New Exploration Potential for Cretaceous Graben-Fill Sediments of Australia's Otway Basin

P. E. Williamson, G. E. O'Brien, M. S. Marlow, M. G. Swift, E. A. Felton, J. Lock, A. S. Scherl, N. F. Exon

The Otway basin is the westernmost of three Bass Strait basins in southern Australia. The basin straddles Australia's southern margin for 500 km, has up to 10 km of sedimentary section and an area of 25,000 km² in water depths under 200 m. The offshore Otway basin had 16 exploration wells, drilled predominantly in the 1960s and 1970s to test graben-fill prospects. Seismic definition of prospects was usually poor, and although hydrocarbon shows were common, no major discoveries were made.

Extensive multichannel seismic data collected in the basin by the Australian Bureau of Mineral Resources Marine Division allowed regional correlation with good-quality data. A reappraisal of the basin uses the new BMR data and existing industry data for structure and seismic stratigraphic analyses. These data combined with studies of geohistory and source reservoir facies, have allowed the prospectivity of the Cretaceous graben fill to be redefined both for previously explored and deeper levels delineating significant Lower and middle Cretaceous rift-related leads.

Prospectivity within the graben fill relates to sourcing from the thick chloritic mudstones and shales of the Early Cretaceous Eumerella Formation into reservoirs in the underlying quartzose to lithic continental Pretty Hill formation or the overlying continental to paralic middle Cretaceous Warree formation. Geohistory analyses indicate that in the central basin, sourcing of traps would be in situ in essentially their present configuration. Where shallow burial occurs, as in the western basin, sourcing is from unknown deep zones as indicated by hydrocarbon shows. The present study thus suggests substantial untested prospectivity in Cretaceous graben-fill sediments in Australia's Otway basin.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.