We combine two- and three-dimensional seismic stratigraphic interpretation with paleobathymetric analysis from benthic foraminifera to understand the genetic significance of prominent seismic discontinuity surfaces typically mapped as "sequence boundaries" and "flooding surfaces" in the late Paleogene-early Neogene Northern Carnarvon Basin. The progradational succession, dominated by heterozoan carbonate sediments, is divided into five northwest-prograding clinoformal sequences and 19 sub-sequences. Clinoform fronts progress from smooth to highly dissected, with intense gullying apparent only after the mid Miocene optimum. Once initiated, gullies become the focus for sediment distribution across the front. Bottomsets remain relatively sediment-starved without the development of aprons on the lower slope and basin. Small-scale variability suggests heterogeneous sediment dispersal through the slope conduits. Along-strike sediment transport superimposed on progradation changes from south-directed in the late Oligocene to north-directed in the late mid-Miocene suggesting a major reorganization of circulation in the southeastern Indian Ocean. Prominent seismic discontinuity surfaces represent both intervals of shallow paleo-water depth and flooding of the shelf. Partial exposure of the shelf indicated by karst morphology is coeval with middle to outer neritic paleo-water depths on the outer shelf. Rather than build to sea-level, progradation occurs with shelf paleo-water depths at the clinoform rollover >100 m. Therefore, in the Northern Carnarvon Basin onlap onto the clinoform front is not coastal and the sensitivity of the clinoforms to sea-level changes is muted.

High-CO2 gas fields serve as important analogues for understanding various processes related to CO2 injection and storage. The chemical signatures, both within the fluids and the solid phases, are especially useful for elucidating preferred gas migration pathways and also for assessing the relative importance of mineral dissolution and/or solution trapping efficiency. In this paper, we present a high resolution study focussed on the Gorgon gas field and associated Rankin trend gases on Australia's Northwest Shelf of Australia. The Gorgon field is characterized by a series of stacked reservoirs (Figure 1), and is therefore well placed to characterize CO2 migration, dissolution and reaction by looking at geochemical signatures in the different reservoirs. Hydrological data at the Gorgon field also suggests that many of the major faults possess very low transmissivities, which should prevent or limit mixing of reservoir fluids with different chemical imprints. The gas data we present here reveal correlatable trends for mole %-CO2 and --C CO2 both areally and vertically as observed by Edwards et al. (2007). We suggest that the observed relationships are imparted due to mineral carbonation reactions that occurred along the CO2 migration pathway. These results have important implications for carbon storage operations and suggest that under certain conditions mineral sequestration might occur over longer migration distances and on shorter timescales than previously thought.

In this paper, we present a high resolution study focussed mainly on the Gorgon field and associated Rankin Trend gas fields, Carnarvon Basin, Australia (Figure 1). These gas fields are characterized by numerous stacked reservoirs with varying CO2 contents and provide a relevant natural laboratory for characterizing CO2 migration, dissolution and reaction by looking at chemical characteristics of the different reservoirs (Figure 2). The data we present reveal interesting trends for CO2 mol% and -13C both spatially and with each other as observed by Edwards et al. (2007). Our interpretation of the data suggests that mineral carbonation in certain fields can be significant and relatively rapid. The Gorgon and Rankin Trend fields natural gases may therefore be a unique natural laboratory, which give further insights into the rates and extent of carbonate mineral sequestration as applied to carbon storage operations.

The Australian Government, through the Department of Resources, Energy and Tourism, has supported Geoscience Australia in undertaking a series of regional-scale, geological studies to assess the CO2 storage potential of sedimentary basins, including the Petrel Sub-basin. The studies form part of the National Low Emissions Coal Initiative designed to accelerate the development of CO2 transport and storage infrastructure near the sources of major energy and industrial emissions. The Petrel Sub-basin was identified as a high-priority region for a future pre-competitive work program by the national Carbon Storage Taskforce. The Carbon Storage Taskforce also recommended the release of greenhouse gas assessment permits, which were released within the Petrel Sub-basin in 2009. As a component of the studies at Geoscience Australia, the numerical simulation was hypothetically designed to dynamically model the reservoir behavior and CO2 migration during the injection and post-injection stages using an in-house built 3D geological model of a represented injection site. 14 million tonnes per annum (MTPA) of CO2 was injected into the lower Frigate/Elang/Plover reservoir over 30 years and CO2 plume migration was simulated up to 2,000 years from the initial injection. The injection rate of 14 MTPA of CO2 used in this study was based on the predicted 2020 CO2 emissions of the Darwin Hub, a figure defined by the Carbon Storage Taskforce (2009). The poster highlights the simulation results including CO2 plume migration distance, CO2 trapping mechanisms and reservoir pressure behavior.

