ABSTRACT: Direct Hydrocarbon Detection in Bottom-Waters of the Australian Continental Margin: Implications for Hydrocarbon Exploration

HEGGIE, D. T., G. W. O'BRIEN, J. H. BISHOP, and G. A. BICKFORD, Division of Marine Geosciences and Petroleum Geology, Bureau of Mineral Resources, Canberra, Australian Capital Territory, Australia

The Bureau of Mineral Resources conducts surface (geochemical "sniffer") surveys from RV Rig Seismic--to seek evidence of hydrocarbon seepage from underlying petroleum source rocks or hydrocarbon reservoirs into the bottom-waters of the Australian Continental Margin. These data are released to the exploration community. In addition, the results of geochemical "sniffer" surveys carried out by commercial contractors under terms of the P(SL)A will, when conditions of confidentiality expire, also be released.

Approximately 15,000 line-km of bottom-water DHD data have been collected from Rig Seismic so far. Many anomalies have been detected. Some are evident for only a few (1-3) km while others persist for distances of tens of kilometers.

All basins with significant seepage into bottom-waters, e.g., Gippsland, Barrow subbasin, the Vulcan graben and the Petrel subbasin, contain known accumulations of petroleum hydrocarbons. Therefore, evidence of thermogenic seepage is an unequivocal demonstration of petroleum hydrocarbon generation in a sedimentary basin. Cross-plots of methane vs. percent hydrocarbon wetness of anomalies distinguish potential sources (dry thermogenic gas, gas-condensate and liquids) of anomalies, and hence are an effective and rapid tool to rank the gas/liquids potential of a basin and perhaps different hydrocarbon prospects. Consequently, bottom-water detection of light hydrocarbon anomalies is a useful reconnaissance scale exploration tool.

Several basins surveyed show no evidence of seepage, e.g., Stansbury, North Bass, and Bass basins and part of the Dampier Subbasin over the Wanaea and Cossack oil fields and the Angel gas field. A lack of seepage predicts low hydrocarbon prospectivity (North Bass and Stansbury basins) with the caveat that an absence of seepage may result from restricted or inadequate hydrocarbon migration pathways through the overlying sedimentary strata (Dampier subbasin).

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)