Bight Basin Research Articles

Investigating relationships between shoreline process regime, shelf-margin architecture, and deep-water sand delivery: Insights from the early post-rift Hammerhead shelf margin (Bight Basin, southern Australia)

Shelf margins represent a crucial area along source-to-sink systems where sediments are partitioned from the shelf to slope and basin-floor areas. Reconstructing the evolution of these depositional systems is key for interpreting the interplay between past accommodation and sediment supply, and sediment dispersal mechanisms into deep water. In the Bight Basin, on the southern margin of Australia, the Hammerhead shelf margin prograded during the Late Cretaceous following break-up between Australia and Antarctica. This understudied interval offers important insights into source-to-sink processes in a post-rift, greenhouse, high sediment supply setting. A dynamic stratigraphic approach using high-resolution 3D seismic data across the Hammerhead shelf margin has been used to quantitatively characterise 28 clinothems developed over ∼ 1.9 Myrs each with an average duration of ∼ 67,000 years. By applying a shallow-marine process-based classification to shorelines, alongside quantitative analysis of the architecture of their coeval deep-water deposits downdip, statistical relationships and clear links between shallow-marine processes, stratigraphic architecture, and deep-water sand delivery are revealed. A statistically significant relationship between fluvial dominated shorelines, high slope gradients, and mass-transport deposit development is demonstrated, as is a requirement for fluvial influence at the shoreline for the initiation of long run-out turbidite systems. These long run-out turbidite systems are interpreted to have been formed by repeated density flows which lead to greater sediment transfer efficiency and increased sediment supply. This research has direct application to improve prediction of reservoir locations within the Bight Basin for resource exploration and/or carbon sequestration and may also be applied to improve deep-water sediment predictability in other basins worldwide developed in similar tectonic and climatic settings.

Climatic and weathering conditions in southern high latitudes during the Turonian-Santonian interval: New insights from IODP Site U1512 (Bight Basin, Southern Australia)

A detailed study, based on a coupled mineralogical and geochemical approach, was done on IODP Site U1512 sediments (Bight Basin, Southern Australia). The mineral assemblages of the clay size fraction (<2 μm), obtained by X-ray diffraction, have been compared to inorganic (major and trace elements) geochemical signals, obtained from both whole rock and clay size fraction, and Sr/Nd isotopes, obtained from clay size fraction to determine climatic and weathering conditions during the Turonian to Santonian interval (∼94–84 Ma, Late Cretaceous) in the southern high latitude band. This study reveals that the mineral assemblages of clay fraction consist of R0-type illite/smectite mixed-layers (considered as smectites), illites, and kaolinites associated with opal-CT and clinoptilolites. Electron microscopy observations have demonstrated a negligible impact of burial diagenesis on clay assemblages while authigenesis contributions of clay minerals are limited and associated with minor volcanic contributions. Clay assemblages would be thus dominantly primary in origin. Based on Sr and Nd isotopic values, clay minerals are interpreted as the resulting from the weathering of rocks and pedogenic blankets of Eastern Australian Highlands and from the remobilization of older sediments from nearby uplifted basins. The chemical compositions of both whole rock and clay size fraction show a relative stability during the studied interval with two exceptions: during the Middle/Late Turonian times for Si, P and Ba and at the Turonian-Coniacian boundary for Ti, K and Zr, associated respectively to an increase of biogenic fraction and to an enhanced input of terrigenous material. The clay assemblages, dominated by smectites, show weak variations along the borehole reflecting stable climatic conditions during the 10-Myr Turonian-Santonian interval. The dominance of smectites, coupled to the absence of chlorite and the low proportions of illite, which are both considered as primary clay minerals, indicates a dominant warm/temperate and sub-humid climate during the studied interval. However, changes in weathering conditions occurred during the Turonian-Santonian interval in southern high latitude. The Early Turonian, characterized by high Chemical Index of Alteration values (>75), is followed by an interval of lower chemical weathering with enhanced biogenic silica during the Middle/Late Turonian. The slight increase of kaolinites, recorded during Coniacian and coeval with increase of Ti/Al and Zr/Al ratios, probably reflects an increase in coarser terrigenous inputs associated with a sea-level fall.

