THE RANKIN TREND NORTHWEST SHELF, WESTERN AUSTRALIA

Abstract

Recent gas/condensate discoveries on the Rankin Trend in the southern part of the Northwest Shelf of Western Australia have provided considerable encouragement to exploration effort in Western Australia and have demonstrated that the area could become a major hydrocarbon producing region.

The Rankin Trend is located near the outer part of the Dampier Sub-basin, a northern offshore extension of the Carnarvon Basin. Geologically, the Dampier Sub-basin consists of a block-faulted Lower Mesozoic substructure overlain by thick Upper Mesozoic and Tertiary sediments.

Thick clastic Triassic and Jurassic sediments were subjected to extensive faulting, which resulted in the formation of several structural trends related to northeast-southwest faults downthrowing towards the present coastline. Later Jurassic and Cretaceous deposition covered this block-faulted substructure with deltaic progradation seaward from the present coastline. Progradation continued through the Tertiary with deposition of up to 10,000 ft. of carbonates and clastics.

The Rankin Trend consists of several en echelon fault blocks containing Triassic sediments overlain by Cretaceous and Tertiary shales and carbonates. Structural development continued during the Cretaceous and Tertiary causing differential compaction and drape over the pre-existing features.

North Rankin No. 1 was drilled in 401 ft. of water to a depth of 11,593 ft. on a culmination along the trend. The well proved a net Upper Triassic sandstone pay 1020 ft. thick within a gross 1851 ft. thick Upper Triassic column. Drillstem tests flowed up to 14 million cu.ft. per day gas from selected intervals, with an average condensate ratio of 30 bbls. per million cu.ft.

A second well, Rankin No. 1, drilled in 303 ft. of water on a separate culmination of the trend some 29 mi. southwest of North Rankin No. 1, proved a similar stratigraphic column and a net 216 ft. of Upper Triassic sandstone pay. A thin basal oil column produced 1062 bbls. oil and 10.9 million cu.ft. of gas per day. The gas reservoir flowed 15.9 million cu.ft. per day with condensate at the rate of 35 bbls. per million cu.ft. A third well, Goodwyn No. 1, on another culmination between the North Rankin and Rankin wells, has proved additional gas/condensate reserves.

The Northwest Shelf Group, operated by B.O.C. of Australia Limited, is currently studying the feasibility of commercial hydrocarbon production.