ONSHORE GIPPSLAND GEOCHEMICAL SURVEY: A TEST CASE FOR AUSTRALIA

Abstract

Drilling for oil in Gippsland probably commenced about 1886 with a test bore to 175 feet near an alleged oil seep. It was however not until 1924 with the discovery of gas and minor heavy oil in a well near Lakes Entrance that petroleum exploration really commenced. Since then about 125 wells have been drilled onshore, half of which were in the Lakes Entrance area where two non-commercial oil pools were delineated. The wells in the remainder of the onshore basin were dry.

Some potential remains in stratigraphie wedge-outs. Because of the low prospectivity of the area and the difficulty of locating these targets with conventional methods, a geochemical survey was carried out in early 1974.

The Gippsland Geochemical Survey covers about 1,100 square kilometres and comprises 1,117 samples. The samples were analysed and the results computer processed. A regional background and anomalous threshold was defined for each homogeneous set of samples. The final map is a contour map of the product of the relative anomalous value by the probability of obtaining an anomalous sample in a small area.

The gas map shows six main anomalies of which five are linear and parallel to the postulated wedge-outs. One anomaly probably relates to the residual oil pools near Lakes Entrance, another is weak and probably spurious. The remaining four anomalies warrant further investigation, preferably by detailed geochemical surveying, followed hopefully by the drill.

Geochemical surveying is a valid and inexpensive tool in on-sh ore petroleum exploration, provided that:

the possibilities and restrictions are understood;

a large enough area is covered with a regular grid to establish proper contrast;

care is taken with data processing, map preparation and interpretation;

it is recognised that the method is additional and does not replace seismic, etc.

Australia has over 1,000,000 square kilometres of moderately prospective onshore basins. The author believes that consideration should be given by the Australian petroleum explorers to employ the potential of this cheap exploration tool more fully.