Authors / CoAuthors
Nicholas, W.A. | Howard, F.J.F. | Siwabessy, P.J.W. | Picard, K. | Radke, L.C.
Abstract
Geoscience Australia is investigating the suitability of offshore sedimentary basins as potential CO2 storage sites. In May 2012 a seabed survey (GA0335/SOL5463) was undertaken in collaboration with the Australian Institute of Marine Science to acquire baseline marine data in the Petrel Sub-basin, Joseph Bonaparte Gulf. The aim was to collect information on possible connections (faults and fluid pathways) between the seabed and key basin units, and to characterise seabed habitats and biota. Two areas were surveyed (Area 1: 471 km2, depth ~ 80-100 m; Area 2: 181 km2, depth ~ 30-70 m), chosen to investigate the seabed over the potential supercritical CO2 boundary (Area 1) and the basin margin (Area 2), with Area 2 located around Flat Top 1 Well. Data analysed include multibeam sonar bathymetry and backscatter, seabed samples and their geochemical and biological properties, video footage and still images of seabed habitats and biota, and acoustic sub-bottom profiles. Pockmarks, providing evidence for fluid release, are present at the seabed, and are particularly numerous in Area 1. Area 1 is part of a sediment-starved, low-relief section of shelf characterised by seabed plains, relict estuarine paleochannels, and low-lying ridges. Facies analysis and radiocarbon dating of relict coastal plain sediment indicates Area 1 was a mangrove-rich environment around 15,500 years ago, transgressed near the end of the Last Glacial period (Meltwater Pulse 1A). Modern seabed habitats have developed on these relict geomorphic features, which have been little modified by recent seabed processes. Seabed habitats include areas of barren and bioturbated sediments, and mixed patches of sponges and octocorals on hardgrounds. In the sub-surface, stacked sequences of northwest-dipping to flat-lying, well-stratified sediments, variably incised by palaeochannels characterise the shallow geology of Area 1. Some shallow faulting through these deposits was noted, but direct linkages between seabed features and deep-seated faults were not observed. Area 2 is dominated by carbonate banks and ridges. Low-lying ridges, terraces and plains are commonly overlain by hummocky sediment of uncertain origin. Pockmarks are present on the margins of banks, and on and adjacent to ridges. Despite the co-location of banks and ridges with major faults at depth, there is a lack of direct evidence for structural connectivity, particularly because of significant acoustic masking in the sub-surface profiles of Area 2. While no direct structural relationship was observed in the acoustic sub-bottom profiles between these banks, ridges and faults visible in the basin seismic profiles, some faults extend through the upper basin units towards the seabed on the margin of Area 2. No evidence was detected at the seabed for the presence of thermogenic hydrocarbons or other fluids sourced from the basin, including beneath the CO2 supercritical boundary. The source of fluids driving pockmark formation in Area 1 is most likely decomposing mangrove-rich organic matter within late Pleistocene estuarine sediments. The gas generated is dominated by CO2. Additional fluids are potentially derived from sediment compaction and dewatering. Conceptual models derived from this are being used to inform regional-scale assessments of CO2 storage prospectivity in the Petrel Sub-basin.
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nonGeographicDataset
eCat Id
79178
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Cnr Jerrabomberra Ave and Hindmarsh Dr GPO Box 378
Canberra
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Keywords
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- External PublicationAbstract
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- bathymetry
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- geological storage of CO2
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- habitat
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- continental shelf
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- seabed
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- marine
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- NT
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Marine Geoscience
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- Published_Internal
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2014-01-01T00:00:00
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geoscientificInformation
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This abstract is for a presentation at the Australian Earth Science Convention 2014, Newcastle, NSW, describing the results of the marine survey GA0335/SOL5463 of the seabed above the Petrel Sub-basin, Joseph Bonaparte Gulf. The survey is described in the post survey report: Carroll et al., 2012. Seabed Environments and shallow geology of the Petrel Sub-basin, Northern Australia. Geoscience Australia Record, 2012/66, 112 pages. (GeoCat No. 74627).
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[-13.0, -12.0, 129.0, 130.0]
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