New insights on the gravity-driven deformation of late Albian – early Turonian stacked delta collapse systems in the Ceduna sub-basin, Bight Basin, southern margin of Australia

Gravitational collapse of delta systems is thin-skinned deformation due to sedimentary loading that induces spreading or sliding above a very weak detachment layer and typically consists of up-dip listric extensional faults linked to down-dip a compressional system of toe-thrusts. Detailed mapping and analysis of a modern high-quality, 3D depth-migrated seismic data set from the offshore Ceduna sub-basin, southern Australian margin has shown that gravitational collapse in this sub-basin occurred by multiple failure episodes. We show that the late Albian – early Turonian succession consists of two stacked collapse features; a previously unrecognised Albian–early Cenomanian system 1 and the overlying system 2 of the Cenomanian–early Turonian White Pointer delta of the Ceduna sub-basin.Collapse system 1 is located beneath the extensional domain of the Cenomanian – early Turonian White Pointer collapse system 2, which itself consists of three structural sub-domains that are classified based on fault architectures and geometries, and are named after their relative location as proximal, central, and distal sub-domain. Individual sub-domain consists of scoop-shaped listric normal faults linked downwards to thrust faults. The main faults of these sub-domains detach on multiple decollement surfaces of shales, presumably overpressured. This study proposes an investigation into the development of stacked and complex basinwards sequential formation of gravity collapse features in delta systems on passive margins. The research documents the complex structural styles and evolution of the late Albian – early Turonian stacked delta collapse systems and proposes a revised structural and stratigraphic framework of the Ceduna sub-basin. A four-dimensional (4D) tectonostratigraphic models for the evolution of the late Albian – early Turonian succession within the Ceduna sub-basin is also proposed.

3D seismic reflection constraints on the emplacement of mafic laccoliths and their role in shallow crustal magma transport: A case study from the Ceduna sub-basin, Great Australian Bight

Magma plumbing systems transport and store magma in the subsurface, as well as playing an important role in feeding eruptions and inducing overburden deformation. Whilst the important role of sills and dikes in these systems is well documented, the three-dimensional (3D) morphology of mafic laccoliths and their role in magma transport and eruption is less well understood. This is due to a paucity of well-documented examples from 3D seismic reflection data, and the limited exposure provided by field examples. Here we use 3D seismic reflection data to document for the morphology and emplacement of a multi-lobed laccolith of inferred mafic composition (the Hammerhead Laccolith (HHL)). This middle Eocene-age intrusion was emplaced at <320 m beneath the paleosurface within the intraplate Bight Basin Igneous Complex located offshore southern Australia. The HHL is composed of multiple convex-upwards, flat-based, laccolith-shaped lobes between 5 and 270 m thick and 2–14 km long. Inflation of the lobes resulted in the formation of a ≤150 m high, compound forced fold at the paleosurface, whose morphology mirrors that of the underlying laccolith. Subsequent drainage of magma from the HHL to feed small-volume lava flows resulted in the formation of collapse faults which focus on the margin of a magma reservoir, similar to those associated with calderas. To the best of our knowledge, this is the first study using 3D seismic reflection data to document the morphology of a shallow mafic laccolith and its relationship to its erupted products, whilst the HHL is notable for possessing characteristics of both classic laccoliths and shallowly emplaced, multi-lobate mafic sills. We highlight the important role of laccoliths in magma plumbing systems, and their potential for inducing both brittle and ductile host-rock deformation. Finally, our detailed mapping of the HHL and its associated forced fold provides a valuable exemplar that may assist the identification of mafic laccoliths using 3D seismic reflection data in energy-rich sedimentary basins.

Long-term and short-term liquid fuel security in Australia – What role for the Great Australian Bight?

