The Northern Carnarvon Basin, and in particular the Barrow and Dampier Sub-basins, is regarded as the premier hydrocarbon basin of Australia, and is one of the more intensely explored areas of the country. The basin lies mainly offshore, extending north from the Pilbara Craton to the continental–oceanic crust boundary, and covers about 500,000 km2.
The Northern Carnarvon is transitional to and overlies the predominantly onshore Southern Carnarvon Basin. Several islands provide excellent locations for production facilities and bases in the basin (e.g. Barrow Island, Airlie Island, Varanus Island and Thevenard Island).
The onshore part of the Northern Carnarvon Basin is readily accessible from the North West Coastal Highway. The towns of Carnarvon, Exmouth, Onslow, Dampier, Karratha and Port Hedland provide excellent support facilities for offshore exploration and development. Karratha is the loading terminal for Woodside’s LNG exports and a processing centre for supplying gas to domestic markets including Perth, Bunbury and the Eastern Goldfields.
Geological setting
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 depocentres of the southern North West Shelf, containing up to 15 km of Mesozoic sedimentary rocks. In these sub-basins, Mesozoic and Cenozoic successions overlie (commonly at considerable depth) Paleozoic sedimentary rocks that extend north from the Southern Carnarvon and Canning basins. The Peedamullah Shelf and Lambert Shelf flank the depocentres shoreward, while a mid-basin arch consisting of the Rankin Platform and the Alpha Arch flank them seaward. The Kangaroo Trough, Dixon Sub-basin, and Investigator Sub-basin lie further offshore.
Exploration history
Oil was discovered in the first modern well drilled in the Carnarvon Basin, Rough Range 1, at the eastern edge of the Exmouth Sub-basin, in 1953. Follow-up 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 increased following a steep decline in 2001 and 2002.
The offshore portion of the basin that is in the State’s jurisdiction has a reasonable regional and detailed seismic grid, mainly north of Exmouth Gulf where 3D surveying is now a common tool in both exploration and development scenarios. Some offshore areas still receive only minimal exploration, especially in the south, with exploration remaining sparse over most of the onshore basin, except for the Rough Range – Cape Range area. The nearshore is highly prospective but largely unexplored owing to the difficulty of conducting seismic and drilling operations in a shallow-water, environmentally sensitive zone.
Petroleum prospectivity
Numerous oilfields and gasfields in the Northern Carnarvon Basin demonstrate the petroleum potential of the region, particularly offshore.
Oil is produced primarily from the Barrow Group and sandy intervals (Windalia Sand, Mardie Greensand and Birdrong Sandstone) of the lower Winning Group, all of which occurred post-breakup. The Lower Cretaceous Barrow Group has excellent reservoir characteristics, and Middle Miocene faulted anticlines provide structural traps. The main source rock for post-breakup accumulations is considered to be the Upper Jurassic Dingo Claystone. This source is estimated to have the capacity to expel 1.27 TL (8 Bbbl) of oil, just over 10 per cent of which has been discovered 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.
Within the Dampier Sub-basin, production of gas, condensate, and associated minor oil, 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, which are probably sourced by the Triassic Locker Shale and intra-Mungaroo shales.
Although parts of the Northern Carnarvon Basin are intensely explored, further discoveries continue to be made at various intervals, both within the proven hydrocarbon-rich Barrow and Dampier Sub-basins and in the less explored surrounding sub-basins. Successful exploration includes different play types and extensions of known discoveries and models.
Find out more about the petroleum prospectivity of the Northern Carnarvon Basin (page 43)
For information on the Commonwealth areas of the Northern Carnarvon Basin see Geoscience Australia’s website