GELTWOOD BEACH — A CASE HISTORY

Abstract

The regional geology of south eastern South Australia is briefly described; and the geophysical work which led to the discovery of a large anticline at Geltwood Beach, and to the selection of the location for the deep test.

The well was drilled in the latter part of 1963 to a depth of 12,300 feet, and bottomed in Aptian sediments. After penetrating a normal Tertiary succession, the well entered a monotonous succession of sandstones, siltstones, mudstones and admixtures of the three, in which it continued to total depth. These sediments represent a dump fill of the Gambier sub-basin with terrigenous material washed in in flood quantity from the elevated basin rim.

Flat dips in a vertical hole showed that the well remained on-structure to total depth.

The well did not find commercial production, nor any strong shows of hydrocarbons. It gave a prospect of producible reservoirs, but left the cap rock question unsolved; it did not find any indisputable oil source rocks, nor carrier beds.

The results of the reflection and refraction seismic surveys were explained.

An isopach map of the Knight Group can now be drawn from which deductions can be made about the probable position of the Cretaceous wedge-out, about the structure, and very hypothetically, about a possible explanation of the marine oil seepages.