CRAYFISH GROUP HYDROCARBONS—IMPLICATIONS FOR PALAEOENVIRONMENT OF EARLY CRETACEOUS RIFT FILL IN THE WESTERN OTWAY BASIN

Abstract

Recent hydrocarbon discoveries in Early Cretaceous (pre-Aptian) reservoirs of the western Otway Basin offer encouragement to future petroleum exploration and also contain clues to the palaeoenvironments and early evolution of this rift basin. The Late Jurassic/Early Cretaceous Casterton Formation and the Early Cretaceous Crayfish Group have traditionally been regarded as fluvial and lacustrine deposits. Indeed, the source rock characteristics inferred from the geochemistry of the Katnook Field condensate and the oils from Wynn-1 and Sawpit-1 are those of siliciclastic freshwater facies. However, the biomarker assemblage of the Troas-1 condensate implies that its source beds were deposited in a marginal marine setting. Even more unexpected are the biomarker compositions of reservoir bitumens from Crayfish-Al and Zema-1 which provide evidence for the existence of saline to hypersaline palya lakes during the early rift phase of the Otway Basin.