The Australian Geological Survey Organisation (AGSO) carried out a regional water column geochemical 'sniffer' program on the NW Australian Margin, from Dampier to the west of Darwin, over the period 21 September to 23 October 1998.. The overall objective of the survey was to acquire bottom water hydrocarbon data of the Northern Carnarvon Basin, offshore Canning Basin, Yampi Shelf and the Southern Bonaparte Basin, particularly in regard to detecting natural hydrocarbon seepage. A total of 4788 line km of sniffer data were acquired during the program, using the contract vessel TSMV Pacific Conquest. 160 survey lines were completed.Weak to moderate concentrations of methane and moderate to strong concentrations of ethane were detected in the waters surrounding the Wandoo B Platform, Northern Carnarvon Basin. Elsewhere in the northern Carnarvon, Skua, AC/P20, AC/P25-26, AC/P27 and AC/P23 survey areas, detected hydrocarbon seepage was close to background concentration indicating that seepage was either absent or of very low level.Moderate to strong methane and ethane concentrations were detected on parts of the Yampi Shelf. It is significant to note that there is a measure of repeatability of methane and ethane sampled between S176, recorded in 1996 and these data recorded in 1998. The levels measured, coincident with AGSO seismic lines S165, confirm hydrocarbon leakage where the regional seal thins onto the Kimberley Block basement (O'Brien et al., 1998a).
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