ABSTRACT: Evidence for a Permian petroleum system in the Timor Sea region, northwestern Australia

Kennard, John, M.¹, Dianne, S. Edwards¹, Tim, E. Ruble², Chris J. Boreham³, Roger E. Summons¹, Gareth, T. Cooper⁴, Malcolm R. King⁵, and Mark Lisk⁶
(1) Australian Geological Survey Organisation, Canberra, ACT, Australia 
(2) CSIRO Division of Petroleum Resources, Sydney, NSW, Australia 
(3) Australian geological Survey Organisation, Canberra, ACT, Australia 
(4) Santos Ltd, Adelaide, SA, Australia 
(5) Shell/Woodside Energy Ltd, Perth, WA, Australia
(6) CSIRO Division of Petroleum, Perth

Petroleum exploration in the Timor Sea region, Bonaparte Basin, has focussed on plays that are charged and sealed by Mesozoic sediments. However, there is increasing evidence that Palaeozoic oil and gas have contributed to the hydrocarbons found in this area. Geochemical appraisal of the palaeo-oil column at Torrens-1 on the Londonderry High, the gas/condensate reserves in the Petrel and Tern fields in the Petrel Sub-basin, and the gas accumulation at Kelp Deep-1 on the Sahul Platform in the Zone of Co-operation, have revealed that these hydrocarbons are most likely generated from organic-rich Permian sediments.

Oil stains and fluid inclusion oil from the Permian Highland Bay and Fossil Head formations at Torrens-1 have similar biomarker compositions indicating a terrestrial source affinity. Carbon isotope values from bulk and compound-specific analyses show that the Fossil Head-reservoired oil is enriched in ¹³C (d¹³C_sat=-26.1 o/oo). Despite having lost its lower molecular weight hydrocarbons, the n-alkane isotope profile exhibits a strong negative slope, and is similar to the Petrel and Tern condensates reservoired in the Hyland Bay Formation. Onshore in the Petrel Sub-basin, Weaber-4 oil has an isotopic signature enriched in ¹³C, as observed at Petrel and Tern. These data are distinct from those of the Carboniferous-sourced Barnett/Turtle oils (offshore Petrel Sub-basin) which have d¹³C_sat values in the order of -28 o/oo.

Dry gas from Kelp Deep-1 in the Hyland Bay Formation has an isotopic composition in keeping with generation from land-plant-rich source rocks, and its stratigraphic and structural position is suggestive of a Permian origin.

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