SOURCE, MIGRATION AND ENTRAPMENT OF HYDROCARBONS AND CARBON DIOXIDE IN THE OTWAY BASIN, VICTORIA

Abstract

Salinity distribution, structure, carbon dioxide occurrences, facies and thermal gradient variations and gas isotope studies in the onshore Victorian Otway Basin indicate that the migration of fluids from the Eumeralla Formation source rocks, and the entrapment of hydrocarbons in the Waarre Formation reservoir sands of the Port Campbell Embayment, are closely associated with the presence of three major fault systems. Hydrocarbon accumulation appears to take place where high salinities and normal rift-valley faults are in closest proximity to a structural trap. The large carbon dioxide occurrences are thought to be associated with volcanism and deep vertical fault conduits.

Although there are, as yet, considerably fewer exploration wells in the Tyrendarra Embayment/Portland Trough, present studies are nevertheless indicating that broadly similar migration and salinity/fault relationships probably exist in this western portion of the basin. Drilling and analytical results also show this end of the basin to be prospective for not only gas but also oil.