Petroleum geological summary
Release areas S12-1, S12-2 and S12-3
Ceduna Sub-Basin, bight basin, southaustralia
HIGHLIGHTS
· Under-explored large frontier Jurassic–Cretaceous basin
· Shallow to ultra deep water depths 150–3,700m
· Large range of structural and stratigraphic plays associated with Upper Cretaceous marine and deltaic successions and Jurassic–Lower Cretaceous non-marine rifts
· Multiple potential source rocks, including Cenomanian–Turonian oil-prone marine organic-rich rocks
Release Areas S12-1 to S12-3 are large areas located over the northern and central parts of the Ceduna Sub-basin in the eastern Bight Basin. The Release Areas offer a range of plays in water depths generally greater than 500m. The Release Areas lie adjacent to four exploration permits granted in January2011.
The Ceduna Sub-basin contains a sedimentary section in excess of 15km thick. The key to the petroleum prospectivity of the sub-basin is the distribution of the Upper Cretaceous marine and deltaic facies. Dredging of upper Cenomanian–Turonian organic-rich marine rocks has confirmed the presence of high quality potential source rocks in this section. These rocks are mature in the central part of the Ceduna Sub-basin and are likely to have generated and expelled hydrocarbons since the Campanian. Excellent reservoir rocks and potential intraformational seals are present in the Upper Cretaceous deltaic successions, and regional seals could be provided by Upper Cretaceous marine shales.
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Location
Release Areas S12-1 to S12-3 (Figure1) are situated in the central Great Australian Bight, off southern Australia. The areas are located mostly within the frontier Ceduna Sub-basin, in the eastern part of the Bight Basin. The Release Areas are located adjacent to petroleum exploration permits EPP37–40, which were awarded to BP Exploration in January 2011. Release Area S12-1 is approximately 300to 450km southwest of Ceduna; Release Areas S12-2 and S12-3 are approximately 250to 450km west of Port Lincoln and 190to 400km southwest of Ceduna. Water depths range from 150 to 3,700m.
The graticular block maps and graticular block listings for the Release Areas are shown in Figure2. Release Area S12-1 comprises 233 graticular blocks and covers a total area of approximately 16,621km2. Release Area S12-2 comprises 264 graticular blocks with a total area of approximately 18,752km2. Release Area S12-3 comprises 202 graticular blocks covering a total area of approximately 14,154km2.
Release Area Geology
The Release Areas are located in the northern and central parts of the Ceduna Sub-basin (Figure3). The east-southeast-trending Ceduna Sub-basin is the major depocentre of the Bight Basin, extending over an area of 126,300km2 and containing at least 15,000m of syn-rift and post-rift Middle Jurassic–Upper Cretaceous sediments (Figure4; Bradshaw et al, 2003). The deepwater Recherche Sub-basin adjoins the Ceduna Sub-basin and extends west along the southern margin. The Ceduna Sub-basin is flanked to the northwest by the half-graben systems of the Eyre Sub-basin, the easternmost part of which extends into Release Area S12-1. The Duntroon Sub-basin adjoins the Ceduna Sub-basin to the southeast, and consists of a series of oblique extensional depocentres. Thin platform cover areas—the Madura and Couedic shelves—are located to the north and east of the Ceduna Sub-basin respectively.
Local tectonic setting
The Ceduna Sub-basin can be divided into three structural domains:
· A steeply-dipping hinge zone along the northern and eastern margins of the sub-basin containing extensional and transtensional Middle Jurassic–Lower Cretaceous half-graben systems (Figure5, Figure 6 and Figure7)
· A broad central depocentre characterised by detached extensional faulting and fault reactivation, forming a complex series of rotated fault blocks (Figure6, Figure7 and Figure8).
· An outer basin high underlain by a zone of complex, mainly contractional, Cenomanian deformation, which was the focus of subsequent extensional faulting and shelf-margin instability (Figure7 and Figure8).
The northern and eastern margins of the Ceduna Sub-basin have a steep ramp-like geometry and are characterised by Middle Jurassic–Lower Cretaceous half-graben. The basin thickens rapidly seaward where the succession is dominated by a thick Lower–Upper Cretaceous marine-deltaic section (Figure7 and Figure8). Structural architecture over much of the sub-basin is controlled by a series of generally northwest-southeast-oriented, listric normal faults that formed as a result of shale tectonics during deposition of a major delta system in the Cenomanian. Dominantly southwest-dipping regional faults detach onto a décollement formed in Albian–Cenomanian shales (Figure6). Farther down-dip, this extension is compensated by contractional faulting and folding. A transitional zone of complex deformation lies between these extensional and contractional zones. This region forms an outer high at the seaward edge of the Ceduna Sub-basin (Figure7 and Figure8). Compaction-related faulting and/or reactivation of the listric extensional faults occurred during the Turonian–Santonian prior to the initiation of sea-floor spreading in the latest Santonian. Some reactivation occurred on selected faults in the Campanian–Maastrichtian, particularly in the more outboard parts of the sub-basin (Figure8). During this period, new gravity-slide structures also formed at the outer edge of the sub-basin as a result of collapse of the gravitationally unstable, palaeoshelf margin (Figure8).
