Fiji forms part of the South-West Pacific island arc system which marks the boundary between the Indo-Australia and Pacific plates. The territorial waters cover almost 1.3 million kmē and contain two shallow-water Tertiary sedimentary basins. Bligh Water Basin, covering some 9500 kmē, has sediment thicknesses in excess of 5km and has excellent potential for hydrocarbons. Bau Waters Basin is also prospective, having a shallow-water area of about 1600kmē, with sediment thicknesses up to 4km.
Fiji lies on the same regional play trend of Miocene reefs which produce oil in Irian Jaya, Indonesia and gas/condensate in offshore Papua New Guinea. Indeed Fiji's basins have many similarities with the oil and gas producing, arc-related basins of Southeast Asia.
Source rocks of Oligocene, Miocene and Pliocene age are exposed onshore in Fiji and have been encountered by drilling in the offshore basins. An oil seep in Bligh Water Basin and oil and gas shows in wells provide evidence that hydrocarbons have been generated in the basins. Modelling studies indicate peak oil generation to be at about 2.6km below sea floor.
Miocene and Pliocene reefal limestones from spectacular outcrops in Fiji and represent the best potential reservoirs. Reefs of the same age have been identified on seismic data from the offshore basins and represent attractive targets for exploration. Common forms are reefal mounds and prograded platforms.
Over twenty structural reefal traps have been identified on seismic lines in the Late Miocene and Pliocene sequences, mostly in Bligh Water Basin. Estimates of potential unrisked recoverable reserves are 270 million barrels of oil (mmbo) per structure. If structural-stratigraphic trapping occurs, recoverable reserves could increase to over 1 billion barrels of oil per structure. There is considerable scope for more reefal structures in the deeper Oligocene-Middle Miocene interval which cannot be resolved on the existing seismic data, and in areas where seismic coverage is sparse.
Limestone turbidite lobes have also been identified on seismic data. These constitute a secondary play and may contain estimated recoverable reserves of 100-200 mmbo per structure.
There are no exploration or production licences at present. Fiji has comprehensive petroleum legislation and the Government seeks to encourage exploration investment by oil companies. All reports and data are available in the Fiji Petroleum Data Package which may be ordered from SOPAC Petroleum Data Bank, Canberra.
Fiji first gained attention as an area of petroleum potential in 1968 following the discovery of oil seeps in neighbouring Tonga. Subsequently, there have been two stages of exploration. The first period from 1969 to 1977 commenced with reconnaissance mapping by Shell Internationale and Magellan Petroleum in 1969 and 1970, which provided the first assessments of source rocks and reservoirs. Following this, the first exploration licence, covering Bligh Water Basin (Figure 1), was awarded to a partnership with Southern Pacific Petroleum as operator in 1969. A total of 1590km of regional deismic data was acquired from which sediment thicknesses and general basin depocentres were established.
In 1971 three licences were awarded to Offshore Oil Exploration, Atlantic and Oceanic Resources, and Investment Corporation of Fiji; and a fourth to International Petroleum in 1972. These licences covered the western Yasawa Platform, central Lau Ridge, Bau Waters Basin and Baravi Basin respectively (Figure 1). A total of 1585km of seismic data was acquired in these concessions from 1971 to 1975. A further 4433km of regional speculative seismic data were acquired by Amoco and Western Geophysical in 1972 and 1973.
From these surveys the Bau Waters Basin and western Bligh Water Basin were outlined and general sediment thicknesses determined on the Lau Ridge. By 1977 all licences granted during the first period of exploration had expired.
Encouraged by high world oil prices, the second period of exploration took place from 1977 to 1987. In 1977, Dakota Exploration was awarded a concession in the Bau Waters Basin and western Koro Sea. Over 1400km of seismic data were acquired offshore which permitted more detailed structural interpretation.
In 1978 three exploration licences were awarded to Pacific Energy and Minerals covering Bligh Water Basin, the Yasawa Platform and Great Sea Reefs Platform. Following farm-out agreements with, or options taken by, Bennet Petroleum, Chevron and Mapco, 6050km of seismic data were acquired which formed the basis of more detailed structural interpretations. Geochemical investigations of sea-bottom sediments in Bligh Water showed one pentane anomaly indicating thermogenic, migrated hydrocarbons.