New 2D seismic data acquired in the Mentelle Basin by Geoscience Australia in 2008-09 has been used for a seismic facies study of the post-rift succession. The Mentelle Basin is a large deep to ultra deep-water, frontier basin located on Australia's southwestern margin about 200 km southwest of Perth. The study focused on the post-rift sequences deposited following the breakup between Australia and Greater India. Stratigraphic wells DSDP 258 and DSDP 264 provide age and lithological constraints on the upper portion of the post-rift succession down to mid-Albian strata. The depositional environment and lithology of the older sequences are based on analysis of the seismic facies, stratal geometries and comparisons to the age equivalent units in the south Perth Basin. Fourteen seismic facies were identified based on reflection continuity, amplitude and frequency, internal reflection configuration and external geometries. They range from high continuity, high amplitude, parallel sheet facies to low continuity, low amplitude, parallel, subparallel and chaotic sheet, wedge and basin fill facies. Channel and channel fill features are common in several facies along with a mounded facies (probably contourite) and its associated ponded turbidite fill. A progradational sigmoidal to oblique wedge facies occurs at several stratigraphic levels in the section. A chaotic mound facies, probably comprising debrite deposits, has a localised distribution.

High-CO2 gas fields serve as important analogues for understanding various processes related to CO2 injection and storage. The chemical signatures, both within the fluids and the solid phases, are especially useful for elucidating preferred gas migration pathways and also for assessing the relative importance of mineral dissolution and/or solution trapping efficiency. In this paper, we present a high resolution study focused on the Gorgon gas field and associated Rankin trend gases on Australia's Northwest Shelf of Australia. The gas data we present here display correlate-able trends for mole %-CO2 and %C CO2 both areally and vertically. Generally, CO2 % decreases and becomes depleted in %C (lighter) upsection and towards the north; a trend which also holds true for the greater Rankin trend gases in general. The strong spatial variation of CO2 content and %C and the interrelationship between the two suggests that processes were active to alter the two in tandem. We propose that these variations were driven by the precipitation of a carbonate phase, namely siderite, which is observed as a common late stage mineral. This conclusion is based on Rayleigh distillation modeling together with bulk rock isotopic analyses of core, which confirms that CO2 in gases are genetically related to the late stage carbonate cements. The results from this study have important implications for carbon storage operations and suggest that significant CO2 may be reacted out a gas plume over short migration distances.

Regional geological properties of sedimentary basins play a significant role in determining the safety of CO2 storage. Four major trapping mechanism have been identified: Structural and stratigraphic trapping is the containment of supercritical CO2 by low permeability / low porosity rocks and is the dominant mechanism during injection and initial storage phase. Residual or capillary trapping is the retention of supercritical CO2 in the pore space between grains and tends to be most relevant on a scale of tens to thousands of years. Solubility trapping is the uptake of CO2 into the formation water, which is considered to be the most important trapping mechanism over hundreds to millions of years (1). Mineral trapping leads to the permanent immobilization of carbon through the precipitation of carbonate minerals. This study assesses the conditions for solubility trapping in major Australian sedimentary basins. The total dissolved solid (TDS) concentration of the formation water has been compiled from over 900 wells as it, along with pressure and temperature, is a key variable controlling CO2 solubility and the associated change in fluid density. Fluid density is a critical factor in driving fluid advection which determines the rate of solubility trapping and downward migration in the formation. This process is vital in reducing the amount of highly mobile supercritical CO2 at the top of the formation and storing it as dissolved CO2 in deeper parts of the formation.

