sedimentary basins
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Legacy product - no abstract available
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Geoscience Australia is currently conducting a study under the National CO2 Infrastructure Plan (NCIP) to assess suitability of the Vlaming Sub-basin for CO2 storage. It involves characterisation of the potential seal, the Early Cretaceous South Perth Shale (SPS), by integrating seismic and well log interpretation into a sequence stratigraphic framework. The SPS, conventionally described as a regional seal deposited during a post-rift thermal subsidence phase, consists of a series of prograding units deposited in a deltaic to shallow marine setting. Mapping of the SPS has revealed differences in the geometries of progradational sequences between the northern and southern areas, related to the type and distance to the sediment source as well as the seafloor morphology. In the northern area, deltaic progradation and aggradation occurred over a flat topography between the two uplifted blocks. The succession is composed of prograding sequences commonly exhibiting sigmoidal to oblique geometries, prograding from the north-east to south-west. In the southern area the topography is more complex due to the presence of several paleotopographic highs associated with pre-existing structures. These sequences are sigmoidal to oblique in cross section. They were deposited in fan shaped lobes, successively infilling paleotopographic lows. Direction of the progradation is from southwest to northeast. The thickness of the SPS varies from 200 m between topographic highs to 700 m in the lows. Sedimentary facies are interpreted to vary from sandy delta front to muddy slope and prodelta deposits. These findings will be used in a 3D geological model for assessing CO2 storage potential.
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The seismic stacking velocity data in the Otway Basin are a useful dataset for calculating depths and sediment thicknesses. This work presents time-depth relationships computed from unsmoothed stacking velocities and compares these with functions obtained from sonobuoy refraction data and exploration well sonic logs. The comparison suggests that a total sediment thickness over-estimate for the Otway Basin of about 15% can be expected from the depths derived from stacking velocities alone. On the other hand, for sediment thickness calculations down to ~3 s two-way travel time below sea floor, stacking velocity data give comparable depths to those obtained from the sonic logs. A piece-wise formula is offered which scales the time-depth function for the Otway Basin in order to compensate for the depth overestimate inherent in using stacking velocities to calculate total sediment thickness.
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The term 'modelling while interpreting' refers to the use of 3D models during the interpretation of reflection seismic data in order to inform that process. Rather than using 3D models at a final stage of the project just to display results, new software tools are emerging to enable development of 3D models in parallel with the seismic interpretation work. These tools provide additional means to help interpreters make informed decisions such as where to pick basement and to check the 3D integrity of their geological models. Applications of this new workflow are illustrated through a recently completed petroleum prospectivity assessment of the Capel and Faust frontier deep-water basins located 800 km to the east of Brisbane. Geoscience Australia acquired 2D geophysical data across these basins in 2007 and subsequently mapped the complex distribution of sub-basins by integrating 2D time-domain seismic interpretation with 3D gravity modelling. Forward and inverse 3D gravity models were used to inform the seismic interpretation and test the seismic basement pick. The identification of basement was problematic due to a lack of wells and the likelihood that acoustic basement represented older sedimentary material intruded by igneous rocks. Sonobuoy refraction data were modelled to achieve conversion of travel times to depth and estimate densities. Modelling gravity while interpreting reflection seismic data improved confidence in the mapping of the extent and thickness of sediments in these basins, and has potential to be used more widely in mapping projects to reduce exploration risk.
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The breakup of Gondwana during the Mesozoic resulted in widespread basin formation along Australia's southern margin, of which the Bight Basin is a component. In contrast to many other extensional margins, the Australian southern margin has been classified as a non-volcanic rifted margin, despite the reported occurrence of scattered volcanic and intrusive rocks in the geological literature. Public release of the Flinders 2D seismic survey data in the Bight Basin has allowed the accurate mapping of widespread sills, dykes, lava flows and volcanoes.
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A recent Geoscience Australia sampling survey in the Bight Basin recovered hundreds of dredge samples of Early Cenomanian to Late Maastrichtian age. Given the location of these samples near the updip northern edge of the Ceduna Sub-basin, they are all immature for hydrocarbon generation with vitrinite reflectance - 0.5% RVmax, Tmax < 440oC and PI < 0.1. Excellent hydrocarbon generative potential is seen for marine, outer shelf, black shales and mudstones with TOC to 6.9% and HI up to 479 mg hydrocarbons/g TOC. These sediments are exclusively of Late Cenomanian-Early Turonian (C/T) in age. The high hydrocarbon potential of the C/T dredge samples is further supported by a dominance of the hydrogen-rich exinite maceral group (liptinite, lamalginite and telalginite macerals), where samples with the highest HI (> 200 mg hydrocarbons/g TOC) contain > 70% of the exinite maceral group. Pyrolysis-gas chromatography and pyrolysis-gas chromatography mass spectrometry of the C/T kerogens reveal moderate levels of sulphur compounds and the relative abundances of aliphatic and aromatic hydrocarbons predict the generation of a paraffinic-naphthenic-aromatic low wax oil in nature. Not enough oom for rest of Abstract
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The Tasman Frontier region includes c. 3,000,000 sq km of seabed that is thought to be underlain by crust with continental affinities: the Lord Howe Rise, Bellona Trough, Challenger Plateau, Dampier Ridge, Middleton Basin, Fairway Basin, New Caledonia Trough, Norfolk Ridge System, Reinga Basin, and deep-water parts of Taranaki and Northland basins. We have compiled and interpreted c. 100,000 line km of archival seismic reflection data. Using seismic stratigraphy tied to Deep Sea Drilling Project (DSDP) wells, we identify a tectonic and stratigraphic event that we refer to as the 'Tectonic Event of the Cenozoic Tasman Area' (TECTA). This Middle Eocene to Late Oligocene event involved regional uplift followed by 1-2 km of tectonic subsidence of topographic highs, and >2 km of tectonic subsidence in the New Caledonia Trough. Strata below the TECTA reflector (or seismic unit in some places) are locally folded or reverse faulted. We present seismic-stratigraphic evidence that numerous islands were transiently created by uplift on the Lord Howe Rise during the TECTA event. We suggest that the underlying cause of the TECTA event was initiation of the subduction system that has since evolved into the Tonga-Kermadec system. Note: Abstract for initial submission; acceptance to be confirmed.
