Petroleum Geology of the Timor Gap Zone of Co-Operation (Area A)

David B. Whittam

The discovery of the Elang/Kakatua oilfield and Undan/Bayu gas-condensate accumulation has demonstrated the presence of a significant new hydrocarbon province in Area A of the Zone of Co-Operation between Australia and Indonesia (ZOCA). The western part of ZOCA is underlain by a north-west trending Triassic to mid-Cretaceous sag basin (the Northern Bonaparte Basin) which is overprinted by a series of east-west faults. The basin is related to Jurassic rifting and continental separation and is overlain by a seaward thickening, passive-margin wedge of Upper Cretaceous marls and Tertiary carbonates. The east-west faults form horst blocks which are the principal structural trapping style.

Hydrocarbons have been encountered in fluvial and shallow marine sandstone of mid-Jurassic age (Bathonian to Callovian). These reservoirs are sealed by marine claystones and shales of Oxfordian to Berriasian age which are also potential petroleum source rocks containing a mixture of Type II and III organic matter. The discovered oils are very light (57° API), mature crudes with low producing gas/oil ratios.

The understanding and prediction of the controls on reservoir quality and access to mature source rocks are key factors in the successful exploration of the area. Poor seismic data quality is a major obstacle to the accurate mapping of structure at the level of top reservoir. Several traps are interpreted to have failed due to breaching and remigration of hydrocarbons caused by Miocene/Pliocene faulting related to the collision between the Australasian and Eurasian plates.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California