential source kitchen. For example, palaeogeographically-inferred lacustrine, fluvial and estuarine source rocks for the Beagle Sub-basin, deposited in the Triassic to Middle Jurassic, are likely to be oil-prone and, for example, to have sourced the oil at Nebo 1 (Stephenson et al, 1998). Upper Triassic Mungaroo Formation claystones, intersected at Poissonnier 1 and Cossigny 1 to the south, have little or no source potential. However, source quality for the formation is expected to improve in more outboard areas. This may be analogous to the plateau setting further south where coaly facies of the Mungaroo Formation are thought to have sourced significant gas accumulations. Recent exploration activity there is based on models of gas charge from deeply buried Mungaroo Formation coals and carbonaceous claystones with peak generation interpreted to occur at depths greater than 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 14 of 24 5 km (Bussell et al, 2001). Equivalent Carnian prodelta sediments intersected on the Wombat Plateau had TOC values up to 2.5%, although they were thermally immature (von Rad et al, 1992b). Lower–Middle Jurassic Murat Siltstone and Athol Formation claystones at Depuch 1, North Turtle 1 and Picard 1, all to the south, have good source potential, and along with their thickness, distribution and inferred maturity level, make these the prime potential source units for the Beagle Sub-basin (Blevin et al, 1994). Wigmore 1 (Kerr-McGee NW Shelf Australia Energy Pty Ltd, 2003) identified these units as gas prone and as being early to midmature. Expulsion and migration is likely to have occurred during the Late Cretaceous and Cenozoic, although in more deeply buried depocentres potential charge could predate emplacement of the sealing Lower Cretaceous Muderong Shale/Forestier Claystone. Oil recovered at Nebo 1 was sourced from either oil-prone deltaic coals or lacustrine mudstones of presumed Early–Middle Jurassic age, and does not show a generic affinity with any oil from the nearby Dampier Sub-basin or greater North West Shelf (Edwards and Zumberge, 2005). A source rock extract of Pliensbachian shale (TOC = 4.6%) from 3840–3850 m in Picard 1 (“Picard Shale”; Murat Siltstone equivalent) is reported to have a similar biomarker character to that of the Nebo 1 oil (Geotech, 1994; Scott and Hartung-Kagi, 1998). Partridge (1988) argued, however, that this shale unit was deposited in a restricted marine environment. The post-breakup Upper Jurassic to Lower Cretaceous marine organic-rich sediments (Dingo/Forestier claystones) are a major source within the Dampier Sub-basin to the south. Seismic mapping suggests a wide distribution but of thinner development northwards (Figure 5 and Figure 6). The organic-rich Dingo Claystone has not been sampled by wells (see Darwin 1 WCR (Apache Oil Australia Pty Ltd, 1995) where a possible very thin unit is inferred) and, therefore, may be absent. Also, geochemical analyses are limited across the region’s wells (Blevin et al, 1994). However, a condensed section of the Dingo Claystone and thicker Forestier Claystone my be present in the deeper adjoining depocentres and, if buried deeply enough, may have generative potential. The Lower Cretaceous Muderong Shale has fair to good TOC and is generally immature, but may be more mature within the Beagle Sub-basin depocentres (Blevin et al, 1993b). Reservoirs Good to excellent quality potential reservoirs occur at several levels, including the Triassic Mungaroo Formation (e.g. 10-20% porosity in Norian-Carnian ditch cuttings in Delambre 1) and Middle Jurassic Legendre Formation sands (e.g. Darwin 1, Ermine 1, Grey Rabbit 1 and Wigmore 1). Upper Jurassic reservoirs are at risk of having been eroded, or not deposited, on the structural highs (e.g. Dampier Sub-basin reservoir Angel Formation absent at Nebo 1 and very thin at Bligh 1 further to the south). A secondary potential 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 15 of 24 reservoir is the post-breakup Callovian Calypso Formation (e.g. Nebo 1 and Darwin 1), although it too may be absent on structural highs due to erosion at the Valanginian Breakup Unconformity. Both the Lower Jurassic-Upper Triassic North Rankin and Brigadier formations are inferred to be of good reservoir quality, although the former was found to be silt-prone at Wigmore 1. Seals Regional seals are provided by the Lower Triassic Locker Shale, Middle Triassic Cossigny Member of the Mungaroo Formation (e.g. Phoenix 1), Lower–Middle Jurassic Athol Formation (e.g. Wigmore 1) and the Lower Cretaceous Forestier Claystone and Muderong Shale (e.g. Delambre 1). Intraformational seals occur within the Middle Jurassic Legendre Formation and the Callovian Calypso Formation (e.g. Nebo 1). Play Types Dominant play types for the northeast Exmouth Plateau area are tilted faultbounded sequences within, and along, Triassic-Jurassic horsts, as well as the possibility of post-breakup Upper Jurassic basin-floor fan sands (e.g. Dampier Sub-basin Angel Formation equivalent), along their flanks. Structural highs hosting Upper Jurassic reservoir