Areas W07-8, W07-9, W07-10 and W07-11
Yampi-Leveque Shelf, Browse Basin
Petroleum Potential
Source rocks
A comprehensive assessment of the source rock potential of the Browse Basin
was undertaken by Boreham et al (1997), and the results summarised by Blevin
et al (1998a, b). These studies recognised organic-rich rocks with fair
to moderate oil potential at multiple stratigraphic levels within the Permian–Early
Cretaceous section, and some local, thin, high-quality coals and pro-delta
shales occur within the Early–Middle Jurassic, fluvio-deltaic Plover
Formation (Figure 6).
Blevin et al (1998b) noted that although many potential source units within
this succession have liquids potential (HI values of >200 mg hydrocarbons/gTOC),
they contain less than 2% TOC (Figure
6). At these low-to-moderate TOC levels, any generated oil may remain
within the source rock (ie, not be expelled from the source rock) and may
be subsequently cracked to gas at higher maturities.
Figure 7 shows generalised distribution maps of Late Jurassic (Vulcan Formation) and Early Cretaceous (Echuca Shoals and Jamieson formations) potential source units in the basin. The Late Jurassic section is generally thin throughout the Browse Basin, with major sediment thickening restricted to the Heywood Graben in the northeast, where restricted marine source facies are likely to be best developed. Localised thickening of Late Jurassic sediments also occurs on the Leveque Shelf and Prudhoe Terrace (Figure 7a), but here the section is dominated by deltaic facies with poorer quality terrigenous organic matter. Thick sections of Early Cretaceous sediments occur within both the Caswell and Barcoo sub-basins (Figure 7b, 7c and 7d), and contain mixed marine and terrestrial organic matter with moderate to good source potential. However, available pyrolysis data suggests that these sediments have better liquid potential within the Caswell Sub-basin (HI=150–350 mg hydrocarbons/gTOC) than the Barcoo Sub-basin (HI=100–250 mg hydrocarbons/gTOC; Kennard et al, 2004).
Potential source facies also occur within the thick succession of Early–Middle Jurassic sediments (Plover Formation) that extend throughout the basin and reaches a maximum penetrated thickness within the Barcoo Sub-basin (920 m in Barcoo 1). This section is dominated by fluvio-deltaic facies, including pro-delta shales and coastal plain shaly coals that have significant source potential (Blevin et al, 1998a). However hydrocarbons generated from this section are likely to be dominated by gas or gas rather than oil.
Reservoirs and seals
Reservoir facies are best developed within the fluvio-deltaic Early–Middle Jurassic Plover Formation, and submarine fans and ponded turbidite mounds of Berriasian, Barremian, Campanian and Maastrichtian age. Late Jurassic (Vulcan Formation) and Early Cretaceous (Echuca Shoals Formation) claystones provide regional seals throughout much of the basin. Potential intraformational shale seals occur within the Early–Middle Jurassic Plover Formation (Blevin et al, 1998a), and Late Cretaceous claystones of the Puffin (or Turnstone) Formation provide potential seals for Campanian–Maastrichtian ponded turbidites and unconfined fans (Benson et al, 2004).
Petroleum systems
Geochemical analysis of oils, oil stains, fluid inclusion oils, condensates, gases and source rocks from the Browse Basin have been undertaken by AGSO and Geotech (2000), Boreham et al (1997, 2001), Blevin et al (1998a, b), Edwards et al (2000, 2004, 2006), Edwards and Zumberge (2005) and Volk et al (2005). Figure 8 demonstrates that the stable 13C isotopic data of gases and oils can be used to discriminate the different sources of hydrocarbons in this basin. These isotopic datasets, together with molecular analyses, provide evidence that at least three hydrocarbon families/petroleum systems are present in the Caswell Sub-basin (Kennard et al, 2004).
- An outer sub-basin, relatively dry gas-prone system sourced from mixed
terrestrial and marine organic matter (Torosa, Brecknock and Calliance
fields: condensate/gas ratios of 10–20 bbls/MMscf). The Argus gas
accumulation (CGR <10 bbls/MMscf; Keall and Smith, 2004) probably represents
a northern extension of this system (Kennard et al, 2004). Edwards et
al (2004) proposed that the Early–Middle Jurassic Plover Formation
was the most likely source for these gases, whereas a Permo-Triassic source
has been modelled by Belopolsky et al (2006).
