GORGON 1 - SOUTHERNMOST RANKIN PLATFORM GAS DISCOVERY

Abstract

The Gorgon 1 gas discovery is located on the western margin of the Barrow Sub-basin in the Carnarvon Basin of Western Australia. The well was drilled by West Australian Petroleum Pty Limited (WAPET) in late 1980 on the southern extremity of the Rankin Platform. The location is 72 km west-northwest of Barrow Island and the water depth is 259 m. The first discovery on the Rankin Platform was made in 1971 by the Burmah Oil Company of Australia (BOCAL), now Woodside Offshore Petroleum Pty Ltd. Since 1971, both WAPET and Woodside have made several gas/condensate discoveries to the northeast of Gorgon.

The Gorgon structure consists of an elongate, uplifted, fault-bounded block of Triassic sediments about 5 km wide and 35 km long and oriented approximately north-south. The southern limit of the block is defined by the convergence of the bounding faults on the east and west sides of the structure.

The reservoir section, as in West Tryal Rocks and other Rankin Platform accumulations, consists of Late Triassic fluviatile and deltaic sands in a sequence known as the Mungaroo Formation. Secondary and shallower objectives in Gorgon 1 were reservoirs in the deltaic Flacourt and Malouet Formations of the Barrow Group. These were mapped in a dip-closed, faulted anticline at the Neocomian unconformity which is overlain by the widespread Muderong Shale.

The primary source of the hydrocarbons is believed to be the thick sequence of Late Jurassic shales (Dingo Claystone) in the Barrow Sub-basin to the east of the Gorgon Structure.

Gorgon 1 was drilled to a depth of 4401 m and encountered massive, fluvio-deltaic sandstone bodies within a gas column of 409 m in the Late Triassic Mungaroo Formation. Six drill stem tests were conducted over selected intervals within the Mungaroo Formation. These produced flow rates of up to 770 500 m3 of gas per day and 2.5 kl of condensate per day at a surface flowing pressure of 16 300 kPa on 2 X 13 mm surface chokes. A seventh drill stem test conducted in the Malouet Formation produced a flow rate of 267 700 m3 of gas per day and 23.3 kl of condensate per day at a surface flowing pressure of 10 500 kPa on a 1 X 13 mm surface choke.

The reservoir sands in the Mungaroo Formation are medium to coarse grained and have an average porosity of 12 per cent and a permeability range of 0.1 to 527 millidarcies.