The Northwest Shelf Province lies in the onshore and offshore jurisdiction of Western Australia. A natural gas pipeline, south to Perth, has been in operation from Barrow Island and adjacent fields since 1984. Offshore islands are used for storage and transmission facilities. This pipeline has been extended to include new fields as they have been developed. Many large discoveries have not as yet been produced and are shut-in or abandoned due to market conditions or technology costs.

The Northwest Shelf has been involved in several continental rifting periods characterized by shifts in the location of rifts, failure of rifts, and changes in the direction of stress. These rifts are associated with Cambrian age separation of Australia from Chinese continental blocks forming the paleo-Tethys Ocean (Baillie and others, 1994), and Carboniferous to Permian age separation of China-Burma-Malay-Sumatra continental blocks forming the neo-Tethys Ocean (Veevers, 1974).

Paleozoic failed rifts, with a present-day northwest-southeast trend, including the Canning Basin (intersecting the northern end of this province), Joseph Bonaparte Gulf (northeast of this province), and Southern Carnarvon Basin (southwest of this province), have continued to influence structural development. These failed rifts were connected by a vast and persistent sag basin, or continental seaway, roughly along the modern Australian northwest shelf. Land offshore of the modern continent and the continent itself shed clastics into the seaway from both sides. Marine to nonmarine sediment has accumulated since the Carboniferous to a thickness of 12-15 km (Stagg and Colwell, 1994). Two reasonably stable basement cratons, the Pilbara Block south of the Canning Basin and the Kimberley Block north of the Canning Basin, are currently exposed onshore and anchor the modern continental margin (Fig. 1).

Carboniferous and Permian sediments are known from penetrations onshore, nearshore, and from the Exmouth Plateau, and are inferred from seismic data (Bentley, 1988; AGSO North West Shelf Study Group, 1994). These sediments record the waning Gondwana glaciation as glacial drift and tillites of the Lyons Group. Shelf carbonates and siltstones were deposited during a subsequent sea level rise, followed by fluvial, deltaic, and shoreface deposits of the Wooramel Group. Another marine transgression is represented by shales, siltstones, and sandstones of the Byro Group (Hocking, 1988). The Late Permian Kennedy Group consists of barrier bar sandstones interbedded with coals.

The persistent continental seaway accumulated Triassic sediments beginning with deposition of the regional Locker Shale source rock over a transgressive sandstone (Fig. 3). The overlying Mungaroo Formation deltas and shorelines prograded from the Pilbara highlands area across the seaway perpendicular to the modern coast. Interpretations of deep-seismic profiles acquired since 1990 (AGSO North West Shelf Study Group, 1994) show that the entire Triassic section, which is approximately 4 km at the Exmouth Plateau, is thin where it overlies the offshore

The close of the Triassic is represented by a regional and angular unconformity involving faulting and tilting of fault blocks across the Exmouth Plateau. At this time the Northwest Shelf divided into several tectonic provinces that influenced subsequent depositional events and provided for the generation and accumulation of hydrocarbons.

During Jurassic time, a string of en echelon, faulted sub-basins formed a sub-basin trend along the inner portions of the continental seaway separating the Exmouth Plateau, Rankin Platform trend on the west from the complex shelf margin formed by fault-block terraces that edge the craton to the east (Fig. 2). The down dropping and subsidence of these sub-basins provided space for deposition of the thick, Jurassic Dingo Claystone in three sequences, lower, middle and upper (Fig.3). This sub-basin trend was 500-1000 m deep and marine circulation was restricted by emergent land offshore and at the north end of the trend. Subsidence in this trend was greatest in the Barrow sub-basin and decreased northward to where continental deposition and erosion intermittently characterized the Roebuck Basin. Transgressive sands, regressive deltas, and deep-water fans were deposited in the sub-basins from erosion of the continent and exposed portions of the Northwest Shelf.

Jurassic age shale was intermittently deposited in troughs and lows formed by tilted fault blocks on the Exmouth Plateau when the plateau was partially submerged (Stagg and Colwell, 1994). As subsidence in the sub-basins slowed, deposition of the Upper Dingo Claystone overtopped most of the Exmouth Plateau as a thin veneer.

Another major unconformity, due to Tithonian tectonic fault reactivation and uplift of the Exmouth Plateau, is overlain by the Early Cretaceous, 1500-2000 m thick (Exon and von Rad, 1994) deltaic Barrow Group (Fig.3). The Barrow Group contains several producing reservoir members from delta bottomset and topset sandstones and from delta-front and deep-water deposits. Detailed sequence stratigraphy indicates two major depocenters, an older one to the northwest over the Exmouth Plateau and a younger one to the southeast over the sub-basin trend (Ross and Vail, 1994). The Indian plate to the southwest was the source area for clastics that prograded along the sub-basin trend and across the Exmouth Plateau, filling in remaining relief. The delta front of the delta complex was roughly perpendicular to the modern shoreline and prograded as far east as northern Barrow Island. The end of Barrow deposition marks Late Neocomian movement of the Indian plate away from the west coast of Australia. The subsequent Late Valanginian Muderong Shale contains the transgressive Mardie Greensand and Birdrong Sandstone Members, which are important reservoirs (Fig.3). The Muderong Shale records a rise and stillstand in sea level and the restoration of a coast-parallel sequence trend and also acts as a regional seal.

Carbonate deposition has dominated the Australian northwest continental margin since formation of the Aptian-Albian unconformity (Fig. 3). Up to 3 km of post- unconformity carbonate strata has been deposited. Several hydrocarbon accumulations in this interval were fed by faults cutting the regional seal. Tectonic movements of the region in Late Cretaceous through Miocene time were relatively minor.