ORIGIN OF THE WINDALIA OIL, BARROW ISLAND, WESTERN AUSTRALIA

Abstract

Although Windalia oil from Barrow Island, Western Australia is deficient in n-alkanes most other chemical properties of this crude are not characteristic of typical bacterially-altered crudes. In this study geochemical properties such as the alkane, aromatic and NSO content, sulphur content, level of maturity, API gravity, n-alkane distribution, and overall molecular weight distribution are compared for Windalia oil and typical bacterially-altered crude oils. In addition, the geological history of Barrow Island is considered to assess whether the Windalia reservoir has been flushed with meteoric waters.

To investigate this problem further the gas chromatographic distribution of compounds in both the whole crudes and the aromatic fractions are compared for Mardie and Windalia oils. Mardie is a bacterially-altered oil located near Barrow Island. These distributions further support the contention that Windalia oil is not bacterially altered.

Carbon isotope "petroleum type-curves", presented for Windalia, Mardie and the paraffinic Jurassic oil from Barrow Island, suggest that there is no genetic relationship between Windalia and the Jurassic oil, but that a strong relationship exists between Mardie and Windalia oil. Therefore, Windalia may be a parent of the Mardie oil and not an altered Jurassic oil.

Because geochemical and geological evidence is inconsistent with the contention that Windalia oil is bacterially altered, an alternative mode of formation is proposed which involves : (a) early migration; (b) burial of the sediments to depths greater than their present depths; (c) reservoir catalysis; and (d) uplift and subsequent weathering of the sediments.