Petroleum Potential of Basins Flanking Viti Levu, Fiji

Beggs, J. M.¹, V. M. Stagpoole², W. A. Brook³ (1) GeoSphere Ltd, Lower Hutt, New Zealand (2) GNS Science, Lower Hutt, New Zealand (3) Akura Ltd, Nadi, Fiji

The Fiji Platform includes a system of sedimentary basins that were first recognized as petroleum-prospective in the 1970's. The initial stages of basin development occurred within an island arc setting in association with subduction along the Pacific plate margin from Eocene to mid Miocene. In the late Miocene and Pliocene, the Fiji segment of the arc system was deformed by clockwise rotation, resulting in up-warping of the large (10,000km3) mountainous island of Viti Levu. Erosion has cut into intermediate plutonic bodies and substantial river systems contribute to mixed siliciclastic and carbonate sedimentation in down-warped basinal areas overlying former intra-arc depocentres to the east (Rewa-Bau Waters Basin) and west (Nadi Bay Basin), and the shortened former fore-arc depocentre to the north (Bligh Water Basin).

Five exploration wells and two stratigraphic wells drilled in these basins in the early 1980's failed to encounter viable reservoirs, but the occurrence of oil and gas shows at shallow levels in several wells attests to the existence and maturation of petroleum source rocks. Seismic data from previous exploration give some indications of possible traps, including inferred drowned reef features.

One-dimensional thermal modeling calibrated by thermal gradients observed in the exploration wells indicates the viability of oil and gas charge from inferred source rocks either within the mid-Cenozoic intra-arc basin sequence, or possibly from Paleocene marine source rocks within pre-arc section rifted from the northern New Zealand margin.