THE GEOLOGY OF THE WEST TRYAL ROCKS GAS FIELD

Abstract

The West Tryal Rocks gas field is located offshore at the western margin of the Barrow Sub- basin, in the Carnarvon Basin of Western Australia. It was discovered by West Australian Petroleum Pty Ltd ( WAPET) in 1973 on a southwesterly extension of the Rankin Platform where, farther north, a number of major gas/condensate discoveries have been made by Burmah Oil Company of Australia Ltd (BOCAL) since 1971.The productive structure at West Tryal Rocks lies at a depth of 3200 m in about 150 m of water. It consists of an elongate north - trending uplifted block of Triassic and possibly Lower Jurassic reservoir rocks called the Mungaroo Beds.The block is unconformably capped by the Lower Cretaceous Muderong Shale which also provides the lateral seal across the bounding faults. The reservoir section dips to the north at a greater rate than does the sealing unconformity so that progressively younger pre-Cretaceous sediments subcrop the unconformity in that direction.Shales of Middle to Late Jurassic age in the Barrow Sub-basin to the east are believed to be the primary source of hydrocarbons, although the overlying Muderong Shale cannot be ruled out.Three main gas-bearing sands have been encountered by the two wells drilled to date. The second well was drilled up-structure from the first and penetrated 144 m of net gas pay of which 133 m is contained in the three sands. The sands are mainly medium to very coarse grained and possess good porosity and permeability.Preliminary reoervvs estimates indicate that the field contains in excess of 28 x 109 m3 (1 x 1012 ft3) of gas (Playford, 1975).The West Tryal Rocks gas field is unique compared to the other Northwest Shelf fields, in that it is slightly overpressured and contains up to 28% of non-combustible gases - predominantly carbon dioxide and nitrogen. Additionally, the field possesses relatively fresh underlying formation waters with high concentrations of bicarbonate ions.