This article hypothesises that Australia's long-term and short-term liquid fuel security will not be improved by the exploitation of petroleum resources in the frontier Great Australian Bight (GAB) basin. This analysis demonstrates that significant challenges arise regarding Australia's energy policy which promotes the development of the GAB, due to corporate imperatives, global economic conditions, and localised challenges such as depth, distance, and prevailing weather. Furthermore, the closure of over half of Australia's refineries has placed structural demands on Australian refinery capacity. Therefore, even if oil were to be found in the GAB and developed, Australia's current policy approaches to downstream refining capacity mean that any oil discovered would likely be exported. Short term, such policy frameworks would cause a significant deficit in Australia's International Energy Agency (IEA) stockpile capacity, requiring the leasing of oil stocks from the US Strategic Petroleum Reserve to meet IEA demands. Given these insecurities, this article postulates that Australia's energy security policy, and particularly its liquid fuel security policy, should be revisited, revised, and reformed to ensure liquid fuel security for all sectors of the economy.

The characterisation and provenance of crude oils stranded on the South Australian coastline. Part II: Potential parent petroleum systems

In 2014–2016 more than 600 specimens of semi-solid crude oil were recovered from 30 ocean beaches along the coastline of South Australia, as part of the recently completed Great Australian Bight Research Program. All are believed to be products of submarine oil seepage. Their source-specific biomarker signatures provide the basis for their assignment to sixteen oil families, some previously unrecognised. Two of these families (asphaltite and asphaltic tar) likely originated from Cretaceous marine source rocks in the offshore Bight Basin. The others comprise waxy oils of lacustrine, fluvio-deltaic and marine source affinity. Their biomarker characteristics do not match those of any Australian crude oil. However, they are strikingly similar to those of oils found in Cenozoic and Mesozoic basins throughout the Indonesian Archipelago and elsewhere in Southeast Asia.

Post-rift magmatism and hydrothermal activity in the central offshore Otway Basin and implications for igneous plumbing systems

Spatially and volumetrically extensive igneous rocks are exposed onshore in southeastern Australia and provide important information about the Cenozoic evolution of the region. However, the morphology and distribution of igneous rocks are still not well constrained in the ~70 km wide continental shelf of the central Otway Basin. Within this region, we present seismo-geomorphological characteristics of 30 volcanoes and 27 hydrothermal vents, as well as several lava flows and magmatic sills, based on 2D and 3D seismic reflection data. Hydrothermal vents formed mainly during the mid-Eocene, while igneous complexes developed between the mid-Eocene to late Miocene, both post-dating continental break-up in the basin. The distribution and morphology of igneous rocks and hydrothermal vents is influenced by rift-related faults, as most of these features are linked to the underlying faults by near-vertical zones of disruption and are elongated in an NW-SE direction that parallels the primary fault trend. Magma for volcanoes and heat sources for hydrothermal vents within the study area are likely supplied by deep-sourced dykes, as magmatic sills only have a scattered distribution in this region. This implies a dyke-dominated plumbing system for magmatism in the Otway Basin. In comparison, the nearby Bight Basin, which has a thin basement but a relatively thick sedimentary sequence, displays magma propogation more prone to forming igneous sills. There are significant lithological and tectonic similarities between the Otway and Bight basins; thus, we propose that a relatively thin sedimentary sequence over a thick basement in the Otway Basin produced sufficient magma pressure for a predominantly dyke-dictated igneous plumbing system. This work highlights the critical role of basin structures, such as the thickness of basement and overlying strata, in the control of igneous plumbing styles and the distribution of post-rift igneous complexes along magma-poor continental margins. Our work, therefore, aids the estimation of different magmatic components within the sedimentary basins and facilitates the understanding of their diverse impacts on the exploration of frontier basins. This result also provides some constraints on the prediction of possible future eruption centres within active volcanic fields.

