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Geoscience Australia has recently completed a survey searching for evidence of natural hydrocarbon seepage in the offshore northern Perth Basin, off Western Australia. The survey formed part of a regional assessment of the basin's petroleum prospectivity in support of ~17,000 sq km of frontier exploration acreage release in the region in 2011. Multibeam bathymetry, sub-bottom profiler, sidescan sonar and echosounder data were acquired to map seafloor and water column features and characterise the shallow sub-surface sediments. A remotely operated vehicle (ROV) was used to observe and record evidence of seepage on the seafloor. 71 sediment grabs and 28 gravity cores were collected and are currently being analysed for headspace gas, high molecular weight biomarkers and infaunal content. Survey data identified an area of high 'seepage' potential in the northernmost part of the study area. Recent fault reactivation and amplitude anomalies in the shallow strata correlate with raised, high-backscatter regions and pockmarks on the seafloor. A series of hydroacoustic flares identified with the sidescan sonar may represent gas bubbles rising through the water column. The ROV underwater video footage identified a dark-coloured fluid in 500 metres water depth proximal to the sidescan flares which may be oil that naturally seeped from the seafloor. The integration of the datasets acquired during the marine survey is indicative of natural oil seepage and provides additional support for the presence of an active petroleum system on this part of the continental margin.

A six-week geological cruise of the West German research vesselSonne, under the auspices of the Australian-German Science Agreement, hasyielded a considerable amount of information about the Exmouth Plateau, theWallaby Plateau, and the intervening Cuvier Abyssal Plain. The main aim ofthe cruise was to sample pre-Quaternary strata cropping out on the plateaumargins; subsidiary aims were to sample the Quaternary sequence from conti-nental shelf to abyssal plain on two representative profiles, to obtainQuaternary cores for gas analysis, and to search for manganese modules. Allfour aims Were successfully accomplished. During the cruise 31 single-channel seismic profiles were run tohelp select sampling targets. On the Exmouth Plateau and adjacent areas 102stations were successfully occupied, 31 yielding pre-Quaternary rocks. On the Wallaby Plateau and adjacent areas 18 stations were successfully occupied,13 yielding pre-Quaternary rocks. Samples were obtained from depths rangingfrom 100 m to 5200 m. The sampling of pre-Quaternary strata has shed new light on thearea, On the northern Exmouth Plateau thick sequences of Early Jurassicshelf carbonates and Middle Jurassic coal measures were found beneath themain (?Late Jurassic) unconformity. Cretaceous shelf and pelagic sedimentswere also shown to exist. Above the main unconformity there is a condensedsequence of Cainozoic pelagic carbonates. On the northern margin of theWombat Plateau there is a volcanic sequence at least 300 m thick beneaththe main unconformity. On the northwestern Exmouth Plateau Albian andMiocene carbonates were sampled. On the southern margin results were disa-ppointing, but Mesozoic sandstone and shale and Tertiary pelagic carbonateswere sampled. The layered sequence below the main (?Neocomian) unconformity,both on the eastern Wallaby Plateau and on the "Sonne Ridge", which extends northward from the plateau into the Cuvier Abyssal Plain, was shown to consistof interbedded weathered "basalts", tuffs, breccias, and volcaniclasticsediments. This suggests that a thick volcanic pile of Early Cretaceous ageforms much of the Wallaby Plateau. A variety of Quaternary cores, almost all of biogenic carbonates,were obtained on profiles from Rowley Shoals to the Argo Abyssal Plain, andfrom southwest of Barrow Island to the Cuvier Abyssal Plain. Quaternarycores in the central Exmouth Plateau were sampled for gas analysis; pre-liminary results indicate that methane is generally present, but in verylow amounts. Manganese nodules were obtained from the southern and easternmargins of the Wallaby Plateau, and preliminary determinations of contentsof nickel and copper suggest that they are above Indian Ocean averages, butwell below a grade of economic interest.

Legacy product - no abstract available

Legacy product - no abstract available

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Legacy product - no abstract available

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Legacy product - no abstract available