Petroleum geology of Fiji

Abstract

Most petroleum exploration in Fiji has been directed towards the shallow-water sedimentary basins around and extending onto Viti Levu, the Bligh Water and Bau Waters Basins. A large amount of commercial multi-channel seismic-reflection data has been acquired across these basins since 1971 and five deep wells were drilled offshore and on Viti Levu from 1980 to 1982. All the wells were dry, although some had minor shows of gas and oil fluorescence. The wells penetrated over 2500 m of sedimentary rocks largely of Miocene and younger age, but some Oligocene or older volcanogenic rocks were also intersected. The well sections comprise mainly volcaniclastics, and Lower to Middle Miocene shallow-water limestones, which were the primary targets, were not encountered. A large amount of recently released commercial seismic data from around Viti Levu is interpreted here to enable an assessment of petroleum potential to be made. Up to about 4 km and 2 km of Late Miocene and younger strata are interpreted in the Bligh Water and Bau Waters Basins, respectively, but strata underlying an angular unconformity, believed to be of Late Miocene age, are not seismically resolved and thus the total sedimentary thickness is unknown. The sedimentary sequences off Viti Levu are cut by large faults, and pre-Pliocene rocks are affected by folding which is locally quite intense. No source rocks have been identified in the deep wells nor in tests of outcrop and stratigraphic borehole material from Viti Levu and vanua Levu, but anomalous amounts of pentane in seabed sediments off northern Viti Levu suggest that some thermogenic hydrocarbons have been generated. Information on subsurface reservoirs is sparse, but limestones are generally considered to have better reservoir potential than coarse volcaniclastics and are the more desirable targets. Because of the limited seismic resolution, no traps with Lower to Middle Miocene shallow-water limestone facies can presently be identified in the basins offshore Viti Levu. Late Miocene and younger sequences contain numerous structurally formed potential traps, and several Pliocene reef-like seismic mounds also occur in the western Bligh Water Basin and form potential traps. Mudstones are widespread in the offshore basins and seals are likely to be good, although some relatively recent faulting may have breached some potential traps. Commercial and non-commercial reconnaissance geophysical surveys have also been conducted across other sedimentary basins in Fiji, including the deep-water Suva and Baravi Basins and basins around vanua Levu and on the Lau Ridge.