Northeastern Exmouth Plateau
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 500m to 4500m.
· Play types include horsts, reefs and basin-floor fan sands.
· Infrastructure in place over 100km to the south.
· Active petroleum system present at Nebo1 to the south, but may indicate a local, rather than a regional, origin.
Location
Release Areas W11-7, W11-8 and W11-9 lie 250-450km north of Dampier, Western Australia, across the northeastern Exmouth Plateau and Beagle Sub-basin of the Northern Carnarvon Basin (Figure1). Water depth across the Release Areas deepens from the south to the north over the range 500-4500m. 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 Figure2. A total of 488 blocks is divided across the Release Areas: 167 blocks (approximately 13655km2) in W11-7, 154 blocks (approximately 12585km2) in W11-8 and 167 blocks (13555km2) 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 sub-basins 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 Wigmore1 well has been drilled in the Release Areas, and it was unsuccessful. In the immediate vicinity, however, Nebo1 discovered a sub-commercial oil accumulation and this suggests, at least, a localised active petroleum system. In addition, Phoenix1, 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 100km to the south, and the large Woodside Energy Ltd-operated Angel gas field, a further 25km 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.
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 Sub-basin (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 (Figure3). 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 10-15km 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 10km 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 (Figure3; Blevin et al, 1993a, 1993b, 1994).
The Rowley Sub-basin represents a major westward-thickening upper Paleozoic-Cenozoic depocentre containing about 9km of Permo-Carboniferous, or older, strata overlain by up to 6km 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 large-throw rift faults descending to the Argo Abyssal Plain.
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 (Figure4; 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. Delambre1). 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 (Figure5 and Figure6).
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; Figure4) and shelf and reefal limestones (Williamson et al, 1989; Haq et al, 1990; Exon and von Rad, 1994). Further inboard, where intersected (Delambre1), 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 Wigmore1). For the Release Areas, information on these units is limited to that obtained from Wigmore1. 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. Wigmore1 and Nebo1).
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. Manaslu1 and Nebo1), 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 (Figure4; ODP Leg122, Wigmore1, Nebo1, Whitetail1 and Manaslu1). This interval is largely represented in the inboard sub-basins, to 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 Darwin1, 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. Nebo1), but it is absent (Wigmore1) or thin (Whitetail1 and Manaslu1) 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.
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 fault-controlled 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 (Trafalgar1 in 1988 and Aurora1 in 1990), together with two wells along the northern margin of the adjacent Dampier Sub-basin (Bounty1 in 1983 and Calypso1 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 sub-basin. Nebo1 (1993) encountered thin oil-bearing sands in the Callovian Calypso Formation (Figure4), 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 (Cimba1, Darwin1 and Halo1).
To the south, within the northernmost Dampier Basin, Mutineer1B discovered oil in the Upper Jurassic Angel Formation, which encouraged further exploration in the adjacent Beagle Sub-basin. Woodside subsequently drilled four unsuccessful wells (Serval1, Ermine1, GreyRabbit1 and Tayra1) in the western-central sub-basin in 1999–2001, and IB Resources drilled two unsuccessful wells (Manaslu1 and Huascaran1) in the central sub-basin in 2001–2002. Wigmore1 (2002) was drilled by Kerr-McGee in the outer, deep-water, northwest portion of the sub-basin, and was also unsuccessful.
To the east of the Release Areas, within the Roebuck Basin, Whitetail1 (2003) and Huntsman1 (2006) (Figure1) 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 Huntsman1 had not been released.