South-eastern Australia Surface Geochemistry II: Light Hydrocarbon Geochemistry in Bottom-waters of the Gippsland Basin, Eastern Otway Basin, Torquay Sub-Basin and the Durroon Sub-Basin Vols 1 and 2

The Bureau of Mineral Resources (BMR) collected 1430 line-km of bottom-water
Direct Hydrocarbon Detection (DHD) data during a survey aboard R.V. Rig Seismic in
the Durroon Sub-basin, the Otway Basin, the Torquay Sub-basin, and the Gippsland
Basin, during late September and early October of 1991.
No significant bottom-water anomalies were detected in the Durroon Sub-basin.
Anomalous concentrations of light C2+ hydrocarbons were detected in the eastern
Otway Basin. The anomalies were not extensive, comprising only a few data points
representing a few kilometres in extent. One anomaly (of methane, ethane and
propane) was accompanied by high levels of the biogenic hydrocarbons, ethylene and
propylene, suggesting in-situ biogenic activity in the water column. However,
anomalous concentrations of C7 and C8 hydrocarbons were also found here and at
three other locations, and are from an unknown 'source'.
A weak bottom-water anomaly was detected in the Torquay Sub-basin in the same
location as an anomaly detected during an earlier survey (Rig Seismic Survey 89), two
years previously. The weakness of the anomaly prevents a confident interpretation of
the potential 'source' of the hydrocarbon anomaly, but the data suggests it is derived
from a gas/condensate, or dry thermogenic gas 'source'.
Several strong bottom-water anomalies were detected in the Gippsland Basin. Bottomwater
anomalies were found near the Sunfish and Tuna oil/gas accumulations, in
similar locations to anomalies found on Rig Seismic Survey 89, two years earlier.
However, another previously-detected anomaly (near Barracouta) was not reproduced.
Additional anomalies were found near Flathead, and to the west of Wahoo. The
anomaly west of Wahoo was weak and in a similar area to that detected on Survey 89.
The composition of most bottom-water hydrocarbon anomalies in the Gippsland Basin
are indicative of a liquid-prone hydrocarbon 'source', while one anomaly in the
northern sector of the survey area is indicative of a gas/condensate 'source'.