Publication Name: PESA's Eastern Australasian Basin Symposium II
Authors: L. Svendsen, T.H. Payenberg, P.J. Boult and J.G. Kaldi
Date Published: December 2004
Number of Pages: 31
Reference Type: Book Section
Abstract:
The Early Cretaceous Eumeralla Formation is regarded as an ineffective regional top seal in the Otway Basin, largely due to sand interbeds providing preferential hydrocarbon migration pathways. A deep-water lacustrine unit consisting of interbedded siltstone and claystone and massive claystone is developed at the base of the Eumeralla Formation in some wells. This interval (Unit VI) was deposited within a transgressive to early highstand systems tract and has better sealing properties than the overlying Eumeralla Formation intervals.Minor lithological variations within the Eumeralla Formation cannot be recognised clearly on log signatures.
The gamma ray log is generally unable to identify sandstone interbeds due to the sand's high volcanoclastic content. An integration of log data with seismic data, palynology, conventional core, and sidewall core and cuttings descriptions has enabled the seal geometry of the basal Eumeralla Formation to be determined. The lacustrine Unit VI is locally developed in the St Clair Trough and on the flank of the Merino High. It is absent in the central Penola Trough and the Robe Trough, but potentially could be developed within the undrilled southern Penola Trough and the Tantanoola Trough. Unit VI occurs preferentially in basins formed along the footwall of half grabens during late rift to early sag. Mercury injection capillary pressure analysis suggests that the lower Eumeralla Formation has a higher seal capacity than the sandrich upper Eumeralla Formation facies. Risk of top seal fracturing is likely to be higher in the onshore troughs where brittle coal beds are developed at the base of the Eumeralla Formation.