sands are also a proven play type developed within the intervening troughs (e.g. Nebo 1 within the Thouin Graben). The regionally extensive Lower Cretaceous Muderong Shale/Forestier Claystone could act as the main sealing facies if of sufficient thickness. For the Beagle Sub-basin, company seismic interpretation reports have identified various leads and prospects. These reports include: Marathon Petroleum’s Angel and Boronia surveys (1982 and 1983 respectively); Lasmo Energy’s Erica and Honorine surveys (1985 and 1986 respectively); Ampol Exploration’s Katrina survey (1989); Premier Oil’s Cocker survey (1998); Kufpec Australia’s Mahakan and Mohaku surveys (1992 and 1993 respectively); and Woodside Petroleum’s Reynard and Remus surveys (1999). The structure contour maps highlight the complex nature of faulting and associated structural closures at various levels, although the fidelity of the contouring has been flagged as an issue given the likely strike-slip component of the faulting. Reefal stratigraphic traps, such as porous patch reefs laterally sealed by finegrained lagoonal facies, provide another play type, as described by Williamson et al (1989) for the Wombat Plateau. An assessment of play types in basins with a strong strike-slip component to faulting should consider Riedel-shear related faulting and folding, resulting from differential movements within and around fault complexes, and the possibility of secondary shallow hydrocarbon entrapment from primary, more deeply breached traps (Chen et al, 2010). Critical Risks The discovery of a small oil accumulation at Nebo 1 proves the existence of an active petroleum system within the region, though it may be locally 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 16 of 24 restricted. The general absence of significant hydrocarbon shows across the region’s wells, within the Middle to Upper Jurassic, is possibly a consequence of the lack, or thin development, of adequate regional-scale source rocks, particularly of the post-breakup shales (i.e. Dingo and Forestier claystones), which results in a high charge risk. However, this is unlikely to be the case with the deeper, extensive Triassic source units. Fault-trap breaching of seals by late-stage faulting and inadequate migration pathways have been suggested to explain several well failures (Blevin et al, 1993b, 1994). The likely effects of oblique extension leading to Callovian breakup, and later re-activation due to Valanginian breakup, in terms of the horst-graben architecture and bounding fault style, raises complexities in trap development and preservation as a significant risk. Typical extensional horstand-graben rift-related play concepts developed for the Dampier Sub-basin are unlikely to extend northwards without considerable modification. Furthermore, regional reactivation of faulting during the Miocene might have also affected trap integrity, as well as the shape of existing structures. The nature of fault movement and fault orientations suggests that migration pathways that use fault conduits would be complex (Blevin et al, 1994). 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 17 of 24 FIGURES Figure 1: Location map of Release Areas W11-7, W11-8 and W11-9, showing existing petroleum permits, oil and gas fields, wells and pipelines. Figure 2: Graticular block map and graticular block listings for Release Areas W11-7, W11-8 and W11-9. Figure 3: Structural elements of the northeastern Exmouth Plateau region, also showing Release Areas, AGSO regional deepseismic lines (see Figure 5 and Figure 6), oil and gas fields and the outline of relevant 3D seismic grids. The more detailed mapping within the Beagle Sub-basin (i.e. yellow horsts) is based on the work of Blevin et al (1994). Figure 4: Generalised stratigraphy of the northeastern Exmouth Plateau, based on the Northern Carnarvon Basin Biozonation and Stratigraphy Chart (Nicole et al , 2010. Geoloci Time Scale after Gradstein et al (2004) and Ogg et al (2008). Figure 5: AGSO regional deep-seismic line 120/14 across the outer Beagle Sub-basin into the northeastern Exmouth Plateau (see Figure 3 for location). Figure 6: AGSO regional deep-seismic line 128/10 across the Swan Canyon of the northeastern Exmouth Plateau (see Figure 3 for location). 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas W11-7, W11-8 and W11-9, Northeastern Exmouth Plateau, Northern Carnarvon Basin, Western Australia Release Area Geology Page 18 of 24 REFERENCES APACHE OIL AUSTRALIA PTY LTD, 1995—Darwin-1 Well Completion Report, unpublished. BARBER, P., 1988—The Exmouth Plateau deepwater frontier: a case study. In: Purcell, P.G. and Purcell, R.R. (eds), The North West Shelf, Australia: Proceedings of the Petroleum Exploration Society of Australia Symposium, Perth, 173–87. BHP BILLITON, 2008—[Web page] Investment presentation, Australian Analysts’ Tour, 27 October 2008, Petroleum Australia presentation. http://www.bhpbilliton.com/bbContentRepository/docs/petroleumSitePresentat ion08.pdf (last accessed 1 December 2010). BLEVIN, J.E., STEPHENSON, A.E. 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