Structural evolution and depositional history of the area
The Ceduna Sub-basin developed in response to repeated periods of extension and thermal subsidence leading up to, and following, the commencement of sea-floor spreading between Australia and Antarctica (Totterdell and Bradshaw, 2004). Deposition commenced during the Middle–Late Jurassic to earliest Cretaceous as a result of northwest–southeast to north-northwest–south-southeast intracontinental extension (Figure4). A series of oblique extensional and transtensional half-graben formed along the northern and eastern margins of the Ceduna Sub-basin, as well as in the Bremer, Eyre and Duntroon sub-basins. This section has not been intersected by wells drilled in the Ceduna Sub-basin, but the fill of the basal half-graben imaged on seismic data is assigned to the Sea Lion and Minke supersequences by analogy with the Eyre Sub-basin. In Jerboa1 in the Eyre Sub-basin, the rift-fill comprises a fluvial–lacustrine sandstone, siltstone and mudstone succession.
The extensional phase was followed by a period of slow thermal subsidence that lasted throughout most of the Early Cretaceous (Figure4), and resulted in deposition of the Southern Right and Bronze Whaler supersequences. Depositional environments were largely non-marine, although some marine influence is evident late in the phase. The Southern Right Supersequence has not been drilled in the Ceduna Sub-basin, but is inferred to be present based on seismic interpretation and correlation with the Eyre and Duntroon sub-basins. The Bronze Whaler Supersequence generally consists of an aggradational succession of fluvial and lacustrine sediments. Where interpreted in the Ceduna Sub-basin, the succession has an onlapping, sag-fill geometry.
An abrupt increase in subsidence rate in the mid-Albian signalled the start of the third basin phase (Figure4), during which up to 10,000m of deltaic and marine, predominantly fine-grained sediments were deposited in the central Ceduna Sub-basin. This period of accelerated subsidence, which continued until the commencement of sea-floor spreading between Australia and Antarctica in the late Santonian, coincided with a period of rising global sea level (Figure4). This combination of factors resulted in a high rate of creation of accommodation, the first major marine flooding event in the basin and the widespread deposition of marine silts and shales of the Albian–Cenomanian Blue Whale Supersequence. Marine conditions extended along the southern margin from the open sea in the west towards the Otway Basin in the east. Progradation of deltaic sediments (White Pointer Supersequence) into this seaway commenced in the Cenomanian. Deposition was rapid, resulting in a short-lived period of shale tectonics throughout the northern half of the Ceduna Sub-basin. The Blue Whale Supersequence is interpreted to have had a pre-deformation thickness of about 1,500 to 2,000m (Totterdell and Krassay, 2003). The thickness of the unit is now highly variable, reaching a maximum of about 4,000m adjacent to some growth faults. The White Pointer Supersequence has a maximum thickness of approximately 5,000m within growth fault-bounded depocentres. Interpretation of seismic facies suggests that a broad band of coaly sediments is present within the White Pointer Supersequence in the inner part of Ceduna Sub-basin. The Cenomanian deltaic sediments are overlain by the marginal marine, deltaic and open marine sediments of the Turonian–Santonian Tiger Supersequence. On seismic data, this supersequence has an overall flat-lying aggradational character, with some progradational seismic facies evident in its upper part. The Tiger Supersequence has a maximum thickness of approximately 4,500m in the eastern Release Areas.
The start of continental break-up in the late Santonian was followed by a period of thermal subsidence and the establishment of the southern Australian passive margin (Figure4). During this phase a second large deltaic system developed, represented by the latest Santonian–Maastrichtian Hammerhead Supersequence (Krassay and Totterdell, 2003). This sand-rich delta is characterised by strongly prograding stratal geometries. The up-dip portion of the basal sequence boundary records widespread incision, including several large incised valleys systems. The Hammerhead Supersequence has an overall progradational–aggradational character. Two lower sequence sets are strongly progradational in character, reflecting a consistently high rate of sediment supply from the late Santonian through the Campanian. The thick, stacked deltaic sequences of the upper sequence set were deposited during a period of balance between the rates of creation of accommodation space and sediment supply. In the Release Areas the Hammerhead Supersequence reaches a maximum thickness of about 5,000m.
