Western Officer Basin Research Articles

Using potential field data to map salt distribution in the Western Officer Basin, Western Australia.

SUMMARY The Neoproterozoic western Officer Basin has a total fill of up to 8 km and a depositional history with similarities to other Australian basins, particularly the Amadeus Basin. Exploration has been limited due to the size and remoteness of the basin; therefore potential field data can be a useful and cost-effective tool to assess petroleum prospectivity. Salt distribution and mobilisation in the Officer Basin has been significantly underestimated due to a lack of quality seismic data. This study uses satellite, digital terrain, magnetic, gravity and seismic data to show the existence of mobilised salt much further west than previously suggested, with significant implications for future exploration in the region.

Cenozoic salt tectonics in the Officer Basin, Western Australia: implications for hydrocarbon exploration

The Neoproterozoic western Officer Basin has a total sedimentary fill of up to 8 km and a depositional history with similarities to other central Australian basins, particularly the Amadeus Basin. The size and remoteness of the basin has traditionally been an impediment to exploration, and only sparse seismic and well data are available. In such areas, potential field data can be a powerful exploration tool to assess petroleum prospectively. Salt distribution and mobilisation in the Officer Basin is poorly understood and has been significantly under-estimated due to a lack of quality seismic data. Examination of the existing aeromagnetic, gravity and seismic data, along with satellite and Shuttle Radar Topography Mission (SRTM) data, indicate that surface and shallow salt is abundant in the northern and central parts of the basin. Remobilisation of salt is greatest in the eastern part of the study area, decreasing towards the west, although the extent of salt occurrence to the west is unclear. Salt diapirs occur along structural trends; east to west in the northeastern (Gibson) part of the basin, and northwest to southeast in the central (Yowalga) area. Neotectonic features such as surface lineaments and recent earthquake data suggest that minor tectonic reactivation is occurring in the present day, and is consistent with a present-day stress orientation of approximately 095°. Miocene to recent stress orientations suggest that structures in the Gibson area may have been reactivated as right lateral faults, whereas those in the Yowalga area are reactivated as left lateral faults. Potential trap styles in the western Officer Basin include structural plays related to salt movement, such as drape folds, diaper overhangs, and thrusts. Late-stage movement of salt must, therefore, be considered when assessing the timing of migration pathways and possible seal breach. An improved understanding of the extent of salt in the Officer Basin, and the degree of reactivation during the Cenozoic, is vital for successful exploration in the region. Acquisition of high-resolution magnetic and gravity data would be a cost-effective exploration tool for better definition of salt and associated hydrocarbon traps.

Structural and stratigraphic architecture of Australia's frontier onshore sedimentary basins: the Western Officer and Southern Carnarvon basins, Western Australia

During the past five years, the Onshore Energy Security Program, funded by the Australian Government and conducted by Geoscience Australia, in conjunction with state and territory geological surveys, has acquired deep seismic reflection data across several frontier sedimentary basins to stimulate petroleum exploration in onshore Australia. This extended abstract presents data from two seismic lines collected in Western Australia in 2011. The 487 km long Yilgarn-Officer-Musgrave (YOM) seismic line crossed the western Officer Basin in Western Australia, and the 259 km long, Southern Carnarvon Seismic line crossed the Byro Sub-basin of the Southern Carnarvon Basin. The YOM survey imaged the Neoproterozoic to Devonian western Officer Basin, one of Australia's underexplored sedimentary basins with hydrocarbon potential. The survey data will also provide geoscientific knowledge on the architecture of Australia's crust and the relationship between the eastern Yilgarn Craton and the Musgrave Province. The Southern Carnarvon survey imaged the onshore section of the Ordovician to Permian Carnarvon Basin, which offshore is one of Australia's premier petroleum-producing provinces. The Byro Sub-basin is an underexplored depocentre with the potential for both hydrocarbon and geothermal energy. Where the seismic traverse crossed the Byro Sub-basin it imaged two relatively thick half graben, on west dipping bounding faults. Structural and sequence stratigraphic interpretations of the two seismic lines are presented in this extended abstract.

Vines 1 revisited: are older Neoproterozoic glacial deposits preserved in Western Australia?

The stratigraphy in Vines 1, a 2017.5 m-deep cored stratigraphic hole drilled by the Geological Survey of Western Australia in 2001 near the Western Australian – South Australian border, has been reinterpreted with implications for the Neoproterozoic to Cambrian geological history of the Officer Basin. A previous interpretation considered the intersected succession as a conformable stratigraphic package, the Vines Formation. An assemblage of palynomorphs, found throughout the hole and previously used to infer an age of no older than earliest Cambrian, is now thought to consist of contaminants. An older assemblage, which is considered to be reworked and inherited from underlying rocks, provides a new maximum age constraint of mid-Neoproterozoic. Based on sedimentological interpretations and comparisons with other drillholes in the western Officer Basin, and the succession in the eastern Officer Basin, the Vines 1 succession is reinterpreted as four discrete sedimentary packages, the Pirrilyungka (new name), Wahlgu, Lungkarta and Vines (redefined) Formations, in ascending order. The Pirrilyungka and Wahlgu Formations include glacigenic sediments and may correlate with similar glacial successions in Supersequences 2 and 3 (mid to late Cryogenian) of the Centralian Superbasin, and the Sturt Tillite and Elatina Formation and their equivalents in the Adelaide Rift Complex of South Australia, respectively. The eolian Lungkarta Formation and fluvial Vines Formation are considered, on regional evidence, to be most likely of Ediacaran to earliest Cambrian age.