Exploration drilling followed with Chevron's wells Bligh Water- 1 and Great Sea Reefs-1 in 1980 (Figure 1). During 1981 and 1982 Bennet Petroleum drilled four wells: Buabua-1 and Buabua-2 located on an island in western Bligh Water Basin, followed by Maumi-1 and Cakau Saqata-1 in Bau Waters Basin. Finally, Worldwide Energy as operator of the Pacific Energy and Minerals group drilled well Yakuilau Island-1 in 1982 in western Bligh Water Basin.
All seven wells were drilled to test Tertiary reefal limestone objectives but, none reached its target and a re-evaluation of seismic data shows that six of the wells did not drill valid structural traps. Consequently, Tertiary reefal limestones are still an untested play with considerable potential. On a more positive note, drilling has shown the presence of source rocks and shows of oil and gas in the offshore basins of Fiji (Figure 2; see Petroleum Geology).
By 1987 the second group of licences had expired. In the same year the Fiji Government's Mineral Resources Department completed a major source rock sampling programme. Two earlier evaluations by SOPAC (refs 1 & 2) have been superseded by a major new evaluation (Ref. 3), the results of which are summarised here. Further details of Fiji's exploration history are given in an earlier publication (Ref. 4).
At present (April 1993) there are no exploration concessions held in Fiji.
Fiji is situated at the boundary of the Indo-Australian Plate and the Pacific Plate. It is the complex interaction of these plates that provides the framework for understanding the geology and petroleum potential of Fiji.
The most recent plate tectonic reconstructions (Refs 5 and 6) show that from the early Eocene to Late Miocene, Fiji formed part of the continuous Outer Melanesian Arc which extended from Papua New Guinea through the Solomon Islands, Vanuatu, Fiji and Tonga/Lau, to New Zealand (Figure 3). This was a migratory arc system (Ref. 7) that 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, the oceanic Ontong-Java Plateau collided with the Solomon Islands section of the Outer Melanesian Arc (Figure 3). Several major events followed that resulted in the break-up of this arc. Firstly, the direction of subduction beneath the Solomon Islands and Vanuatu arcs was reversed. Subduction of the Pacific Plate ceased, to be replaced by eastward subduction of the back-arc basins beneath the Solomons and Vanuatu Arcs. This was followed by rapid opening of the North Fiji Basin. The Hunter Fracture Zone acted both as a transform and as a oblique subduction zone to accommodate the break-up of the Outer Melanesian Arc.
Finally, subduction of the Pacific Plate beneath the Tonga Ridge from the Pliocene onwards has resulted in the opening of the Lau Basin which now separates the Lau and Tonga Ridges.
Fiji's geological history can be divided into three distinct periods of island arc development: Eocene, Late Oligocene-Early Miocene and Late Miocene-present day (Figure 4).
In the Early Eocene Fiji probably originated as part of the Outer Melanesian Arc (Figure 3). However, the oldest rocks actually exposed on Fiji are of Late Eocene age (Figure 4). These form the Yavuna Group consisting of island-arc volcanics which were uplifted to permit the deposition of shallow-water, platform limestones. The subsequent initiation of the South Fiji Basin in the Early Oligocene coincides with a stratigraphic break.
This hiatus was followed by the second phase of arc development represented by the Late Oligocene to Middle Miocene Wainimala Group (Figure 4). This forms the southern half of Viti Levu and is exposed in islands of the Yasawa Platform. Age equivalents of the Wainimala Group have also been encountered in the offshore basins by wells Bligh Water-1, Great Sea Reefs-1, Maumi-1 and Cakau Saqata-1 (Figure 2).
