Petroleum Systems
Magoon and Dow (1991) defined a petroleum system as a mature source rock and all its generated hydrocarbon accumulations. A survey of organic-rich rocks (ORRs) (TOC>2.0%) in the Browse Basin by Blevin et al (1997) and summarised by Blevin et al (1998b), indicated that there may be more than 20 potential source intervals ranging in age from Permo-Carboniferous to Early Cretaceous (Figure 6 [PDF, 47KB]). The ORRs were grouped by Blevin et al (1997, 1998b) into:
- Early Cretaceous petroleum systems and play families;
- Late Jurassic petroleum systems and play families;
- Early to Middle Jurassic petroleum systems and play families;
- Permo-Carboniferous to Late Triassic petroleum systems and play families.
One or both of the Jurassic petroleum systems sourced the giant gas fields in the Caswell Sub-basin, while the oil fields on the Yampi Shelf were sourced from the Early Cretaceous petroleum system (Boreham et al, 1997, 2001; Blevin et al, 1998a, b; Edwards et al, 2000, 2004; Kennard et al, 2004).
In the Barcoo Sub-basin the key petroleum systems are the Early to Middle Jurassic and Permo-Carboniferous to Late Triassic systems, as the Late Jurassic succession is relatively thin and marginally mature, and the Early Cretaceous succession is immature to marginally mature (Blevin et al, 1998a; Kennard et al, 2004).
Source Rocks
Thick sections of Late Jurassic and Early Cretaceous sediments occur within both the Caswell and Barcoo sub-basins (Figure 7 [PDF, 200KB]), and contain mixed marine and terrestrial organic matter with moderate to good source potential. A significant thickness of mostly Barremian age shale was deposited in a topographic low between Barcoo 1 and South Brecknock 1. However, the effectiveness of this'pod' to source structures in the Barcoo Sub-basin is dependent on maturity. Geochemical maturity indicators show that the oil window lies at a depth of 3800 m in Barcoo 1, and that the Lower Cretaceous succession is immature for oil (Blevin et al, 1997, 1998a).
Thick accumulations of Early to Middle Jurassic (pre-Callovian) organic rich sediments are preserved in the western and northeastern Barcoo Sub-basin (Figure 8 [PDF, 129KB]; Blevin et al, 1997). Early Jurassic marine facies in the sub-basin are predicted to have the greatest source potential of the Early to Middle Jurassic sequence in the Browse Basin, particularly at third and fourth order sequence boundaries where reconnaissance samples show TOC values exceed 2% (Blevin et al, 1998b).
Permo-Carboniferous to Late Triassic source intervals are not well documented, as this succession is poorly constrained by well data. Initial source rock richness shows that most of the intervals sampled from this succession have only poor to fair source potential (Figure 6 [PDF, 47KB]), although these samples clearly do not reflect the source potential for facies within the deeper half-graben (Blevin et al, 1998b). For example, Late Triassic highstand shales preserved at Barcoo 1 thicken toward the east into the central Barcoo Sub-basin, and are expected to contain viable source facies (Figure 9 [PDF, 140KB]; Blevin et al, 1998b).
Expulsion and Migration
Recent hydrocarbon expulsion modelling suggests the Late Jurassic to Cretaceous succession in the Barcoo Sub-basin is immature to marginally mature, with only minor gas expulsion from the central part of the sub-basin (Kennard et al, 2004).
Source rocks in the Early to Middle Jurassic Plover Formation in the Barcoo Sub-basin are mature for oil and wet gas generation. Petroleum systems modelling suggests that these source intervals expelled gas in the northern and eastern parts of the sub-basin in the Cenozoic (Kennard et al, 2004).
The Carboniferous to Triassic succession is considered to be overmature in the depocentres of the Browse Basin due to the thickness of sediment deposited during Permo-Triassic extension and early post-rift basin phases (Blevin et al, 1997). However, hydrocarbon generation could be occurring where source facies of this age are not as deeply buried. For example, petroleum systems modelling suggests the Late Triassic succession intersected in Lomardina 1 is currently in the oil window (Blevin et al, 1997).
Post-Eocene migration of hydrocarbons was primarily towards the Late Triassic anticlinal highs and fault blocks in the western Barcoo Sub-basin, and fault traps, late stage anticlines and onlap plays in the eastern part of the sub-basin and on the Leveque Shelf (Blevin et al, 1997).
Reservoirs
Late Triassic and Early to Middle Jurassic fluvio-deltaic sequences provide the main reservoirs for outboard structural and stratigraphic traps. Potential reservoirs for onlap plays on the eastern margin range from Early Jurassic to Late Cretaceous age (Blevin et al, 1997).
Seals
Early Cretaceous (Echuca Shoals Formation) and Late Jurassic (Vulcan Formation) highstand shales provide regional seals for the Barcoo Sub-basin. The Late Triassic highstand shales in the central Barcoo Sub-basin are expected to contain seal facies for a potential Permo-Carboniferous petroleum system (Blevin et al, 1998b). Potential intraformational shale seals occur within the Early-Middle Jurassic Plover Formation, but are poorly documented (Blevin et al, 1998b).
Play Types
Potential play types in the Barcoo Sub-basin are shown by'play number' on Figure 10 [PDF, 22KB] (Blevin et al, 1997; 1998b) and include the following:
- Play 1, Carboniferous to Permian extensional fault-related plays;
- Play 2, Late Triassic tilted fault blocks and associated anticlines;
- Play 3, Early Cretaceous onlaps, erosional truncation seal and associated traps;
- Play 4, Miocene fault reactivation plays.
Late Triassic faulted anticlines, which host the giant gas fields along the outer margin of the Caswell Sub-basin, have been tested unsuccessfully at Barcoo 1, where the tested structure lacked closure.
A gas accumulation was intersected in Early Cretaceous drape of erosional basement highs on the Leveque Shelf at Psepotus 1, but it is unclear whether the gas was sourced from the Barcoo Sub-basin to the northwest of the well, or from the Caswell Sub-basin to the north.
The prominent Late Tertiary fault-reactivation anticlines along the margin between the Barcoo Sub-basin and the Leveque Shelf (e.g., Trochus, Lynher, Lombardina and Sheherazade structures) have proved unsuccessful, with the possible exception of the inferred hydrocarbon column at Arquebus 1 ST1 (Haston and Farrelly, 1993).
Critical Risks
A key risk for oil prospectivity in the Barcoo Sub-basin is the absence of source rock of suitable maturity. The Late Jurassic succession is considered to have the best oil source potential on the North West Shelf, but this is absent from the Browse Basin because major rifting did not occur in the region at that time and consequently, the restricted conditions that favour development of marine source rocks did not develop (Longley et al, 2002; Keall and Smith, 2004). The oil accumulations that have been discovered in the Browse Basin are sourced from the Early Cretaceous Echuca Shoals Formation in the Caswell Sub-basin, but this succession is immature to marginally mature in the Barcoo Sub-basin.
Evidence of gas at Arquebus 1 ST1, and the gas accumulation at Psepotus 1, suggest that source, maturity and relative migration/trap timing are not critical risks for gas prospectivity in the Barcoo Sub-basin and the adjacent Leveque Shelf. Key risks for the large fault-reactivation anticlines along the boundary between the Barcoo Sub-basin and Leveque Shelf are the lack of updip lateral seal and/or reactivation and breach by Pliocene faults (Blevin et al, 1997).