A RE-EVALUATION OF THE HYDROCARBON HABITAT OF THE NORTHERN PERTH BASIN

Recent oil discoveries at Hovea, Cliff Head, Jingemia and Eremia have challenged the perception that the northern Perth Basin is largely gas prone and renewed interest in the identification of oil source rocks in the succession. A major geochemical reassessment of all units from the Middle Triassic to the base of the Permian shows that the principal oil prone source rocks are restricted to a discrete zone within the basal Kockatea Shale, the Sapropelic Interval of the Hovea Member (defined herein). The source zone, typically 10–40 m thick, is laterally continuous over much of the onshore northern Perth Basin. It immediately overlies the Permian-Triassic boundary and may form part of a global anoxic event. These are some of the best oil source rocks ever identified in Australia. A combination of diagnostic biomarkers and compound specific isotopic data consistently link the Hovea Member source rocks to all oils analysed from the northern Perth Basin. Gas prone source rocks are developed at several levels in the Permian, and particularly in the Irwin River Coal Measures. In most areas of the basin, oil charge from the Triassic is in direct competition with any gas generated from the Permian, as seen in the large Warradong Kitchen, southeast of the Dongara Field. This has resulted in a predominance of gas discoveries, often with a thin oil leg. The smaller but oil prone Jingemia Kitchen, southwest of Dongara, has sourced the recent oil discoveries at Hovea, Jingemia and Eremia. There is evidence that the Hovea Member source rocks are also widely distributed offshore in the Abrolhos Sub-basin where they appear to have sourced the Cliff Head oil discovery and strong oil shows in Morangie–1 and Livet–1, some 300 km northwest of Dongara.