Fault controlled focused fluid flow in the Ceduna Sub-Basin, offshore South Australia; evidence from 3D seismic reflection data

This paper investigates the spatial link between fluid escape pipes and underlying faults in the Ceduna Sub-basin. The Ceduna Sub-basin is the largest depocentre of the Mesozoic Bight Basin that formed as a result of rifting between Australia and Antarctica beginning in the Middle Jurassic. The study area is in the northwest of the Ceduna Sub-basin, within which sets of fluid escape pipes were identified in 3D seismic data along the study area's south-western edge. Set 1 pipes are observed within the siliciclastic Wobbegong and carbonate Dugong Supersequences of Paleocene to Pleistocene age, while Set 2 pipes are observed within the top 400 ms (~485 m) of Santonian to Maastrichtian deltaic Hammerhead Supersequence. Dimensions of Set 1 and Set 2 pipes range from 50 ms (60 m) to 400 ms (485 m) in height and 70–180 m in width. Roots of the fluid escape features overlie the upper tips of normal faults within the Hammerhead Supersequence. The spatial distribution of fluid escape features and predominant underlying faulting style within the Hammerhead Supersequence varies within the study area; randomly distributed fluid escape features overlie polygonal faults to the northeast, whilst linear clusters of fluid escape features overlie normal faults in the southwest. Spatial statistics analysis of fluid escape features and underlying faults support the interpretation that the faults have acted as fluid migration pathways. The polygonal and normal faults developed in the Late Cretaceous and are interpreted to have transmitted fluids probably sourced from the Lower-Upper Cretaceous Blue Whale Supersequence, and the Late Cretaceous White Pointer and Tiger Supersequences. These supersequences have been identified as containing potential source rocks, and geochemical analysis them suggest that hydrocarbon generation initiated as early as the mid-Cretaceous (~95 Ma) and continued through to the Cenozoic (~15 Ma).

Geochemical characterisation of migrated hydrocarbons reveals insights into the source rocks of the Bight Basin

The Ceduna Sub-basin in the offshore Bight Basin of southern Australia is under-explored with no significant hydrocarbon discoveries. Validation of potential petroleum systems is limited to geochemical data from minor shows. Fluid inclusion (FI) analysis was used to investigate the possibility of hydrocarbon migration throughout the penetrated sedimentary sections. Low abundances of FIs were identified in sandstones from several intervals in the Gnarlyknots-1A and Greenly-1 wells, indicative of oils trapped along migration pathways. The trace quantities of oils contained within FIs were successfully analysed by the molecular composition of oil inclusions (MCI) technique. The Gnarlyknots-1A FI oil provides the first geochemical characterisation of oil generated in the central Ceduna Sub-basin. It originated from source rocks that contain both terrestrial and algal organic matter deposited in a sub-oxic environment. The biomarker signature of the Greenly-1 FI oil reflects its derivation from a fluvial-deltaic, clay-rich source rock containing predominantly terrestrial higher plant detritus deposited under oxic conditions. These results reveal that several Lower to Upper Cretaceous source rock intervals in the Tiger, Blue Whale and White Pointer supersequences have potentially generated hydrocarbons, a finding which significantly enhances the petroleum prospectivity of the Bight Basin.

The sedimentological evolution and petroleum potential of a very thick Upper Cretaceous marine mudstone succession from the southern high latitudes—a case study from the Bight Basin, Australia

During IODP Expedition 369, a 690 m thick succession of silty claystone spanning the early Turonian to the late Santonian was encountered at Site U1512 in the Bight Basin, offshore southern Australia. Stacking patterns, sedimentary facies and palynological assemblages reveal that the succession was rapidly deposited with hyperpycnal and hypopycnal flows in a marine prodelta setting, which was subject to basin restriction. The dominance of clay-rich facies and phytoclasts in the succession was likely the result of a major river system delivering a high sediment load into the Bight Basin when a warm, wet climate prevailed. A combination of high sedimentation rates (19–272 m/Myr) and accelerated subsidence prevented the delta from rapidly prograding into more distal regions of the basin. The complete Turonian to Santonian mudstone succession yields low total organic carbon (~1 wt%) and Type IV kerogens. However, palynofacies assemblages become progressively marine in character and total organic carbon values vary between 1 and 1.5 wt% with depth. This may indicate that the base of the hole at Site U1512 was close to potential organic-rich black shales associated with Ocean Anoxic Event 2. Low amplitude and irregular reflections on seismic data and disparities between biostratigraphic zonations suggest the upper 350 m of the Turonian to Santonian succession may represent a mass movement that happened during the Pleistocene. This study reveals that Site U1512 material likely represents a near-stratigraphically complete marine mudstone succession from high paleolatitudes, as well as the only depositional record that was fully cored from the Bight Basin.

