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Northern Canarvon Basin
The Northern Carnarvon Basin, and in particular the Barrow and Dampier Sub-basins, is regarded as the premier basin of Western Australia, and is one of the more intensely explored areas of Australia. The basin is mainly offshore, and extends from the Pilbara Craton to the continental-oceanic crust boundary, and covers about 500 000 km2.
The Northern Carnarvon Basin is dominated by a southwest-trending set of troughs – the Exmouth, Barrow, Dampier, and Beagle Sub-basins. These are the major Mesozoic depocentres of the southern North West Shelf, containing up to 15 km of Mesozoic sedimentary rocks.
In the sub-basins, Mesozoic and Cenozoic successions overlie (commonly at considerable depth) Paleozoic sedimentary rocks that extend north form the Southern Carnarvon and Canning Basins. They are flanked shoreward by the Peedamullah Shelf and Lambert Shelf, and seaward by a mid-basin arch — the Rankin Platform and Alpha Arch. The Kangaroo Trough, Dixon Sub-basin, and Investigator Sub-basin lie farther offshore.
The evolution of the Northern Carnarvon Basin was controlled by the breakup of Gondwana. Prior to breakup, several sedimentary sequences were deposited from the Ordovician to the Permian in an elongate basin between the Archean Pilbara Craton and continental blocks to the northwest.
At the end of the Paleozoic, northeastward-trending troughs developed in the Northern Carnarvon Basin, and the present basin framework began to develop, as rapidly subsiding troughs in the Triassic, prior to faulting and breakup in the Jurassic.
Thick siliciclastic sequences accumulated in offshore marine to continental settings. Final continental separation did not occur until the Early Neocomian, further offshore than the attempted rifts along the axis of the Barrow and Dampier Sub-basins, leaving a trailing edge, passive margin basin.
Some time after breakup, in the Late Cretaceous, global oceanic circulation patterns changed and deposition shifted from siliciclastic dominated to carbonate dominated, forming a thick carbonate wedge across the entire offshore basin.
Oil was discovered in the first modern day well to be drilled in the Carnarvon Basin, Rough Range 1, at the eastern edge of the Exmouth Sub-basin in 1953. Followup discoveries of oil at Barrow Island (1964) and of gas in North Tryal Rocks 1 (1971) established the Northern Carnarvon Basin as a major hydrocarbon province.
The level of exploration activity has continued to increase since a steep decline in 2001 and 2002.
During 2006 there were 46 producing fields, several new fields in extension or development drilling, and numerous undeveloped hydrocarbon accumulations. Recent new field wildcat wells are tabulated in Table 7 along with their completion status.
The offshore portion of the basin has a reasonable regional and detailed seismic grid. This is mainly north of Exmouth Gulf where 3D surveying has become a common tool in both exploration and development scenarios. Some offshore areas have still seen only minimal exploration, especially in the south, and exploration has been sparse over most of the onshore Northern Carnarvon Basin, with the exception of the Rough Range – Cape Range and Tubridgi areas.
Nearshore areas are highly prospective, but are largely unexplored because of the difficulty of seismic and drilling operations in a very shallow water, environmentally sensitive zone.
The numerous oilfields and gasfields of the Northern Carnarvon Basin demonstrate the petroleum potential of the region, particularly offshore. Oil is produced primarily from the Barrow Group and Windalia Sandstone, within the post-breakup sequence. The Lower Cretaceous Barrow Group has excellent reservoir characteristics, and Middle Miocene faulted anticlines provide structural traps.
The main source rock for these post-breakup accumulations is considered to be the Upper Jurassic Dingo Claystone. The source rocks are estimated to have the capacity to expel 1.27 teralitres (8 billion barrels) of oil, of which just over 10% has been discovered to date within the Barrow Sub-basin. The sub-basin margins such as the Peedamullah Shelf, Rankin Trend, Exmouth Gulf, and the sub-basin axes may hold the key to a major portion of the undiscovered reserves.
Production of gas and condensate, and associated minor oil, within the Dampier Sub-basin is primarily from pre-breakup sandstones of the Upper Jurassic to mid-Upper Triassic Angel, Brigadier, and Mungaroo Formations. Truncation and fault traps control the pre-breakup accumulations. The source rock for these accumulations is thought to be the Locker Shale.
Although parts of the Northern Carnarvon Basin have been intensely explored, further discoveries continue to be made at different levels in the succession, both within the proven hydrocarbon-rich Barrow and Dampier Sub-basins and in the less explored surrounding sub-basins. Different play types are being successfully explored, as well as extensions of known discoveries and models.
The support facilities present in the Carnarvon Basin now allow the development of offshore fields that have less than 1.5 GL (10 MMbbl) of recoverable product.