Areas W07-18, W07-19, W07-20 and W07-21
Beagle Sub-basin, Carnarvon Basin
Petroleum Potential
The discovery of oil in the upper Calypso Formation sands at Nebo 1 (1993) demonstrated the presence of an active petroleum system within the southern Thouin Graben of the Beagle Sub-basin. This well also confirmed adequate top seal at the Oxfordian Unconformity, and suggested that down-dip potential exists within the Nebo Structure. However, the subsequent failure of the Halo 1 and Huascaran 1 wells to encounter hydrocarbons indicates risks associated with vertical and lateral migration, top seal between the Legendre and overlying Calypso Formation reservoirs, fault seal and fault movement/reactivation timing, or adequate source rock quality and distribution for effective regional charge.
The hydrocarbon potential of the remainder of the Beagle Sub-basin is unproven. The main Middle Jurassic (Legendre Formation) horst plays have been unsuccessfully tested by numerous wells, including many drilled after the Nebo discovery, as have down-dip hanging wall targets (Halo 1 and Tayra 1). The southern Late Jurassic Angel Formation play, which hosts the Mutineer/Exeter, Angel and Talisman accumulations in the adjacent northern portion of the Dampier Sub-basin, has also been unsuccessfully tested by several wells (Jarman 1, Finucane 1, Serval 1, Bounty 1, Calypso 1, Trafalgar 1 and Aurora 1). The deep-water northwestern potential has been unsuccessfully tested by Delambre 1 and Wigmore 1, as well as at Whitetail 1 (2003) and most recently Huntsman 1 (2006) in the adjacent Rowley Sub-basin to the north.
Source rocks
In contrast to the major rift depocentres on the northwest Australian margin, the Beagle Sub-basin succession lacks the development of thick Late Jurassic restricted marine organic-rich source rocks. Late Jurassic claystones are either thin or absent in the Beagle Sub-basin, and where present, are generally immature. Other potential source units within the sub-basin occur within the Early Triassic Locker Shale, Middle–Late Triassic Mungaroo Formation and Early–Middle Jurassic Legendre/Athol Formation, including the ‘Picard Shale’ (Figure 3).
The Early Triassic Locker Shale generally has poor source potential and tends to be silty and immature to marginally mature where intersected along the basin margin (Poissonier 1 and Bruce 1). However, source quality may improve in more outboard areas and it is likely to be mature in the outer portion of the North Turtle Terrace. The Locker Shale can be expected to be overmature in the Beagle and Cossigny troughs, and throughout the outer parts of the sub-basin.
Claystones within the Middle–Late Triassic Mungaroo Formation where intersected at Poissonier 1 and Cossigny 1 have little or no source potential. Source quality may improve in more outboard areas, similar to the Exmouth Plateau where coaly facies of the Mungaroo Formation are thought to have sourced significant gas accumulations.
Claystones and thin coaly sediments within the Early–Middle Jurassic Legendre/Athol Formation at Cossigny 1, Depuch 1, North Turtle 1 and Picard 1 have good to very good source potential and together with its inferred maturity level, make this the prime potential source unit within the Beagle Sub-basin (Blevin et al, 1994b). If these units are sufficiently rich to expel hydrocarbons, expulsion and migration is likely to have occurred during the Late Cretaceous and Tertiary, but in more outboard areas (eg, Wigmore 1) potential charge could predate emplacement of the top Legendre seal (Muderong Shale).
The oil recovered at Nebo 1 is 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 shale (TOC = 4.6 %) from 3840–3850 m in Picard 1 (‘Picard Shale’ or Murat Siltstone/Athol Formation) is reported to have a similar biomarker character to that of the Nebo 1 oil (Geotech, 1994; Scott and Hartung-Kagi, 1998).
The complete absence of significant hydrocarbon shows, including fluid inclusion analyses, in all other wells in the Beagle Sub-basin strongly suggests the lack of effective source rock quality and distribution within the sub-basin, apart from a local source pod within the southern Thouin Graben that generated the oil recovered at Nebo 1.
Reservoir
The Beagle Sub-basin has an abundance of widespread sand facies. Good quality potential reservoirs occur at several levels, including the Middle–Late Triassic Mungaroo Formation, Early Jurassic North Rankin Formation, Early–Middle Jurassic Legendre Formation and Late Jurassic Angel Formation (Figure 3). Secondary potential reservoirs include basal Triassic transgressive sands, the Late Triassic–Early Jurassic Brigadier Formation and the Callovian Calypso Formation.
Seal
Regional seals are provided by the Early Triassic Locker Shale, Middle Triassic Cossigny Member of the Mungaroo Formation, and the Early Cretaceous top seals of the Forestier Claystone and Muderong Shale (Figure 3). Potential intraformational seals occur within the Early Jurassic ‘Picard Shale’ unit of the Athol Formation/Murat Siltstone, and the Callovian Calypso Formation.
Fault trap breaching of seals by late-stage faulting and inadequate migration
pathways have been suggested to explain several well failures (Blevin et
al, 1994a, 1994b), but the general absence of significant hydrocarbon shows
is probably largely a consequence of the lack of adequate source rocks resulting
in a high charge risk for the sub-basin.