Constraining the timing and evolution of hydrocarbon migration in the Bight Basin

Abstract Micron-sized fluid inclusions within quartz grains from Late Cretaceous sandstones, recovered from drill cuttings from four wells in the Great Australian Bight, contain palaeo-fluids such as brine, black oil, light oil, gas condensate, and N2 and CO2-rich hydrocarbon gas, providing evidence of multiple palaeo-hydrocarbon migration in the Bight Basin. A series of microscopic, spectroscopic and thermometric characterisation techniques were applied to the petroleum fluid inclusions in Gnarlyknots-1A, Potoroo-1, Greenly-1 and Duntroon-1 in order to estimate fluid types, palaeo-pressures and palaeo-temperatures (PT) of hydrocarbon entrapment, and to provide constraints on the age of petroleum migration in the central and eastern parts of the Ceduna Sub-basin. The Coniacian interval (Tiger Supersequence) in Gnarlyknots-1A trapped a variety of petroleum fluid types (black oil, light oil and gas-condensate) over an extended period. The earliest phase of oil entrapment recorded in quartz grains took place at a minimum of 58 °C as light oil and, together with pressure, constraints this to the end of the Cretaceous (circa 75 Ma). Source rocks capable of light oil generation at this time occur in the Blue Whale Supersequence. Petroleum compositions evolved toward more gas-rich fluids during the Cenozoic. This, together with some late oil constrained to Miocene age migration, implies generation from at least two source rocks. Source rocks capable of generating gas at this time are interpreted to be in the Blue Whale and White Pointer supersequences, while late oil may have been expelled from the Tiger Supersequence in the central basin depocentre. The palaeo-PT evolution recorded by the fluid inclusions in Gnarlyknots-1A closely follows the modelled burial curve, suggesting that the petroleum fluids were in thermal equilibrium with the rock. In contrast, the oil and gas inclusion assemblages found at Potoroo-1, Greenly-1 and Duntroon-1 were trapped later, with some at temperatures higher than the formation temperatures predicted from burial models. Whilst there are several potential mechanisms that explain these observations, we suggest that these wells are located near major extensional faults that have the potential for conducting fluids upwards. Additionally, gases such as CO2 and N2 associated with hydrocarbons possibly relate to mixing with mantle-derived fluids, which may also have contributed to the aforementioned thermal anomalies.

Revealing oil migration in the frontier Bight Basin, Australia

Abstract The Bight Basin along the southern margin of Australia represents one of Australia's most prospective deep-water frontier hydrocarbon exploration regions, however its 15 km-thick sedimentary succession remains largely untested. Whilst there is some evidence of oil from shows, fluid inclusions and natural strandings of asphaltite along the basin margin, it is unclear if any hydrocarbons were generated in the deep-water Ceduna Sub-basin. Fluid inclusions offer a unique method to test for petroleum migration that would otherwise remain hidden and a study, using CSIRO's Grains with Oil Inclusion (GOI™) technique, was undertaken on Gnarlyknots-1A along with other historic exploration wells. With the exception of Jerboa-1, in the Eyre Sub-basin, the GOI results (

The Bight Basin, evolution and prospectivity II; seismic, structure and balanced sections

The Bight Basin, evolution and prospectivity II; seismic, structure and balanced sections

New insights on the Upper Cretaceous Tiger Supersequence of the Bight Basin from International Ocean Discovery Hole U1512

The Bight Basin is considered to be one of Australia’s most prospective petroleum regions. However, the sparsity of geologic data means that potential petroleum plays in the basin have yet to be fi...

