REGIONAL GEOLOGY AND HYDROCARBON POTENTIAL OF THE MESOZOIC OF THE WESTERN PAPUAN BASIN, PAPUA NEW GUINEA

Abstract

Jurassic and Cretaceous clastics of the Western Papuan Basin provide reservoir and source potential for hydrocarbon accumulation. Early Jurassic coarse clastics from sources in the south and west cover a wide stable platform area, while finer grained equivalents were deposited in a deepening trough along the east and northeast margins. A Middle Jurassic transgression deposited a thick shale unit over most of the basin, followed by a return to sand and silt deposition in the Late Jurassic-Early Cretaceous. Eocene/Miocene structuring in the north formed the Omati Trough which filled with deep water Miocene carbonates. This was followed by shallow water carbonates which cover the Western Papuan Basin. Mountain ranges to the north and northeast were the source for thousands of feet of poorly sorted Plio- Pleistocene sediments which were dumped in the rapidly subsiding Aure Trough and spread as a veneer over the remainder of the Papuan Basin.

Geochemical studies indicate that adequate source rocks exist within the Mesozoic shales and these reach maturity at a depth of about 2700m. The Mesozoic shales correlate more closely with the various oil and condensate seeps than do the Miocene carbonates which are generally too immature over much of the basin to be considered as a significant oil source.

The stable platform, except for the Omati Trough, has undergone very little tectonic movement since the Triassic and this has severely restricted the formation of structural traps suitable for hydrocarbon entrapment. Better structuring exists to the east of a Mesozoic hinge-line running approximately north- south just offshore from the present-day coastline, but lack of good Mesozoic reservoir sands in this area limits the hydrocarbon prospects. The most prospective area is the structured margin of the Omati Trough, where tilted fault blocks provide traps for hydrocarbons generated from the underlying shales of the Cretaceous and Jurassic, but difficult terrain and high exploration costs make for high risk exploration.