Hydrocarbons are found in reservoirs within, directly overlying, and adjacent to (across faults and unconformities) the Jurassic source rock designated for the Dingo-Mungaroo/Barrow TPS 394801. Migration was generally up-dip and hydrocarbon accumulations remain close to the areal extent of the mature source rock. Fault migration into overlying reservoirs is evident and fault migration through the overlying regional seal is an important method to establish accumulations in post-seal reservoirs. Multiple migration is evidenced by minor to extensive residual hydrocarbon columns, fault spill from one closure to the next one up-dip, spill-point migration to adjacent closures, varying oil-water contacts, and oil over gas relationships.

The Locker-Mungaroo/Barrow TPS 394802, includes fault-fed accumulations overlying mature source rock. Fault reactivation is an important mechanism creating traps and providing remigration paths to post-Triassic accumulations.

SOURCE ROCK
The primary petroleum system of Northwest Shelf Province 3948 is the Dingo-Mungaroo/Barrow TPS 394801. This corresponds approximately to the Westralian 2 of Bradshaw and others (1997). A secondary system is the Locker, Mungaroo/Barrow TPS 394802, similar to the Westralian 1 petroleum system of Bradshaw and others (1997).

Jurassic Dingo Claystone of the Dingo-Mungaroo/Barrow TPS 394801, is the source rock for hydrocarbons in the sub-basins and on the Rankin Platform trend. Strata of the same age along the Tethyan margin in Europe, the Middle East and Asia are recognized as major source rocks (Klemme and Ulmishek, 1991). Deposition of the Dingo Claystones on the Northwest Shelf occurred mainly in a restricted marine setting as the sub-basins subsided. The Rankin Platform trend edge of the Exmouth Plateau was occasionally exposed, as was most of the Exmouth Plateau. Three major successions of Jurassic Dingo Claystone, lower (Sinemurian/ Pliensbachian), middle (Toarcian-Bathonian), and upper (Callovian-Kimmeridgian) occupy the sub-basins (Wulff, 1992). The thickest and deepest-water Dingo Claystone intervals were deposited adjacent to the Rankin Platform trend (Wulff, 1992). By Late Kimmeridgian time, the sub-basins were filled and the Dingo Claystone was deposited across most of the Exmouth Plateau. An offshore land area, Argo Land (Bradshaw and others, 1994) and periodic emergence of the Roebuck Basin and the region northeast of the Dampier sub-basin, further restricted circulation. Jurassic deposition in the region northeast of the Dampier sub-basin was dominated by shallow marine and continental coarse clastics so potential hydrocarbon source rocks are terrestrial and localized.

Total thickness of the Dingo Claystone, estimated from seismic data, is 3-4 km (Stagg and Colwell, 1994). Possible net thickness of the Middle Dingo Formation in the Barrow sub-basin is 100-250 m and Upper Dingo Formation thickness is estimated at up to 1000 m (Scott, 1994). The Dingo Claystone is thin on the eastern terraced margin and to the north in the Beagle sub-basin where subsidence was not pronounced. Age-equivalent facies to the north are mostly shallow marine and continental sands. On the Exmouth Plateau, Jurassic rocks occur as a thin veneer with pockets up to a few hundred meters in half grabens on the surface (Stagg and Colwell, 1994).

The transgressive Locker Shale was deposited on an angular and erosional unconformity developed on Permian fault blocks and is underlain by a basal transgressive sandstone (Butcher, 1989). The Locker Shale is widespread and represents marine incursion across the entire shelf. This shale and equivalent shales in adjacent areas are 200-1000 m thick throughout the region. The Triassic Mungaroo Formation overlies the Locker Shale and records a regression beginning in Early Triassic time. The Mungaroo Formation generally coarsens upward and includes interbedded quartzose sandstones, siltstones, shales and coal units. Strata equivalent to the Mungaroo Formation become progressively more marine influenced to the north.

Bradshaw and others (1994) list an average total organic carbon content (TOC) of >2wt% and hydrogen index (HI) of >150 from Jurassic marine source rocks with significant terrestrial input. In the Beagle sub-basin, Jurassic TOCs from coals and algal sapropel range from 0.92 to 13.01wt% and HIs from 28 to 214 (Blevin and others, 1994). Oil shows in the Nebo-1 discovery may be from Early Jurassic source rocks. Scott (1994) reports TOC of 1-5wt% and HI from 100-400 for Upper Dingo Claystone, and TOC 0.2-3wt% and HI 100-250 for Middle Dingo Claystone. Kopsen (1994) discusses Jurassic strata with oil and gas potential in the Barrow and Dampier sub-basins derived from marine shale and coal measures and in the Beagle sub-basin derived from Early Jurassic coal measures.

The oil-and gas-prone Triassic source rocks are generally of moderate quality with local areas of high quality, such as TOCs of 2.5wt % and Rock-Eval S₂ up to 5 mg/g (Scott, 1994). Two wells, reported by Scott (1994), have the following analysis: TOC wt%, 0.91 and 0.90; S₂mg/g, 2.58 and 2.16; HI, 284 and 240; Tmax, 433° and 429°. These variations may be due in part to locally restricted conditions created when the initial Triassic transgression flooded erosional and tectonically produced relief on the underlying Permian surface. In Candance-1 and Kybra-1 wells, the basal Locker Shale has TOCs of 1.0-2.9wt% and HI up to 300 (Warris, 1993). Warris (1993) predicts that the Locker Shale is mature to over-mature in the Barrow/Dampier sub-basin on the downthrown side of the Flinders Fault. The Triassic strata from the Exmouth sub-basin, Barrow sub-basin, Dampier sub-basin and the Beagle sub-basin is generally rated as a major potential oil source interval by Kopsen (1994). Both marine shale and coal measures are indicated. The Locker Shale in the Exmouth sub-basin and along the eastern and southern margins of the Barrow sub-basin is presently mature (Parry and Smith, 1988).

Subsidence of the sub-basins may have pushed the Locker Shale and coals of the Mungaroo Formation into and through the oil window during the Jurassic Period. These hydrocarbons may still be trapped in deep, untested structures or may have migrated and remigrated escaping to the surface during later lowstand unconformities and tectonic activity. Evidence for mixing of these hydrocarbons with younger hydrocarbons is not conclusive. The Triassic Locker Shale is considered the source of gas-condensate in the Cretaceous reservoir at Scarborough-1 and gas and oil shows in Triassic reservoirs at Vinck, Sirius, Endracht, Jupiter, and Zeewulf on the Exmouth Plateau (Fig. 6b).