Seismic evidence for seal and reservoir in the Late Cretaceous Ceduna Delta, Great Australian Bight

The Ceduna Sub-Basin contains a large and underexplored Jurassic - Cretaceous rift, located within the Great Australian Bight Basin. Limited exploration to date has targeted the Late Cretaceous Ham...

A second type of highly asphaltic crude oil seepage stranded on the South Australian coastline

Abstract Strandings of semi-solid to solid asphaltic bitumen along the coastline of South Australia have been reported as far back as the late 1800s. Hitherto only a single variety, now referred to as asphaltite, has been attributed to seepage from the nearby Bight Basin. The geochemistry of the asphaltites suggest they were derived from a marine source rock deposited under anoxic or euxinic conditions, most likely a Cretaceous ocean anoxic event, and were generated within the early/main oil window. Here we identify a new type of semi-solid asphaltic bitumen collected following a severe storm event in 2016. Termed asphaltic tars, these viscous oils bear a strong geochemical resemblance to the asphaltites. Both oil types have high asphaltene contents, identical n-alkane carbon isotopic profiles and near identical source-specific sterane distributions. However, several notable geochemical variations can distinguish these new strandings from the asphaltites. The most notable of these differences include heavier bulk sulphur isotopic composition, extremely high abundances of Re and Os with distinct 187Re/188Os and 187Os/188Os values and thermal maturity parameters consistent with generation in the late oil window. The differences in sulphur isotopic composition and Re-Os systematics could be considered evidence that despite their other source-specific similarities, the asphaltic tars originated from a different source rock. However, alteration of these two parameters can occur due to thermochemical sulphate reduction. Conclusive identification of this alteration process typically relies on further diagnostic parameters which are unfortunately not available in the case of coastal oil strandings. This introduces uncertainty to the correlation of these two types of asphaltic oil. In either scenario, the similarities between these two types of oil suggest their source rock(s) contained highly comparable organic matter inputs. We therefore attribute the origin of these new asphaltic tar strandings to natural seepage from the offshore Bight Basin.

Remotely constraining the temporal evolution of offshore oil systems

An understanding of the temporal evolution of a petroleum system is fundamental to interpreting where hydrocarbons may be trapped in the subsurface. However, traditional exploration methods provide few absolute constraints on the timing of petroleum generation. Here we show that 187Re/187Os geochronology may be applied to natural crude oil seepage to determine when petroleum generation occurred in offshore sedimentary basins. Using asphaltites collected from the South Australian coastline, our determined Re-Os age (68 ± 15 million years ago) is consistent with their derivation from a Late Cretaceous source rock in the nearby Bight Basin, an interpretation similarly favoured by source-specific biomarker constraints. Furthermore, the calculated initial 187Os/188Os composition of the asphaltites, a value inherited from the source rock at the time of oil generation, suggests that the source rock represents the later stage of Oceanic Anoxic Event 2. Our results demonstrate a new approach to identifying the origin of crude oils encountered in coastal environments by providing direct constraints on the timing of petroleum generation and potential source rock intervals in poorly characterised offshore sedimentary basins prior to exploratory drilling.

New insights on Upper Cretaceous stratigraphy and sedimentology of the Bight Basin, Australia from IODP Site U1512