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Sandstone deposits are important sources of uranium, accounting for approximately 20 percent of global production, largely through in situ leach (ISL) mining. Most of this production has come from deposits in the western US, Niger and Kazakhstan. In Australia, uranium is being produced from the Berverley sandstone deposit in the Frome Embayment of South Australia, and a second ISL mine is under development at Honeymoon in the same region. Such deposits occur where uranium-bearing oxidised ground waters moving through sandstone aquifers react with reducing materials. The nature and abundance of organic material in the ore-bearing sedimentary sequence is of critical importance in the formation of sandstone uranium deposits. The locations of ore zones (distance from the margin of the basin) and the sizes of mineral deposits depend, amongst other factors, on the abundance and the reactive nature of the reductant. In sandstones rich in organic material (containing debris of fossil plants or layers of authigenic organic material) the organic materials either reduce uranium directly with bacteria as a catalyst, or result in production of biogenic H2S (Nash et al, 1981; Spirakis, 1996). In sandstones relatively poor in organic material the reduction is caused either by H2S (biogenic as well as non-biogenic) produced from the interaction of oxidised ground water with pyrite (thiosulphate produced initially by oxidation of pyrite breaks down to form reduced sulphur) in the sandstone aquifer or from the introduction of reduced fluids/gases (H2S and/or hydrocarbons) along favourable structures (Granger and Warren, 1969; Nash et al, 1981; Spirakis, 1996). This paper outlines the geology of the world-class sandstone uranium deposits in the Chu-Saryssu and Syrdarya Basins in the south-central portion of Kazakhstan, which are hosted by sandstones relatively poor in organic matter (Fig. 1, Table 1). It highlights the case for the crucial role of hydrocarbons in the formation of these and some other large sandstone type uranium deposits. Based on the model developed, it is concluded that there is considerable potential in Australia for discovery of large sandstone hosted uranium mineralisation, including in little explored regions underlain by basins with known or potential hydrocarbons.
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Rare earth elements (REE), high field strength elements and detrital zircon populations fingerprint and establish the provenance of siliciclastic sedimentary rocks within the Palaeoproterozoic Tanami Group, the main host to gold in the Tanami region, an important Proterozoic gold province in northern Australia. These data also define chronostratigraphic surfaces which provide a framework for constructing sequence stratigraphic models for the Tanami basin. This basin model provides insight into the likely spatial distribution of deep water facies, which are the best trap rocks for epigenetic gold deposition. The largest gold deposits occur in the well-bedded carbonaceous siltstone of the Dead Bullock Formation (DBF), which is the basal unit of the Tanami Group. At the Callie deposit, which is the largest deposit in the region, the host DBF is characterised by: (1) enrichments of MgO, TiO2, Cr, Sc (2) light REE depletion (relative to PAAS), and (3) a dominant population of detrital zircons with an age of ~2500 Ma. In contrast, turbidites of the overlying Killi Killi Formation, which is less strongly mineralised, is characterised by: (1) enrichments in Zr, La/Sc, La/Sm, (2) relatively flat REE patterns, and (3) the presence of a major population of detrital zircons with ages of 1880-1840 Ma. These characteristics are independent of grain size and are interpreted to indicate that the DBF was derived from a provenance dominated by a bimodal suite of Archean mafic and felsic rocks, whereas the Killi Killi Formation was derived from a provenance dominated by felsic rocks from the Palaeoproterozoic. These characteristics, which are best shown, using Zr/Sc versus Th/Sc diagrams, can be used as an exploration tool to differentiate the more favourable DBF from the less prospective Killi Kill Formation. These data combined with regional geologic data provide a first order understanding of the make-up of the Tanami basin, with implications on the distribution of siltstone-dominated stratigraphy that is the best host for gold.
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Lithostratigraphy, grain sizes and down-hole logs of Site 1166 on the continental shelf, and Site 1167 on the upper slope, are analyzed to reconstruct glacial processes in eastern Prydz Bay and the development of the Prydz trough-mouth fan. In eastern Prydz Bay upper Pliocene-lower Pleistocene glaciomarine sediments occur interbedded with open-marine muds and grade upward into waterlaid tills and subglacial tills. Lower Pleistocene sediments of the trough-mouth fan consist of coarse-grained debrites interbedded with bottom-current deposits and hemipelagic muds, indicating repeated advances and retreats of the Lambert Glacier-Amery Ice Shelf system with respect to the shelf break. Systematic fluctuations in lithofacies and down-hole logs characterize the upper Pliocene-lower Pleistocene transition at Sites 1166 and 1167 and indicate that an ice stream advanced and retreated within the Prydz Channel until the mid Pleistocene. The record from Site 1167 shows that the grounding line of the Lambert Glacier did not extend to the shelf break after 0.78 Ma. Published ice-rafted debris records in the Southern Ocean show peak abundances in the Pliocene and the early Pleistocene, suggesting a link between the nature of the glacial drainage system as recorded by the trough-mouth fans and increased delivery of ice-rafted debris to the Southern Ocean.