Vagrant Oils: Geochemical Signposts to Unrecognised Petroleum Systems

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Vagrant Oils: Geochemical Signposts to Unrecognised Petroleum Systems

 

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Publication Name: The Sedimentary Basins of WA

Authors: Roger E. Summons, Marita Bradshaw, James Crowley, Dianne S. Edwards, Simon C. George and John E. Zumberge

Publication Volume: 2

Date Published: December 1998

Number of Pages: 27

Reference Type: Book Section

Abstract:

Biodegraded oils and residual oil stains from wells in the Arafura, Bonaparte and Carnarvon basins have been chemically analysed and the data compared to a reference set for known petroleum systems of the western Australian margin. The biodegraded Wandoo and Stag oils are shown to be chemically related to the Barrow, Harriet, Bambra and Campbell accumulations and consequently part of the prolific Late Jurassic Westralian 2 Petroleum System. The biodegraded Arafura 1 oil shares many characteristics with early Palaeozoic oils of the Canning Basin and is logically classified within the Larapintine 1 Petroleum System. Other samples, termed 'vagrants', evade classification due to geochemical features which distinguish them from previously defined oil families. Residual oil in Dill 1 is chemically similar to the Leatherback 1 oil and is derived from a clastic marine
source rock with a predominant component of terrestrial organic matter. These oils are sufficiently distinct from the bulk of Westralian 2 oils to indicate a different source rock, although it is possibly a basin margin facies of the Upper Jurassic Dingo Claystone. Residual oil in Torrens 1 on the Londonderry High shares geochemical imilarities with oil from Dill and Leatherback suggesting that they were sourced from predominantly terrestrial organic matter. However, local geology suggests a probable Palaeozoic origin for this oil. Oil from the North Herald Field has some subtle features that distinguish it from nearby Westralian 2 oils of the Barrow Sub-basin and it may contain oil residues derived from an older calcareous marine source rock. The Torrens 1 oil appears to represent a new Bonaparte Basin petroleum system while Dill 1 and Leatherback 1 provide evidence for a new system in the Carnarvon Basin.

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