CHEMOSTRATIGRAPHY AND THE COMPOSITION OF OILS IN THE PERTH BASIN, WESTERN AUSTRALIA

Abstract

The known global secular change in the distribu­tion of carbon isotopes between the Palaeozoic and Mesozoic is the basis of a new oil-source correlation tool. The carbon isotopic signatures of n-alkanes, in combination with information about the distribu­tions of diagnostic biomarkers have been used to classify Perth Basin oils according to the age of the source. These data confirm that most of the oil in the northern Perth Basin originates from the basal section of the Kockatea Shale.

Oils exclusively from Triassic sources are isotopically light with n-alkane 813C values near −34%o PDB. Jurassic oil from Gage Roads-1 is isotopically heavy (~24%o) and is also distinctive in its relatively high content of conifer-derived aromatic hydrocar­bons. Condensates from Jurassic source intervals in the Dandaragan Trough are isotopically heteroge­neous with n-alkane 813C values between −25%o and −29%o. The Whicher Range-1 condensate, of appar­ent Permian origin, is isotopically heavy with n-alkane 5l3C values near −25%o. The isotopic data provide information about variation in sedimen­tary facies and possible multiple sources that is not evident from the biomarker signatures. All the Jurassic oils have significant amounts of bicadinanes, resin-derived biomarkers until recently attributed exclusively to tropical angiosperms.

A strong excursion in the isotopic signature of organic carbon is present in core at 2,293 m in the Woodada-2 borehole and occurs with no obvious lithological change. Similar isotope shifts are known to mark the Permian-Triassic boundary globally and have been previously recognised in the Bonaparte, Bowen, Canning, Carnarvon and tenta­tively in the southern Perth basins. The excursion in Woodada-2 is abrupt and suggests a significant time break in sedimentation. However, diagnostic Permian palynoflora or fauna have not been de­tected below 2,293 m in the Woodada-2 core, and hence, the assignment of a Permian-Triassic con­tact cannot be made unequivocally with the exist­ing data.