New reef targets for oil and gas exploration in Fiji, Southwest Pacific

Abstract

Fiji forms part of the Southwest Pacific island arc system which marks the boundary between the Indo-Australia and Pacific plates_ The territorial waters cover almost 1.3 million km2 and contain two shallow water Tertiary sedimentary basins_ Bligh Water Basin, covering some 9500 km2, has sediment thicknesses in excess of 5 km and has excellent potential for hydrocarbons_ Bau Waters Basin is also prospective, having a shallow water area of about 1600 km2, with sediment thicknesses up to 4 km_ 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 2.6 km below sea floor. Miocene and Pliocene reefal limestones form 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 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 million barrels of oil 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 the SOPAC Petroleum Data Bank, Canberra.