Shaly foresets and sandy topsets are present from Saladin field to Cowle field but only sandy topsets are present at Roller field. Barrow reservoirs at Saladin, Yammaderry and Cowle are fluvial channel sandstones (lower sandstones) overlain by tidal back-barrier sandstones (bioturbated and upper sandstones). Roller field reservoir is fluvial lower sandstones only. Upper sandstones of the Barrow at Saladin, Yammaderry and Cowle represent an early marine transgression drowning the Barrow delta following continental separation south of the Exmouth Plateau (McLerie and others, 1991).

The Barrow Group Flag Sandstone produces oil in the Harriet field and accounts for discoveries in Orpheus-1, Plato-1, and Campbell-2 wells. A study by de Boer and Collins (1988) indicates that the Flag Sandstone consists of quartzitic sands reworked into deep-water fans from Barrow delta distributary rivers and transported down the Lowendal Syncline. Four-way closures, due to relief on the top of the Flag fans, are sufficient to trap hydrocarbons at Bambra, Campbell, Rosette, and Harriet fields. Some faulting may help confine these accumulations (McClure and others, 1988). There is an erosional southwest limit just south of Barrow Island, an erosional southeast limit east of Barrow Island, a northwest limit west of Tryal Rocks-1 and a northern shaleout south of Withnell-1. The Flag Sandstone is one of several identified or inferred deep-water fans deposited generally to the north into the basin in front of the Barrow delta. Another of these fans is the reservoir at Scarborough-1 on the Exmouth Plateau.

The Birdrong Sandstone from the onshore Carnarvon Basin is described as a transgressive basal deposit and ranges in age from late Valanginian or early Hauterivian to Barremian and Aptian. It produces gas at Tubridgi and Rivoli fields, oil at Rough Range field and oil and gas at Wandoo field. Mapped isopach thicks occur at Sholl Island-1, Direction-1 Hope Island-1, Whitlock Dam-1, and Resolution-1 (Hocking, 1988). The Birdrong Sandstone, which may be equivalent to the Mardie Greensand, generally overlies the Mardie Greensand where it is present and lies parallel to the present coast and north of North West Cape.

The transgressive Mardie Greensand of the Muderong Shale was deposited unconformably over fluvial or back-barrier Barrow Group after a time gap of several million years (McLerie and others, 1991). The Mardie Greensand may be the younger and northern equivalent of the Birdrong Sandstone, but sometimes it is placed at the top of the Barrow Group under the Hauterivian unconformity. Since the Mardie Greensand is distinct, it may be the lateral equivalent to the Birdrong Sandstone in a slightly differing depositional environment that promoted glauconite development and preservation. The Mardie Greensand represents a deeper-water depositional setting than the Birdrong Sandstone (Hocking and others, 1988) and was deposited over about 15 million years. It is a gas bearing reservoir on the Peedamullah shelf in Mardie-1 and an oil and gas bearing reservoir in Pepper South and Barrow Island fields. It oversteps the Barrow Group to lie on locally present conglomerates on the southeastern parts of the Peedamullah shelf. The Mardie Greensand ranges in thickness from 15-30 m with a maximum thickness of 59 m at Rosily-1A. It pinches out on the northernmost foreset of the Barrow delta and grades into shale. Mapped thicks occur to the southwest of Barrow Island, offshore Exmouth and north of North West Cape (Hocking, 1988).

The Aptian Windalia Sandstone member of the Muderong Shale is interpreted to have originated as a storm-generated, shelf-sand deposited below fair-weather wave base. It is the main hydrocarbon reservoir in the Barrow Island oil field.

In the Dampier sub-basin (Howell, 1988), reservoirs that are hydrocarbon sourced and locally sealed by the Dingo Claystone are present in transgressive sands, regressive deltas (the Bathonian Legendre Formation), and deep-water fans. Other reservoir rocks are found in the Tithonian Dupuy Member of the Upper Dingo Claystone and the deep-water fan Angel Sandstone of the Dupuy Member, which produces at Wanea, Cossack, and Angel fields. The Dupuy submarine fan built into the Barrow sub-basin from the east and southeast across the Dingo Claystone.

The Late Jurassic Angel Sandstone of the northern Barrow-Dampier sub-basin and its equivalent, the Dupuy Member of the Upper Dingo Claystone in the Barrow Island area, are both characterized as deep-water turbidites (Bint, 1991).

Facies of the Triassic Mungaroo Formation include meandering and braided stream, prodelta, distributary mouth bar, distributary/fluvial channel and levee, interdistributary bay, crevasse splay, and coal-swamp deposits (Barber, 1988). The Mungaroo Formation is approximately 3000 m thick and displays three main stacked deltaic sequences capped by alluvial plain deposits within which are several high- frequency sequences (Barber, 1988). This interval contains reservoirs on tectonically high fault blocks on the Exmouth Plateau, Rankin Platform trend, and areas of the eastern edges of the sub-basin trend and the inner structural terraces. Within the sub-basin trend the interval has not been reached by the drill and is not readily imaged by seismic data.

Overall, porosity and permeability are good in all reservoir facies. Primary porosity is largely preserved and secondary porosity can be a contributing factor. Non-marine sandstones of the Mungaroo Formation have porosity averaging 28% with highs of 34% in 30 m thick delta plain units (Barber, 1988). Permeability can be as high as 1000 mD. Burial depth decreases these values and carbonate cementation in the marine intervals affects both porosity and permeability.

Jurassic and Cretaceous reservoirs range in porosity from 16% to 35% and permeability from 27 mD to 3000 mD. Minor quartz overgrowths, authigenic clay and feldspar dissolution are reported.

SEAL ROCK
The Muderong Shale is the regional seal deposited across the Northwest Shelf as a result of a major transgression in late Valanginian (Fig. 3). It is approximately 900 m thick and interfingers with the distal basinal deposits of the Barrow Group and onlaps the top of the succession. Maximum transgression occurred in the Aptian Stage after a slow reworking of the top of the Barrow delta into the Birdrong Sandstone and Mardie Greensand. The Muderong Shale and its sandy members span the Cretaceous Berriasian to Aptian Stages and are overlain by a widespread unconformity (Butcher, 1989).