Petroleum Occurrence
Significant early offshore discoveries include gas at Petrel and Tern in 1969 and 1971 from the Lower Permian Hyland Bay Formation (Fig. 8) (Gunn, 1988b; Mory, 1988) and oil in 1972 from Puffin-1 in the Vulcan graben sub-basin, Territory of Ashmore and Cartier Islands.

Gas discoveries dominate in the central portions of the Petrel sub-basin. Oil gravity ranges from 30° -36° API and 50° API in liquids associated with the Petrel gas discovery. A pipeline is planned for two of these gas discoveries to Darwin (Fig. 2) (DPIE, 1998).

Source Rock
The Keyling, Hyland Bay-Permian TPS 391002, contains one source-rock complex and two assessment units, the Petrel assessment unit 39100201 and the Vulcan assessment unit 39100202 (Fig. 8). This petroleum system includes the commercial gas and condensate discoveries at Petrel and Tern (Fig. 8) sourced by the Lower Permian Keyling Formation and the Upper Permian Hyland Bay Formation (Fig. 5 and Fig. 9). The Keyling Formation consists of delta-plain coals and marginal marine shales deposited in the Petrel sub-basin north of the Carboniferous depocenter (Gunn, 1988b). The Keyling source rock is characterized by high organic carbon and fair source quality, TOC 35 wt% and HI 230 mgHC/gTOC for the coals and TOC 2.8 wt% and HI 95 mgHC/gTOC for the shales (Edwards and others 1997). Mory (1988) defines the formation as the third unit of the Kulshill Group that thickens toward the Malita sub-basin (Fig. 4, C-D and E-F) (DPIE, 1998). It conformably overlies the Treachery Shale and is in diachronous contact with the overlying Fossil Head Formation. The Keyling Formation is as much as 973 m thick in Tern-1 and is present on the Lacrosse terrace and Plover shelf (Fig. 4, E-F) (Mory, 1988).

The Upper Permian Hyland Bay Formation is as much as 520 m thick (Mory, 1988) and contains prodelta shales high in gas-prone organic carbon. These shales contain TOC of 2 wt% and HI of 55 mgHC/gTOC (DPIE, 1998). Other reported values are TOC 1.6 wt%, HI 125 mgHC/gTOC, R_o 0.9% at Petrel-2 and TOC 10 wt%, HI 240 mgHC/gTOC, R_o 0.7% at Bougainville-1 (McConachie and others, 1996). Possible oil-prone source rocks from the Upper Permian section have TOC of 15wt%, HI of 250-500, and R_o 0.6% at the eastern side of the sub-basin.

Peak generation from Keyling and Hyland Bay source rocks occurred during the Middle Triassic and Late Cretaceous-Tertiary with possible expulsion for the Keyling Formation as early as Late Permian (Fig. 9) (Edwards and others, 1997; DPIE, 1998). Late Permian-Early Triassic maturation and migration of hydrocarbons from the Keyling Formation is expected if high quality coaly shales that are present on the eastern, Moyle platform side of the Petrel sub-basin, extend and thicken into the sub-basin (Edwards and others,1997). Maturation modeling suggests peak generation and migration from the Hyland Bay Formation in Late Cretaceous through Tertiary time and (depending on modeled overburden thickness) Early Triassic or Middle to Late Triassic from the Keyling Formation (DPIE, 1998).

Reservoir Rock
Gas and condensate discoveries of the Keyling/Hyland Bay-Permian TPS are located in the central portions of the Petrel sub-basin and include Tern and Petrel fields, each estimated to contain recoverable reserves of approximately 600 billion cubic feet of gas (BCFG) (DPIE, 1998). A gas discovery at Fishburn-1 and gas and condensate shows at Penguin-1 are also reported. Most of the discoveries to date are in the Permian Hyland Bay Formation of the Kinmore Group (Fig. 5) (DPIE, 1998). Paleogeographic maps show that a large lower delta plain, prograding northwestward, occupied the Petrel sub-basin from Latest Carboniferous to Late Permian time (Mory, 1988; McConachie and others, 1996). At Petrel and Tern fields, the Hyland Bay Formation is 400-500 m thick and is characterized by sandstones and mudstones with minor coals and limestones. The main gas reservoirs of Petrel field are three, 5 m-thick, stacked, fluvial-marine deltaic sandstones, with 23% porosity, in the Cape Hay Member of the Hyland Bay Formation (Gunn, 1988b; DPIE, 1998). Good permeability is indicated by high flow rates. Reservoir quality is facies dependent and relies on clay coatings on sand grains to inhibit quartz overgrowth and preserve primary porosity (Bhatia and others, 1984; Gunn, 1988b). The reservoir at Tern field is the sandstone unit of the Tern Member of the Hyland Bay Formation with an average porosity of 20%, where quartz overgrowth has been inhibited by clay coatings, and good flow rates were reported indicating good permeability (DPIE, 1998; Gunn, 1988b; Gunn and Ly, 1989). The Tern Member is interpreted as originating in a barrier bar environment (Bhatia and others, 1984). The reservoir at the Fishburn-1 gas discovery and oil and gas shows at Torrens-1 are also considered to be in the Upper Permian Hyland Bay Formation (Fig. 8) (DPIE, 1998).

Seal Rock
Gas reservoirs in the Petrel field are sealed by intraformational shales in the Kinmore Group (Gunn, 1988b). The seal at the Tern field consists of the overlying shallow marine Mount Goodwin Formation of the Kinmore Group (Fig. 5 and Fig. 9) (Gunn, 1988b; DPIE, 1998). The Mount Goodwin Formation is present across the Londonderry high and the Petrel sub-basin and varies from 150-670 m in thickness (Mory, 1988). Paleozoic claystone seals are proven in the eastern arm of the province and the Paleozoic unconformity in this region may act as a seal where it is coupled with overlying marine shales rather than sandstones (McLennon and others, 1990).

Trap Types
Anticlines are the most common type of trap for accumulations of both oil and gas in the petroleum system. Draping structures are also important. These traps contain approximately 30% of the reported oil equivalent reserves in the province (Petroconsultants, 1996). The Middle Triassic to Early Jurassic regional compression also affected this area, resulting in uplift and the formation of inversion anticlines, large anticlines, and monoclines (DPIE, 1998).