Areas W07-1, W07-2 and W07-3
Londonderry High, Bonaparte Basin
Petroleum Potential
Hydrocarbons and source rocks
The petroleum prospectivity of the Londonderry High relies on either sourcing from the Palaeozoic succession, similar to that reported in the central and inboard Petrel Sub-basin, or long distance migration from Jurassic source kitchens in the adjacent Sahul Syncline and Vulcan Sub-basin. To date, there is evidence for the existence of an Early Carboniferous oil-prone petroleum system from oil-stains in Torrens 1 (to the south of the release areas), and for a gas-prone Permian petroleum system (gas at Prometheus/Rubicon). Seismic amplitude anomalies, interpreted as direct hydrocarbon indicators (DHIs) of gas saturation, have been reported by Gorter et al (1998) in sandstones of the lower Mount Goodwyn Formation in the vicinity of the Plover and Peewit wells. These authors presume that the most likely source of gas is from pre-Triassic sediments, or alternatively long distance migration from Late Jurassic source rocks in the depocentres to the north. Permian sediments are moderately rich in organic carbon and give fair pyrolysis yields on the Londonderry High (Whibley and Jacobson, 1990; Passmore et al, 1997).
In addition to the Palaeozoic-sourced hydrocarbons, presumed Mesozoic-sourced gas is encountered at Ascalon 1A and Saratoga 1, probably being generated from the Sahul Syncline and Malita Graben where claystones of the Plover Formation, Elang/Vulcan Formation and Frigate Shale (Flamingo Group) are thermally mature. Likewise, gas at Halcyon 1 and Delamere 1, on the western margin of the Vulcan Sub-basin, are probably the result of long-range migration from Jurassic depocentres in the Swan Graben (Figure 4). The gas at Halcyon 1 is probably a mixture, and the gas at Delamere 1 is biodegraded (Geoscience Australia, unpublished data).
There is the potential for oil to be found on the Londonderry High as well as gas, since oil stains obtained from the Late Carboniferous Kuriyippi Formation reservoir in Torrens 1 have been loosely correlated with the Early Carboniferous-sourced oils in the Barnett and Turtle wells (Ruble et al, 2000). Analysis of oil-bearing fluid inclusions and stains also from Torrens 1 indicates a 42 m gross palaeo-oil column within the Permian Fossil Head Formation (Lisk and Brincat, 1998). Isotopic and biomarker profiles of a residual oil from this palaeo column are most comparable to the condensates from the Petrel and Tern wells that are attributed to a Permian source (Edwards et al, 2000; Ruble et al, 2000).
Oil shows in the Early Cretaceous Sandpiper Sandstone in Ascalon 1A and gas recovered from the Jurassic Flamingo Sandstone in Saratoga 1 (Geoscience Australia, unpublished data) have most likely been sourced from Jurassic source rocks. Lowry (1995) also reports oil shows in the Flamingo Group at Rambler 1, as well as gas in tight reservoirs of the Plover Formation.
Reservoirs
Gas discoveries on the Londonderry High at Prometheus/Rubicon and Ascalon 1A occur within the Late Permian Hyland Bay Formation (Robinson and McInerney, 2004). At Ascalon 1A, gas also occurs within the Early Triassic Mount Goodwin Formation. Gas indications at Avocet 1 and Eider 1 occur within the Jurassic Upper Plover Formation, with hydrocarbon shows at Halcyon 1, Rambler 1 and Saratoga 1 being reservoired within the Late Jurassic–Early Cretaceous Flamingo Group.
Seals
Two regional seals are present across the Londonderry High; the Cretaceous Bathurst Island Group and the Early Triassic Mount Goodwin Formation.
Play types
On the Londonderry High, two groups of play types were documented by Passmore et al (1997); structural fault-dependent plays and stratigraphic plays. Fault-dependent traps are the most common play type due to the highly faulted character of the region. This play type has reservoir potential in sandstones of Late Permian, Triassic, Jurassic and Early Cretaceous age. The potential for stratigraphic pinchout plays occurs across the Londonderry High where erosion has removed Jurassic and/or Triassic rocks. This play type is prevalent in the southern part of this region, towards the erosional edge of the Sahul Group. Brincat et al (2001) indicated that unbreached traps of the Londonderry High may be difficult to locate and concluded that stratigraphic traps may constitute an alternative play type.
Other potential plays on the Londonderry high include Maastrichtian sandstones
with anticlinal closures (Brincat et al, 2001) and stratigraphic pinchouts
and submarine fan sandstone plays within the Flamingo Group (Whibley and
Jacobsen, 1990). Vertical migration of liquid hydrocarbons due to fault
reactivation could have charged shallower reservoirs and produced valid
exploration targets (Cadman and Temple, 2004).