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Fiji lies at the boundary of the Indo-Australian plate and the Pacific plate and represents the remains of the Outer Melanesian island arc.
From the Early Eocene to Late Miocene, Fiji formed part of this arc which extended from Papua New Guinea through the Solomon Islands, Vanuatu, Fiji and Tonga to New Zealand. It moved eastwards as the Pacific Plate was subducted beneath it.
Several back-arc basins developed, including the South Fiji Basin which separated the Outer Melanesian arc from the rifted continental block of the Norfolk Ridge.
In the Late Miocene subduction of the Pacific Plate ceased, replaced by eastward subduction of back-arc basins beneath the Solomons and Vanuatu island arcs and rapid opening of the North Fiji Basin.
The Hunter Fracture Zone was a transform fault and oblique subduction zone that accommodated the break-up of the Outer Melanesian arc. Finally, subduction of the Pacific Plate beneath the Tonga Ridge from the Pliocene onwards resulted in the opening of the Lau Basin southeast of Fiji which now separates the Lau and Tonga Ridges.
The oldest rocks exposed on Fiji are Late Eocene island-arc volcanics uplifted to permit the deposition of shallow-water, platform limestones deposited during the first phase of arc development.
The second phase of arc development is represented by Late Oligocene to Middle Miocene sediments deposited in a fore-arc basin developed to the north of the volcanic arc axis in which reefs formed on the edge of a shallow-water platform.
A deep-water basin developed to the north of this platform. During the Middle to Late Miocene volcanics were extruded which were eroded to form submarine fans in the deeper waters.
The third phase of arc development, from the Late Miocene to the present day, coincides with the break-up of the Outer Melanesian Arc and the opening of the North Fiji Basin during which time Fiji remained in a back-arc setting. This was accompanied by folding, wrenching and volcanic activity.
Present day shallow-water offshore basins (the Bligh Water Basin and Bau Waters Basin north and east of Vitu Levu respectively) are superimposed on the fore-arc with structural highs providing centres for reef growth. Reefs developed around the margins of structural highs and volcaniclastics were deposited in the basin centres.
The Bligh Water Basin and the Bau Waters Basin, where water depths can be less than 500m, were unsuccessfully explored in the early 1980s. Other basin fragments lie in water depths above 2000m.
Although source rocks have been identified in Fiji the remoteness, depth of water and lack of obvious closed trapping opportunities suggests to Globalshift that Fiji does not contain potentially commercial hydrocarbon accumulations.
Fiji has no history of oil or gas production. However, some exploration activity began in 1968 when oil seeps were recognised in neighbouring Tonga.
From 1969 to 1970 reconnaissance mapping identified a number of offshore basins and the first exploration licence, covering the Bligh Water Basin, was awarded with 1,590 kms of seismic data acquired. In 1971 and 1972 four further licences were awarded covering the western Yasawa Platform, the central Lau Ridge, the Bau Waters Basin and the Baravi Basin.
A total of 1,585 kms of seismic data were acquired in these concessions from 1971 to 1975 followed by 4,433 kms of regional speculative seismic data. All licences expired in 1977.
In 1977 a concession was awarded in the Bau Waters Basin and western Koro Sea and over 1,400 kms of seismic data were acquired here. In 1978 three exploration licences were then awarded over the Bligh Water Basin, the Yasawa Platform and the Great Sea Reefs Platform with 6,050 km of seismic data acquired.
Drilling followed with 2 offshore wells in 1980 and 4 wells in 1981 and 1982, two of which were drilled on an island. All 7 wells were drilled to test Tertiary reef limestones but none were successful and by 1987 all the licences had expired.
The wells did identify source rocks and encountered minor oil and gas shows and in 1987 small quantities of oil were reported in a number of shallow sediment cores taken from the sea bed in the Bligh Water basin off Tavua. The oil is believed to have originated from Tertiary sediments.
However, no drilling has been carried out since 1982 and Globalshift forecasts no oil or gas production is likely in Fiji over the short and medium term.
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