Geoscience Australia Marine Survey GA0332 (SS05/2011): Natural hydrocarbon seepage survey of the offshore northern Perth Basin - %Carbonate and specific surface areas of seabed sediments.

Brief description

In September and October of 2011 Geoscience Australia surveyed part of the offshore northern Perth Basin in order to map potential sites of natural hydrocarbon seepage. The primary objectives of the survey were to map the spatial distribution of seepage sites and characterise the nature of the seepage at these sites (gas vs oil, macroseepage vs microseepage; palaeo vs modern day seepage) on the basis of: acoustic signatures in the water column, shallow subsurface and on the seabed; geochemical signatures in rock and sediment samples and the water column; and biological signatures on the seabed. Areas of potential natural hydrocarbon seepage that were surveyed included proven (drilled) oil and gas accumulations, a breached structure, undrilled hydrocarbon prospects, and areas with potential signatures of fluid seepage identified in seismic, satellite remote sensing and multibeam bathymetry data. Within each of these areas the survey acquired: water column measurements with the CTD; acoustic data with single- and multi-beam echosounders, sidescan sonar and sub-bottom profiler (sidescan not acquired in Area F as it was too deep in places); and sediment and biological samples with the Smith-McIntyre Grab. In addition, data were collected with a remotely operated vehicle (ROV), integrated hydrocarbon sensor array, and CO2 sensor in selected areas. Sampling with the gravity corer had limited success in many of the more shallow areas (A-E) due to the coarse sandy nature of the seabed sediments. This data set comprises carbonate and specific surface area measurements in the upper 2cm of seabed sediments.

Lineage

Maintenance and Update Frequency: asNeeded

Statement: Bottom sediments were collected using a Smith McIntyre grab. The surface sediments (~0-2 cm) within the grab were spooned into falcon vials and the porewaters were removed by centrifugation. Bulk carbonate was determined on residual sediment using the carbonate bomb method. Specific surface areas were determined using a 5-point Brunauer-Emmett-Teller (BET) adsorption isotherm on a Quantachrome NOVA 2200e analyser, with nitrogen used as the adsorbate. The samples were first cleaned of organic matter by slow heating for 12 hours to 350oC. The average %RSD of the carbonate and specific surface area replicates were 1% and 10.5% respectively. The average %RSD of the carbonate and specific surface area replicates were 1.3% and 0.1% respectively.

Thanks to the crew of the RV Southern Surveyor for help with sample collection.