- A central sub-basin, wet gas-prone system, the source(s) of which has
yet to be established (Brewster/Ichthys field; condensate/gas ratios of
60 bbls/MMscf). These accumulations could have been charged from either
underlying Jurassic (Plover or Vulcan formations) or overlying Early Cretaceous
(Echuca Shoals Formation) source rocks, but the lack of an oil leg in
these wells tends to suggest that they did not receive a significant charge
from the oil-prone Early Cretaceous petroleum system. From the similarity
of the δ13C isotopic data of the gas/condensates recovered from the Brewster-reservoir
in the Ichthys field with those of Bayu and Undan in the northern Bonaparte
Basin, a Jurassic source is implied, with the possibility of a contribution
from Late Jurassic Vulcan Formation source rocks.
- An inner sub-basin oil (plus gas)-prone petroleum system sourced from predominantly marine algal and bacterial organic matter within the Early Cretaceous sediments of the Echuca Shoals Formation (Cornea and Gwydion fields, Caswell 2 oil accumulation). Blevin et al (1998a) defined this system as the Westralian (W3) Petroleum System. The Cornea and Gwydion oils and gases are biodegraded to differing extents.
In the Heywood Graben, the Crux gas discovery is interpreted to be sourced from mixed terrestrial and marine organic matter contained within Early–Middle Jurassic source rocks (Edwards et al, 2004). However, the gas is drier than the gases from the Ichthys field (Nippon Oil Exploration, 2001; Kaoru et al, 2004). From biomarker data (George et al, 2000), and the δ13C isotopic evidence presented in Figure 8, the samples of gas and condensate recovered from Crux 1 represent another hydrocarbon family within the Browse Basin.
Timing of generation and expulsion
Hydrocarbon expulsion modelling (Kennard et al, 2004) suggests multiple effective source units for gas expulsion in the basin, whereas effective oil-charge is largely restricted to the Heywood Graben in the northeast, the central and southern Caswell Sub-basin, and possibly the rift section in the deep-water Seringapatam Sub-basin.
Significant quantities of oil are modelled to have been expelled from Jurassic sediments (Plover and lower Vulcan formations) in the Heywood Graben during the Cenozoic and Neogene, respectively. These charges are likely to have sourced the thick palaeo-oil columns interpreted at Heywood 1 and Crux 1 on the basis of fluid inclusion analysis (Eadington and Middleton, 2000; Brincat et al, 2004). Lesser quantities of oil are modelled to have been expelled from the Vulcan Formation in the central and southern Caswell Sub-basin. Indeed an investigation of the fluid inclusions in the sandstone reservoirs of the Browse Basin gas accumulations has shown that the hydrocarbon charge consisted of an early oil charge, filling only the crestal parts of the structures before being displaced or absorbed by gas (Brincat, 2006). Only relatively minor gas expulsion, but no oil, is predicted to occur in the Barcoo Sub-basin where source facies are generally leaner (Kennard et al, 2004).
Recent hydrocarbon generation and expulsion studies of Early Cretaceous (Echuca Shoals and Jamieson formations) source rocks using Small Angle Neutron Scattering (SANS) confirms the existence of potential oil and gas-prone source rocks that are sufficiently thermally mature for generation to occur, but which show little or no evidence of expulsion and effective regional charge (Radlinski et al, 2004).
Similarly, fluid inclusion analysis provides no evidence of an effective regional oil charge to Cretaceous reservoirs in the Caswell Sub-basin (Brincat and Kennard, 2004; Brincat et al, 2004). However, as the organic-rich sediments within this succession occur as thin transgressive sheets deposited in response to fluctuating sea level on a gently inclined ramp margin, detailed understanding of the local expulsion-migration history may require higher resolution (systems tract level) sequence stratigraphic models. Effective oil charge from parts of the Echuca Shoals Formation is confirmed by geochemical analysis of the Cornea, Gwydion 1 and Caswell 2 accumulations, and is postulated as the probable source of the inferred gas accumulation at Marabou 1 (Benson et al, 2004).
Play types
The major play types within the basin are Late Triassic faulted anticlines, Jurassic horsts/tilted fault blocks and associated drape anticlines, Early Cretaceous drape of erosional basement highs on the Yampi and Leveque shelves, and Late Cretaceous basin floor fans and ponded turbidite stratigraphic traps (Figures 4, 5 and 9). To date, the prominent Late Cenozoic fault-reactivation anticlines along the basinward margin of the Leveque Shelf (eg, Trochus, Lynher, Lombardina and Sheherazade structures) have proved unsuccessful, with the possible exception of the inferred hydrocarbon column at Arquebus 1 (Haston and Farrelly, 1993).