Areas W07-16 and W07-17
Exmouth Plateau, Carnarvon Basin
Petroleum Potential
The producing oil accumulations at Lambert, Hermes, Mutineer/Norfolk and Exeter lie no more than 10 km to the east of Area W07-17. The world class North Rankin/Perseus gas accumulation extends to within 10 km of the southern edge of Area W07-17, with other gas accumulations nearby.
Source rocks
Figure 4 shows the petroleum systems elements of the Exmouth Plateau. The thick sedimentary section is dominated by Permian to Triassic sediments. This section has the greatest potential for mature source facies, with possible organic-rich units in the Early Triassic (marine Locker Shale equivalents) and Late Triassic (deltaic Mungaroo Formation facies and marine equivalents, plus carbonate lagoonal facies). Recent exploration activities on the Exmouth Plateau are based on a model that invokes gas charge from the deeply buried coals and carbonaceous claystones of the Mungaroo Formation. Gas charge is typically dry with condensate yields of less than 10 bbls/MMscf. Peak gas generation from these Triassic source rocks is interpreted to occur now at depths greater than 5 km below sea floor (Bussell et al, 2001). This interpretation agrees with an earlier investigation that, based on vitrinite reflectance data (Cook et al, 1985), concluded that the Mungaroo Formation intersected by wells on the Exmouth Plateau is now in the oil generation window.
The Late Jurassic Dingo Claystone was deposited in a deep-water, low energy, anoxic environment, and is the principal source for oil in much of the Carnarvon Basin; however, it is not widespread or thick on the Exmouth Plateau. This is due to erosion or non-deposition during, and immediately following, the Late Jurassic main syn-rift phase.
Reservoir
Potential reservoir facies include Triassic deltaic sands (Mungaroo Formation) and reefal carbonates within fault block traps, Late Jurassic shoreface or fan sands (Jansz/Io) in stratigraphic traps and Tertiary fan sands.
Exploration drilling in and around the release areas has clearly demonstrated the presence of multiple reservoir target levels ranging in age from the Triassic Mungaroo Formation through to the Tithonian Angel Formation (Figure 3).
The giant Jansz/Io giant gas discovery within the thin Late Jurassic section is the most significant result of recent drilling activity in Australia. The entire Oxfordian shoreface sandstone body in the Jansz/Io structure appears to be gas-bearing (Jenkins et al, 2003). The sandstone body, with an area of about 2,000 km2, is the largest in Australia in terms of gas reserves (about 20 TCF or 559 BCM http://www.doir.wa.gov.au/documents/mineralsandpetroleum/final_reserves_2005 (1).pdf) and areal extent.
In the release areas on the Exmouth Plateau, Late Triassic and Early Jurassic tilted fault block plays involve sources from within the Triassic sealed by intra-formational Late Jurassic or Cretaceous claystones. These plays include traps beneath the Jurassic Main Unconformity, roll-over structures and fault traps. Tilted fault blocks were peneplaned, but often form broad anticlinal features (Figure 5).
Delambre 1, drilled about 50 km northeast of Area W07-16, intersected an Early Jurassic oolitic facies. Andromeda 1 also intersected an Early Jurassic limestone with oil indications. To the north and west of the release areas, Late Triassic reefal carbonates have been recovered from ODP drilling and dredging (Exon et al, 1991). Hence, there is the potential for plays in the Late Triassic–Early Jurassic section with carbonate reservoirs and trapping mechanisms such as banks and mounds (Figure 6).
Seal
Both regional and intraformational seals are present in the release areas
with regional seals 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. Intraformational
seals are present within the Triassic Mungaroo Formation.