Seabed environments and shallow geology of the Leveque Shelf, Browse Basin, Western Australia: seabed sediment grainsize by sieve measurement

Brief description

In May 2013, Geoscience Australia, in collaboration with the Australian Institute of Marine Science, undertook a marine survey of the Leveque Shelf (survey number SOL5754/GA0340), a sub-basin of the Browse Basin. This survey provides seabed and shallow geological information to support an assessment of the CO2 storage potential of the Browse sedimentary basin. The basin, located on the Northwest Shelf, Western Australia, was previously identified by the Carbon Storage Taskforce (2009) as potentially suitable for CO2 storage. The survey was undertaken under the Australian Government's National CO2 Infrastructure Plan (NCIP) to help identify sites suitable for the long term storage of CO2 within reasonable distances of major sources of CO2 emissions. The principal aim of the Leveque Shelf marine survey was to look for evidence of any past or current gas or fluid seepage at the seabed, and to determine whether these features are related to structures (e.g. faults) in the Leveque Shelf area that may extend to the seabed. The survey also mapped seabed habitats and biota to provide information on communities and biophysical features that may be associated with seepage. This research, combined with deeper geological studies undertaken concurrently, addresses key questions on the potential for containment of CO2 in the basin's proposed CO2 storage unit, i.e. the basal sedimentary section (Late Jurassic and Early Cretaceous), and the regional integrity of the Heywood Formation (the seal unit overlying the main reservoir). The survey collected one hundred and eleven seabed sediment samples that were analysed for their grain size, textural composition and carbonate content. This dataset includes the results of grain size measurements done by sieve analysis on seabed sediments.

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Maintenance and Update Frequency: asNeeded

Statement: A marine survey (GA0340/SOL5754) of the Leveque Shelf was undertaken in May 2013 as part of work investigating CO2 storage in Australia's offshore sedimentary basins. Seabed surface sediments were collected using either a Smith McIntyre or a Shipek grab sampler. Sediment grabs typically recovered about 0.1 cubic metres of undisturbed sediment, from which a 250 - 500 g sub-sample was taken to a depth of ~5 cm. Sub-samples were described from visual inspection, noting grain size, sorting, composition and Munsell colour then stored in plastic bags and refrigerated. In the laboratory, samples were used for grain size analysis and measurement of carbonate content on separate splits. Retrieved samples were kept chilled until measurement in the laboratory at Geoscience Australia. Sediment grain size in these samples was measured by dry sieving, and sediment fractions are presented as a percentage of dry weight. The main sediment fractions from 4 phi to -5 phi (0.63 mm to 32 mm) were measured by sieve. The less than 63 um fraction (> 4 phi) was measured by laser granulometry using a Malvern Mastersizer 2000, with results expressed as a percentage of the total particle volume based on an average of three measurements on each sample.