Soil-Gas Surveys Help Locate New Oil Fields in Australia

John R. Conolly, William R. Ryall

Surface geochemical surveys that measure the distribution of trace amounts of light hydrocarbon gases at shallow (1 m) depths in soils have been made throughout the desert inland basins of Australia during the past 5 years.

In parts of western Queensland, namely the southeastern flank of the Eromanga basin, these surveys have greatly aided frontier exploration by locating regions of anomalous light hydrocarbon gases in the soils that were subsequently found to overlie structures containing oil. By contrast, structures which contained only a normal background distribution of the light alkane gases, that is, methane, ethane, propane, and butane, commonly proved to be noncommercial. Case histories of soil-gas surveys made from the same region over a period of years show that the distribution of soil-gas anomalous areas remains stable, greatly adding to the reliability of this rapid and inexpensive exploration technique.

Concentrations of up to ten times background of the light alkane gases were found to occur directly above structures which were later found to be oil productive. Several case histories will be described from Queensland. Comment will be made on the difficulties experienced in some basins, such as the Surat basin in Queensland, where thick coal measures and soils with swelling clays produce confusing patterns of soil-gas distribution.

The soil-gas technique is extremely cost efficient. Experience in many different basins in Australia suggests that it is best employed in arid environments and in basins where the obvious oil and gas targets are relatively shallow (1,000 to 2,000 m) or so large that they stand a good chance of being detected at the surface. Case histories of Australian examples show how these data can be integrated with the known basin geology and geophysics.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.