The geological storage of carbon dioxide (CO2) is the process whereby CO2 captured from power plants or other industrial facilities is transported by pipeline to a suitable location and then injected under pressure into a deep geological reservoir formation, where it remains permanently trapped and prevented from entering the atmosphere. The processes by which it is retained in the subsurface are generally those that have trapped oil, gas and naturally generated CO2 for millions of years. The geological formations that can be utilised for this trapping have the same characteristics as those that are able to act as reservoir rocks for petroleum. They have good porosity and permeability and have an overlying sealing formation, which will prevent the trapped fluids migrating out of the storage reservoir and possibly escaping to the surface. In addition, because of the phase behaviour of CO2, efficient storage requires that they are stored at depths greater than 800 below the surface. Unlike oil and gas, which rely primarily on a three dimensional structural trap to prevent them from ultimately rising to the surface, there are additional trapping mechanisms for CO2. Given a sufficiently long migration path within a formation, CO2 will ultimately be rendered immobile by dissolution into the formation water, residual trapping and potentially, over longer time scales, mineralisation. As groundwaters at these depths are generally saline, this type of storage is often termed deep saline aquifer storage. A recent nationwide review by Commonwealth and State geological surveys, as part of the Carbon Storage Taskforce, rated the suitability of geological basins across Australia for geological storage of CO2. The most geologically suitable basins are the offshore Gippsland and North Perth basins but several onshore basins also rate highly. These include the Eromanga, Cooper, Bowen, Galilee, Surat, Canning and Otway basins. The Victorian Government has recently released area for greenhouse gas storage exploration in the Gippsland Basin and the Queensland Government in the Galilee and Surat basins. The aquifers within these basins provide groundwater for human consumption, agriculture, mining, recreation and groundwater dependent ecosystems. The Surat Basin also contains oil and gas accumulations that are being exploited by the onshore petroleum industry. Understanding the existing the groundwater's chemistry and the connectivity between aquifers in the context of its current use is essential in order to determine whether prospective aquifers could be used for geological storage of CO2 without compromising other activities. The potential risks to groundwater from the potential migration of CO2 and changes to groundwater properties that might be expected will also be discussed. Current data gaps include poor hydrogeochemical data coverage for the deeper aquifers and particularly limited data on trace metals and organics. A comparison with experiences learned from enhanced oil recovery using CO2 in North America and the CO2CRC's pilot CO2 injection project in Western Victoria will illustrate some of the unique differences and opportunities for geological storage of CO2 in Australia. Oral presentation at "Groundwater 2010" conference, 31 October - 4th November 2010, Canberra

Cool-water carbonate environments may be responsible for up to one third of the carbonate sediment produced on continental shelves, and are useful modern analogues for many geologically ancient deposits. The extensive southern margin of the Australian continent is recognised as the world's largest modern example of a high energy, cool-water carbonate depositional realm. A number of studies have suggested that Quaternary sediment production here is largely influenced by oceanography, and that wave abrasion strongly limits sediment accumulation. Therefore, in this region the outer-shelf, below the storm wave base, is thought to be the focus of sediment accumulation. The inner shelf is considered a zone of active sediment production due to the proliferation of carbonate secreting organisms, although few studies have investigated sediment production or accumulation in this energetic and dynamic environment. The Recherche Archipelago, which sits at the western margin of the Great Australian Bight (GAB), was examined to better understand Quaternary shelf evolution and the importance of this type of inner shelf as a carbonate 'factory'. Surficial sediments, video, multibeam sonar data, cores and shallow seismics were collected. The present seabed of the archipelago features extensive areas where flat-lying limestones sit over the often irregular granite basement. The Pleistocene erosional surface is overlain by a coarse bivalve and rhodolith dominated gravel lag. Significantly, there are extensive Holocene deposits, up to 7 m thick, throughout the archipelago, particularly in association with granite islands. These deposits comprise cross-bedded gravelly carbonate sands dominated by fragments of calcareous algae (rhodoliths), molluscs and bryozoans. In contrast, the inshore and coast is dominated by terrigenous sediment. Seismic profiles and preserved palaeo-shoreline features suggest that slow but episodic aggradation of marine sediment has occurred on the inner shelf over successive Quaternary sea level cycles, although there are also extensive areas of non-deposition. This accumulation is partly attributable to the sheltering effect of high-relief granitic outcrops and cementation of subaerially exposed carbonate sediments.

Multibeam sonar mapping, drill cores and underwater video data have confirmed the existence of a previously unknown reef province in the Gulf of Carpentaria, Australia. Seven reefs, comprised of coral limestone that support living corals have been mapped so far and as many as 50 other reefs may exist in the region. U/Th ages show that reef growth commenced shortly after limestone pedestals were submerged by rising sea level around 10.5 kyr BP, making them the oldest reefs known in Australia. Reef growth persisted for ~2.0 kyr but it had ceased at most locations by ~8.0 kyr BP. Measurements of reef growth rates (0.95 to 4 m kyr-1), indicate that the reefs were unable to keep pace with contemporaneous rapid sea level rise (>10 m kyr-1), which is consistent with a 'give up' reef growth history. Core samples from reef platforms demonstrate that Pleistocene limestone is exposed in depths of 27 and 30 m below present mean sea level. These depths represent regionally significant phases of reef growth during a prolonged sea level still stand. We conclude that the reefs are therefore mostly relict features, whose major phase of growth and development relates to an earlier, pre-Holocene sea level stillstand.