The Bight Basin, on Australia’s southern margin, is one of the world's most prospective deepwater frontier basins. The 10 offshore wells drilled in the basin had limited success or yielded disappointing results. There has been a strong dependence on seismic data to interpret stratal ages and the regional depositional history because of the limited number of wells, which are all in the more proximal region. In October 2017, the International Ocean Discovery Program Expedition 369 drilled a hole at Site U1512 that straddled the Australian Geological Survey Organisation Survey s065 line 06 on the continental slope, ~67 km south-east of the Jerboa 1 well. The recovered core is the most extensive lithological dataset acquired from the basin and consists of a 10 m thick Pleistocene ooze overlying a 690 m succession of Turonian–Santonian strata. The Cretaceous strata consist of silty claystone with a few thin beds of glauconitic and sideritic sandstone (&amp;lt;32 cm thick). The Tiger Supersequence is substantially thicker than had been anticipated. Preliminary palynofacies analysis indicates a prevailing dysoxic marine environment, with the assemblage dominated by phytoclasts (40–90% of the assemblage). This may have been a consequence of high rates of freshwater runoff into the restricted basin. Rapid sedimentation rates (up to 260 m/Myr), the silt content (2–25%) and the palynofacies suggest the strata were deposited primarily by hyperpycnal and hypopycnal flows. These new datasets will provide a means to re-evaluate the palaeogeography of the basin and its resource potential.

The Bight Basin: a tale of three deltaic megasequences

The Bight Basin developed during the Jurassic and Cretaceous in response to repeated periods of extension and thermal cooling leading up to, and following, the onset of seafloor spreading between present-day Australia and Antarctica. The bulk of the resulting sedimentary basin fill includes up to 15 km of Middle Jurassic–recent sediments comprising three deltaic megasequences: White Pointer, Tiger and Hammerhead. High quality seismic 3D data have enabled detailed mapping of the megasequences, evaluation of the nature of infill and assessment of implications for hydrocarbon prospectivity. The Cenomanian White Pointer Megasequence succeeds a period of widespread mudstone deposition in the basin. Growth fault and complex styles of deposition are a function of the high accumulation rates. The growth faulting gradually ceased towards the top of the megasequence allowing for the delta to advance into the basin. Renewed tectonic activity during accumulation of the Turonian–Santonian Tiger Megasequence led to formation of the Outer High trend in the basin. Basal transgressive mudstones during early Tiger deposition are succeeded by a set of progradational–retrogradational units. The base of the Santonian–Maastrichtian Hammerhead Megasequence records widespread subaerial erosion and formation of incised valleys depicting a distinct drop in relative sea level most likely in response to the start of the Australian and Antarctic breakup. This event is followed by a widespread regional flooding event, before the Hammerhead delta built out, first in a highly progradational and subsequently in a more aggradational style. At the end of the sequence, the delta steps back due to decreasing sediment input. Detailed seismic stratigraphic mapping has improved the understanding of the gross depositional environment developed throughout the three megasequences, and importantly also helped identification of the main reservoir fairways and their distribution through time.

Reservoir Modelling, Structural History and Volumetrics of the Jerboa Area, Eyre Sub-Basin

The Eyre Sub-basin is situated in the Bight Basin in the Southern Margin. It covers an area of approximately 8,000km2. It is one of several half-graben in the Bight Basin that formed during the initial rifting between Australia and Antarctica. Sediments have accumulated in these depocentres from the Middle Jurassic to present. The initial environments of deposition were a series of fluvial, lacustrine and floodplain sequences before becoming open marine environments in the Cretaceous to Recent. Structural, Property and Reservoir Facies Models were created using the seismic interpretation and well logs from Jerboa-1. The models suggest the sequences on the Jerboa high thicken down-flank away from the well. Source rocks capable of generating hydrocarbons complete with oil stains and oil inclusions were found in the Middle Jurassic-Early Cretaceous non-marine sequences. There are several potential stacked reservoir sections in the sequences from the Middle Jurassic to the Berriasian. These have high porosities between 14-30% and clean channel sands as indicated on the Jerboa-1 logs. The Valanginian-Barremian shale dominated sequence is mapped as laterally extensive and has the best potential of sealing the underlying reservoirs. Across fault seal assessment indicates the fault throws potentially seal the reservoirs, although fault reactivation in the Late Cretaceous resulted in a collapsed graben complex along the apex of the Jerboa high and is potentially the cause of the hydrocarbons escaping. Sections in the Eyre Sub-basin that have stacked channel reservoirs, source rocks with adequate burial and traps that have not been breached have the potential to hold an accumulation of hydrocarbons. The Eyre Sub-basin is a working petroleum system.