A dramatic reduction in sediment supply at the end of the Cretaceous saw the abandonment of deltaic deposition. A widespread angular unconformity between the Bight Basin and Eucla Basin successions on the Madura Shelf suggests regional uplift at this time. From the late Paleocene to present, the largely cool-water carbonates of the Eucla Basin accumulated on a sediment-starved passive margin. A short phase of magmatism in the middle Eocene, coinciding with the onset of rapid spreading, affected the central and eastern Ceduna Sub-basin. This magmatic phase was characterised by both extrusive volcanism (volcanoes, flows, volcanic build-ups; Figure5) and the intrusion of sills, dykes and deeper igneous bodies (Schofield and Totterdell, 2008).
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Exploration History
Petroleum exploration in the Ceduna Sub-basin has occurred in three major cycles – the late 1960s to early 1970s, the early 1990s and 2000 to present. In nearly 50 years of exploration in the offshore Bight Basin, approximately 100,000line-km of seismic data have been acquired and only 10 petroleum exploration wells have been drilled. With the exception of Gnarlyknots1, 1A, all the wells were drilled in relatively shallow water near the basin margin and the deeper part of the sub-basin remains largely untested.
During the 1960s and 1970s, exploration was carried out by Shell Development (Australia) (Shell) and Outback Oil. Seismic, shipboard magnetic and aeromagnetic data were acquired. Several prospects were developed from these activities and three exploration wells were drilled, Echidna1 and Platypus1 in 1972 and Potoroo1 in 1975. By 1977, Shell had surrendered all of its Bight Basin exploration petroleum permits. The early 1980s was a period of relatively lacklustre exploration in the central Ceduna Sub-basin, with exploration efforts concentrating on shallower, flanking depocentres. Outback Oil and BP undertook exploration in the Duntroon Sub-basin, resulting in the drilling of Duntroon1, while Esso Exploration and Production Australia (Esso), in joint venture with Hematite Petroleum (Hematite), focused their exploration efforts on the Eyre Sub-basin, acquiring seismic and drilling Jerboa1.
In early 1990, BP flew an Airborne Laser Fluorosensor (ALF) survey that covered the inboard Bight Basin. The initial results were poor, but reprocessing and reinterpretation of the data resulted in the identification of 941 confident fluors (Cowley, 2001). In 1991, BHP Petroleum (Australia) (BHP) commenced an exploration program focusing on the eastern Ceduna and Duntroon sub-basins, east of the current Release Areas. BHP drilled three wells in 1993—Borda1 and Greenly1 (Ceduna Sub-basin), and Vivonne1 (Duntroon Sub-basin). Although all were plugged and abandoned, their results vastly improved knowledge of the basin succession, and gas shows and oil indications in Greenly1 provided some exploration encouragement.
The latest phase of petroleum exploration commenced in 2000, when three petroleum exploration permits were awarded to a joint venture comprising Woodside Energy (operator), Anadarko Australia and PanCanadian Petroleum (now EnCana). The permits, EPP28, EPP29 and EPP30 covered the majority of the current Release Areas. The joint venture acquired a large quantity of 2D seismic data and drilled an exploration well, Gnarlyknots1, 1A. In early 2006, 1,250km2 of 3Dseismic data (Trim 3DSeismic Survey) was acquired over EPP29; however, in 2007, Woodside surrendered its permits. Also during this period, permits were held in the eastern Bight Basin (Duntroon Sub-basin and adjacent portion of the Ceduna Sub-basin) by the Woodside-Anadarko-EnCana joint venture and Santos Offshore. Approximately 2,300km of seismic data was acquired during the exploration programs in these permits, which were both surrendered in 2007.
In 2009, six areas in the central Ceduna Sub-basin were released for bidding, followed in 2010 by the release of two exploration areas in the eastern Ceduna–Duntroon sub-basin. In January 2011, BP Exploration was awarded four permits (EPP37–40) in the central Ceduna Sub-basin. The guaranteed work program for the permits includes four exploration wells and ~12,000km2 of 3D seismic data. In June 2011, Bight Petroleum was awarded the two Ceduna-Duntroon permits (EPP41 and42); the three-year work program includes one well.
Geoscience Australia (GA) and its predecessor agencies have a long history of research in the Bight Basin, conducting several gravity and magnetic surveys and acquiring over 28,000line-km of regional 2Dseismic data. GA’s 2007 Bight Basin Geological and Sampling Survey (Totterdell et al, 2008; Totterdell and Mitchell, 2009) targeted and sampled potential source rocks of late Cenomanian to early Turonian age from the northwestern edge of the Ceduna Sub-basin.
Well control
Only ten petroleum exploration wells have been drilled in the eastern Bight Basin, five of those in the Ceduna Sub-basin (Figure1). There is only one well within the current Release Areas—Gnarlyknots1, 1A in Release Area S12-2. Potoroo1, which lies just to the east of Release Area S12-1, provides a key stratigraphic control point for the Ceduna Sub-basin succession. Oil shows and gas indications in Greenly1 and evidence of a breached accumulation in Jerboa1 (in the Eyre Sub-basin) provide support for the presence of active petroleum systems in the basin.