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ABSTRACT: The origin and alteration of hydrocarbons in the Northern Bonaparte Basin, offshore Northern Australia

Preston, James C.1, Noel A. Newell2, and Dianne Edwards3
(1) BHP Petroleum Pty Ltd, Melbourne, Australia 
(2) Consultant to BHP Petroleum, Melbourne, Australia 
(3) Australian Geological Survey Organisation (AGSO), Canberra, Australia

To better understand the petroleum systems active in the Northern Bonaparte Basin, geochemical data from source-rocks, oils and sediment-extracts were collated and interpreted.

Source-rock data from Mid-Jurassic to Early Cretaceous sediments indicate the presence of moderately organic-rich source-rocks containing mixed organic matter, with an upward decrease in the proportion of land-plant material.

The oils were divided into two families, those from Early Cretaceous fracture systems, and those from Oxfordian-Callovian reservoirs. The latter, including all the commercial accumulations, were divided into two sub-families, the more land-plant-influenced oils from the north, and the relatively marine-influenced oils from the south. The Early Cretaceous oils correlated strongly with source-extracts from adjacent claystones. The Oxfordian-Callovian reservoirs were self-sourced from claystone interbeds. Contributions from source-seals, land-plant-rich in the north, and marine-influenced in the south, may explain the geochemical distinction between the northern and southern sub-families.

Under-saturation of the oils, and the residual saturations observed in most structures, resulted from the selective removal of large amounts of low-molecular-weight compounds. This is better explained by high-temperature water-washing than by fault-leakage. The degree of water-washing increases to the northwest, the Pliocene obduction of Timor onto the Australian continental margin having provided the water-drive mechanism. It is estimated that hydrocarbon volume losses were about 70-90%, consistent with many of the observed residual columns. Such observations have greatly influenced estimates of exploration risk in the Northern Bonaparte Basin.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia