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Southern Carnarvon Basin
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Southern Carnarvon Basin
The onshore, primarily Paleozoic, Southern Carnarvon Basin has seen minimal exploration compared to the adjoining Perth and Northern Carnarvon Basins. The basin extends west from the Yilgarn Craton to the edge of the continental shelf and covers approximately 200 000 km2.
Geological Setting
The north–south elongated Southern Carnarvon Basin is composed of two principal structural elements: the Gascoyne Platform to the west, and the Merlinleigh and Byro Sub-basins to the east. The Gascoyne Platform contains gently folded Ordovician to Devonian strata, unconformably overlain by a veneer of Mesozoic and younger rocks.
In comparison, the Merlinleigh and Byro Sub-basins are characterised by a thick Upper Carboniferous to Permian section, underlain by a Lower Carboniferous – Devonian section and unconformably overlain by a veneer of Cretaceous and younger rocks. To the north, Triassic rocks cover the Permian sequence. Northerly and northwesterly-trending faults are present, and the Paleozoic section is up to 7 km thick.
Seismic data indicate that the breakup of Gondwana during the Early Cretaceous had the greatest impact on the structural evolution of the area. This tectonism produced wrenching that resulted in long-wavelength folds and strike-slip faults, rejuvenated in the Middle Miocene.
Exploration History
Petroleum exploration commenced in the Southern Carnarvon Basin in the 1930s after W.G. Woolnough first drew attention to the Wooramel River area, and when hydrocarbon shows were encountered in shallow water bores in the northern part of the region.
WAPET was the first company with serious exploration programs in the 1950s and 1960s, following its oil discovery at Rough Range, on the boundary between the Northern and Southern Carnarvon Basins.
After early work near Rough Range proved non-commercial, the main exploration activity focus moved north to the offshore Northern Carnarvon Basin. To date, 75 onshore (including 57 stratigraphic tests) and two offshore wells have been drilled in the Southern Carnarvon Basin. No fields or accumulations have been discovered to date
Petroleum Prospectivity
The northernmost part of the Southern Carnarvon Basin is characterised by three large Cenozoic anticlines: Rough Range, Giralia, and Marrilla Anticlines. Several smaller anticlines of the same age are exposed along the eastern and western shores of Lake MacLeod in the central part of the basin, and similar anticlines underlie the peninsulas and islands of the Shark Bay region.
There have been only a few valid tests for hydrocarbon plays in the region. Lower Cretaceous sandstone, which has excellent reservoir characteristics, has been the main objective for oil exploration in the northern part of the basin, but distance from effective source rocks or adequacy of seal are risks. Source rocks are present in the Lower Permian, Upper Devonian, and Silurian strata (Larapintine 2 and 3, Transitional, and Gondwanan petroleum systems).
Lower Permian source rocks are regionally immature to marginally mature, and are mostly gas prone. In comparison, Devonian–Silurian source rocks have been shown to have good potential for both oil and gas generation, although Silurian source beds are thin.
The Cretaceous succession may still offer some potential for hydrocarbon generation and for an oil discovery (Austral petroleum system). The Devonian succession offers the most challenging, yet untested, objective.
Untested structural highs have been demonstrated adjacent to major faults (mainly along the eastern edge of the Gascoyne Platform) that may have provided vertical conduits for migrating hydrocarbons.
