The Gorgon Gas Field
The Gorgon gas field is situated 130 km offshore on the Northwest Shelf of Western Australia. The field was discovered by West Australian Petroleum Pty. Limited (WAPET) in 1980, and two successful appraisal wells were drilled in 1982 and 1983. The Gorgon structure is an elongate horst block trending north-northeast at the southwestern end of the Rankin Trend, which also contains Woodside Offshore Petroleum's Goodwyn and North Rankin gas fields.
The Rankin Trend forms the western margin of the Barrow and Dampier subbasins. It was formed in the Early Jurassic during active rifting, in response to downfaulting along the eastern margins of these half-grabens. The extensional faulting of the Rankin Trend follows two dominant trends. Northeasterly faults parallel the rift axis, and north-northeasterly faults follow a reactivated Paleozoic/Precambrian trend. At the close of the Jurassic, uplifting took place to the south of the Barrow Subbasin in association with the breakup of Australia and India. This provided large volumes of sediment, which moved north and spread out across the Exmouth Plateau and the Barrow Subbasin, forming the deltaic Barrow Group. In the Early Cretaceous, continental breakup took place as India separated from Australia. This cut off the sediment supply to the Barrow Group. The marine Muderong shale was then deposited as a regional seal across the Rankin Trend and the Barrow and Dampier subbasins and adjoining shelves. Cretaceous and Tertiary sediments overlying the Muderong Shale were deposited as prograding slope wedges on the northwest Australian continental margin.
The Triassic Mungaroo Formation, which hosts the gas at Gorgon, is a prerift sequence of meander belt sandstones and overbank siltstones and shales.
The Triassic sandstones dip north within the Gorgon horst, which is unconformably overlain and sealed by Cretaceous shale of the Barrow Group. The discovery well, Gorgon 1, encountered 98 m of net gas pay in a 489-m gross Triassic interval above a gas-water contact at 4111 m subsea. North Gorgon, 23 km north of Gorgon 1, encountered 127 m of net gas pay in an 884-m gross interval above a lowest known gas at 4135 m subsea. Central Gorgon 1, situated between the previous two wells, encountered 35 m of net gas pay in a 443-m gross interval above a gas-water contact at 4049 m subsea.
Most sands in the Gorgon field have tested at over 850,000 m3 (30 million ft3) of gas per day. The sandstones consist mainly of quartz, with kaolinite occurring as diagenetic replacements of feldspar grains. Porosity of the reservoir sandstones decreases with depth from 20 to 12% owing to the growth of authigenic quartz cement.
The Gorgon gas field is at least 45 km long by 5 km wide. It is estimated to contain 312 billion m3 (11 tcf) of gas in place.