Geoscience Australia Survey 267, Post Survey Report: Testing Natural Hydrocarbon Seepage Detection Tools on the Yampi Shelf, Northwestern Australia

Brief description

Geoscience Australia Marine Survey 267 was undertaken aboard the Parmelia K, a 28 m pearling boat out of Broome. The survey departed Darwin on 3 March 2004 and terminated in Broome on 18 March 2004. The length of the survey was curtailed by unfavourable weather conditions associated with tropical cyclones Evan and Fay. The aim of the survey was to test and validate a range of techniques used in the detection/identification of natural hydrocarbon seepage. The Yampi Shelf in the northern North West Shelf was selected for the test bed as it is a known region of intense and widespread hydrocarbon seepage.

One of the most significant findings of the survey was the direct observation of natural hydrocarbon seepage in the Timor Sea region of the North West Shelf, which has previously only been interpreted through remote sensing or automated water sampling techniques (sniffer). Seepage plumes were observed rising from pockmark fields or hard-grounds on the vessel's echosounder and the side-scan sonar. Gas bubbles with oily films were observed at the surface. Active seepage sites were detected in close associated with hydrocarbon related diagenetic zones (HRDZs) interpreted in 3D seismic coverage of the region. A towed fluorometer detected variations in the hydrocarbon concentration of the sea surface over the study areas. Seepage appeared to be most active in association with low-tide, and more suppressed during high tide.

Multi-beam swath bathymetry of the study areas revealed channels between 2 and 20 m deep on the Yampi Shelf headland and in the vicinity of the major HRDZs. Tidal current directions measured by an acoustic doppler current profiler (ADCP) in the Yampi Shelf headland channels suggests that tidal shears over this headland may be giving a response on Synthetic Aperture Radar (SAR).
Sediment samples were collected with a Smith-Macintyre Grab, a dredge and a gravity core, although coring was largely unsuccessful due to carbonate hard-grounds over the seepage sites. Grab and dredge samples suggest that the active seeps are zones of preferential macro-biotic distribution.
Significant down-time was introduced through poorly functioning equipment and a lack of experience on behalf of the staff. This was primarily due to the fact that some of the equipment was leased from Seismic Asia Pacific Ltd. Therefore it is recommended for future surveys that an operator be supplied with leased equipment, or that the equipment is purchased by Geoscience Australia and staff are appropriately trained before the survey. It is also highly recommended that the survey have seismic acquisition capability, as the seismic data proved to be the best proxy for seepage site location.

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