Definition Of The Crayfish Group Unconformity, Western Otway Basin, Offshore South Australia.
Beaumont-Smith, Nicholas H.
Honours Degree, 1994
University of Adelaide
Sandstones of the Late Jurassic to Early Cretaceous Crayfish Group (Otway Supergroup) contain the majority of the hydrocarbon shows and all of the known oil and gas reserves in the South Australian or western portion of the Otway Basin. In particular, sandstones at the very top of the group at Katnook are currently under production to supply gas to Mt. Gambier and some surrounding industrial plants.
The position of the unconformity at the top of the Crayfish Group is of exploration importance in that it influences the distribution of reservoir sands. The exact placement of the unconformity within the stratigraphy of the Penola Trough has been brought into question by recent drilling. The current interpretation places the unconformity between two sand units of similar character; the Katnook Sandstone below and the Windermere
Sandstone Member of the Eumeralla Formation above. The evidence for this position includes the identification of an angular unconformity on seismic data and a palynological break at approximately the same level, the boundary between F. wonthaggiensis and the C. hughesii spore/pollen zones.
The re-evaluation of the unconformity in this study used a number of analytical and investigative techniques including sandstone petrology, X-ray diffraction to determine clay mineralogy of interbedded shales, seismic interpretation, petrophysical log analysis, magnetic susceptibility measurements and a detailed review of palynology.
The results of this study suggest that the unconformity at the top of the Crayfish Group is at the base of the Eumeralla Formation. Seismic reflectors within the Eumeralla Formation downlap onto the unconformity. The Windermere Sandstone Member, however, is not within the Eumeralla Formation, but below the unconformity and is gradational above the Katnook Sandstone. Petrographic provenance studies show the Katnook and Windermere sandstones to be very similar to each other, but distinctly different from the sandstones of the underlying Pretty Hill Formation and the overlying Eumeralla Formation. Detailed palynology of the Windermere Sandstone Member in a cored interval from Katnook #4 shows an overlap between palynomorphs of the F. wonthaggiensis and C. hughesii zones. The lithic-rich Windermere Sandstone from the type section in Windermere #2 across the Merino Uplift in western Victoria is not necessarily the same unit as the sands in the Penola Trough sharing the same name. It appears that in the Penola trough, the Katnook and Windermere sandstones can be treated as one unit for exploration purposes. XRD analysis has identified smectites with potentially expandable lattices which will require explorers to continue to guard against formation damage by selection of suitable drilling fluids.