PETROLEUM PROSPECTS, BLIGH WATER, FIJI

Abstract

The Fiji Archipelago constitutes a segment of the island arc system of the southwest Pacific. The islands in the group comprise a mixture of igneous and sedimentary rocks including carbonates. The hydrocarbon potential of the region has been emphasised since the Tonga oil seepage drew the attention of the oil industry to this part of the globe in 1968.

Southern Pacific Petroleum (Fiji) Limited was granted the first offshore petroleum concession in Fiji by the Government in 1969 and the negotiation for the concession resulted in the formulation of Fijian offshore petroleum exploration legislation. Reconnaissance sparker seismic shooting indicated the existence of stratified rocks beneath Bligh Water and subsequent aeromagnetics and multiple coverage seismic has confirmed the presence of a sedimentary basin.

The oldest dated sedimentary rocks in Fiji are Eocene. They form part of a very thick sequence, including volcanics, ranging into Lower Miocene which regionally has been mildly metamorphosed. Locally this sequence has been intruded by intermediate to basic plutonic stocks which subsequently have been unroofed by widespread intra-Miocene erosion. This sequence constitutes the economic basement in the region. Middle to Upper Miocene and Pliocene clastic, marine, fossiliferous sediments and shallow water carbonates on the two main islands of Fiji were deposited in localised basins, generally in complex association with volcanic rocks.

The sedimentary column contains considerable fine grained, dark coloured, marine elastics. Bioclastic limestone, also present in the sequence, has excellent porosity and permeability. Good reservoir character is similarly indicated for some of the coarser elastics, despite the lack of quartz.

The seismic data, when integrated with the geology of the islands, infer that the deepest continuous reflection horizon represents the intra-Miocene unconformity; additionally, the data show that the Bligh Water Basin is divided into northern and southern components, divided by a basement high. The southern region appears more attractive for exploration as 7000 ft. (2100 m.) of section is indicated and the area is covered by shallow water. Faulting and drape features are the obvious structural forms present. Several discontinuous reflection events, commonly occurring at equivalent positions on different seismic profiles, may indicate presence of carbonate banks or cays similar to the type of carbonate deposits exposed in the succession onshore.