A REVIEW OF GEOPHYSICAL EXPLORATION IN THE POLDA BASIN, SOUTH AUSTRALIA

Abstract

The Polda Basin is an intracratonic basin, at least 5000 m in depth, extending from the edge of the Great Australian Bight nearly 400 km east to the centre of Eyre Peninsula in southern South Australia, it can now be recognised as a structural feature of some antiquity, originating possibly as a crustal downwarp in Proterozoic time during the same orogenic regime that gave rise to the Officer and Amadeus basins of central Australia, with which it has strong affinities. Its internal structure indicates that it has been subject to the same orogenic episodes that have affected southern and central Australia, culminating in rotational block faulting associated with the final pull-apart of Gondwanaland. Extensive salt mobilisation observed on seismic sections, confirmed by the drilling of the Mercury 1 and Columbia 1 wells, supports the concept of the basin's Cambrian or Precam-brian origins.

The exploration history of the basin is a useful example of the application of a number of geophysical disciplines to arrive at a satisfactory interpretation. A reappraisal of geophysical data was undertaken after Outback Oil N.L. drillhole Gemini 1 penetrated 'uneconomic' basement at 856 m, far short of the anticipated sedimentary section of 1830-2440 m. Interpretation of shipborne magnetic data, not previously considered, indicated that a seismic horizon close to where Gemini 1 penetrated igneous rock was related to a thin layer of magnetic material which did not correspond to deep magnetic basement. Further detailed aeromagnetic and seismic studies confirmed this interpretation, which was tested by the drilling of two exploration wells. Mercury 1 and Columbia 1. Although unsuccessful as hydrocarbon discovery wells, these two wells confirmed the overall validity of the interpretation. Stratigraphic information derived from these wells and from further geophysical studies and drilling onshore have led to a new assessment of the basin and its potential.