During this period the axis of the Outer Melanesian Arc passed through southern Viti Levu. A fore-arc basin developed to the north of the volcanic arc axis. Coral-algal reefs formed on the edge of a shallow-water platform north of the arc axis, across the width of Viti Levu (Figure 5). The best exposed example is the Qalimare Limestone which comprises mounds at least 300m thick (Figure 6). The occurrence of massive fore-reef limestones on the northern Yasawa Platform at Sawa-i-Lau suggests that reef development associated with structural highs may have been widespread in the offshore fore-arc basin. A deep-water basin developed to the north of the platform. Low-grade regional metamorphism is restricted to areas adjacent to the volcanic axis in the south. The present day shallow-water offshore basins, Bligh Water Basin and Bau Waters Basin, are superimposed on the larger Late Oligocene to Middle Miocene fore-arc basin. Seismic correlation across Bligh Water Basin suggests that it contains over 1300m of Late Oligocene to Early Miocene strata (Figure 5)m, whilst Bau Waters Basin contains at least 750m. Both basins contain several structural highs which were extant in the Late Oligocene-Middle Miocene fore-arc basin and may thus have provided centres for reef growth.
Seismic data suggest that the deep-water Suva Basin (Figure 7) probably contains up to 1500m of the Late Oligocene-Middle Miocene volcaniclastics. Situated to the south of the arc axis, this is a back-arc basin.
During the Middle to Late Miocene, a major hiatus in Fiji coincides with the intrusion of the basic to intermediate Colo Plutonic Suite and uplift of the arc axis (Figure 4). Ensuing erosion of the arc produced breccias and conglomerates (Tuva Group) which were dumped in the fore-arc basin to the north to form submarine fans. Other large structural highs were active in the Bligh Water and Bau Waters Basins resulting in deposition of up to 1500m of Late Miocene sediment in restricted half-grabens. The large, densely vegetated land areas would have provided an abundant source of plant material.
The third period 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 this time Fiji remained in a back-arc setting. Initial rifting of the arc was accompanied by folding and faulting, often induced by wrench tectonics: the Colo Orogeny.
This deformed the older Wainimala and Tuva Groups and Colo plutonics. Calc-alkaline volcanic activity associated with break-up of the arc occurred in Vanua Levu and northern Viti Levu.
Subsequently, deposition occurred in several basins situated to the north and south of the uplifted Late Oligocene-Early Miocene arc (the Medrausucu, Nadi and Rewa Basins); and between the new volcanic centres (e.g. on Vanua Levu). Coral-algal reefs developed during the Late Miocene and Early Pliocene around the margins of, and on structural highs within, the larger basins, e.g. Lami and Tuvu reefal limestones (Figure 4). Volcaniclastics were deposited in the basin centres.
From the Late Miocene onwards, Bligh Water Basin evolved, bounded to the north and west by the deformed structural highs of the Great Sea Reef Platform and the Yasawa Platform respectively, and to the south by the new Pliocene volcanic centres (Figure 1). The basin probably contains a maximum thickness of 2800m of Late Miocene to Recent sediment which overlies older, Late Oligocene to Middle Miocene strata (Figure 7). Numerous structural highs have been activated during and since the Colo Orogeny. Seismic data indicate that these highs, together with other volcanic highs, favored reef growth in both the Late Miocene and the Early Pliocene.
Bad Waters Basin represents the offshore extension of the Rewa Basin (Figure 1). It contains up to 2800m of Late Miocene to Recent sediment which overlies deformed Late Oligocene to Early Miocene strata. Several fault-bounded structural highs have also given rise to reef growth.
The deep-water offshore Baravi and Suva Basins contain maximum sediment thicknesses of 2700 and 1500m respectively. The sequence is probably of Late Pliocene to Pleistocene age (Figure 7; Ref. 8). These basins developed in a back-arc setting in response to subduction of the South Fiji Basin along the Hunter Fracture Zone during the Pliocene to Late Pleistocene (Ref. 9).
The shallow-water Lau Ridge situated to the east of Viti Levu originated as part of the Outer Melanesian Arc, probably in the Eocene (Figure 3; Ref. 10), and exhibits a Miocene to Early Pliocene geology that is similar to Viti Levu. However, major uplift associated with opening of the Lau Basin in the Late Pliocene has resulted in exposure of the Late Miocene to Early Pliocene reef limestones on the Lau islands. Although several narrow grabens have survived uplift, the total sediment thickness is typically less than 500m.