Palaeomagnetic results from the Lancer 1 stratigraphic drillhole, Officer Basin, Western Australia, and implications for Rodinia reconstructions

A partial record of the positions of Australia during Middle to Late Neoproterozoic time is provided by palaeomagnetic results for samples from the Lancer 1 stratigraphic drillhole in Western Australia. Lancer 1 was drilled vertically to 1501 m, through essentially horizontal Neoproterozoic strata of the western Officer Basin. We studied 123 samples from 28 intervals of drillcore which were oriented by matching features (fractures, cross-beds, etc.) in the core with oriented acoustic scanner images of the drillhole walls. Three new palaeopoles are reported for red mudstones and sandstones (redbeds) of the Browne (44.5°N, 141.7°E, dp = 5.1°, dm = 9.0°), Hussar (62.2°N, 85.8°E, dp = 7.3°, dm = 14.6°), and Kanpa (74.0°N, 128.8°E, dp = 7.7°, dm = 14.8°) Formations of the ca 830 – 720 Ma Buldya Group (Supersequence 1), which exhibit stable, two-polarity magnetisations carried by fine-grained hematite and magnetite. The overlying ca 610 – 590 Ma Wahlgu Formation glaciogenic diamictite (Supersequence 3) yielded dispersed directions and an imprecise palaeopole that overlaps results from the glaciogenic Elatina Formation and other Late Neoproterozoic rock units. The results help to elaborate the Middle to Late Neoproterozoic apparent polar wander path for Australia and indicate, in agreement with palaeoclimatic data and previous palaeomagnetic studies, that the continent was slow-moving and occupied low latitudes at this time. Assuming that Australia and Laurentia were still joined at ca 780 Ma, comparison of the new Hussar Formation palaeopole with coeval Laurentian data favours AUSMEX, rather than SWEAT or AUSWUS, as the most likely configuration of these two continents in Rodinia. This preliminary study of Lancer 1 demonstrates the utility of acoustic scanner logs for orienting drillcores, as well as the scope for additional sampling and palaeomagnetic studies of Lancer 1, and other oriented drillcores, to yield a more continuous record of Australia's past motions and to provide magnetostratigraphic data for enhancing inter-basin correlations.

Palaeoclimate implications from deep drilling of Neoproterozoic strata in the Officer Basin and Adelaide Rift Complex of Australia; a marine record of wet-based glaciers

Two prominent episodes of Neoproterozoic glaciation in Australia are recorded in the Cryogenian (Sturtian < 725 Ma) and during the Ediacaran (Elatina < c. 580 Ma). Deep stratigraphic drilling and coring of Sturtian and Elatina glacial strata within the Adelaide Rift Complex (Blinman 2), the eastern Officer Basin (Nicholson 2) and the western Officer Basin (Vines 1 and Empress 1/1A) permits detailed sedimentary logging of lithofacies in more than 3000 m of continuous drill core. Glacial intervals in widely spaced drill sites show a commonality of facies types consisting of diamictites intimately interbedded with graded conglomerates, sandstones and siltstones that were deposited by subaqueous sediment gravity flows below wave base. Direct evidence for a glacial source of sediment is provided by striated and outsized clasts in fine-grained laminated facies. Soft-sediment deformation structures produced by downslope slumping, together with intraclasts of reworked sediment, are common. Facies suggest a depositional setting such as that found on submarine fans within Pleistocene glaciated basins strongly influenced by floating ice and the delivery of poorly sorted debris by the rainout of ice rafted debris, fall out of fine-grained suspended sediment and episodic downslope resedimentation. The large volume and great thickness of these subaqueous glacial facies within the Adelaide Rift Complex and Officer Basin indicates that Neoproterozoic glaciers in Australia delivered large fluxes of meltwater and sediment to offshore basins in similar fashion to wet-based glaciers typical of younger Phanerozoic glaciations. Evidence of meltwaters as a key agent of transportation is difficult to reconcile with climate models of a fully permafrozen planet where the hydrological cycle was shut down by extreme cold and normal sedimentary processes were suspended.

Salt tectonics in the Officer Basin: implications for trap formation and petroleum exploration

The Neoproterozoic Officer Basin in Western Australia is an inland frontier area that covers about 300 000 km² in Western Australia and 225 000 km² in South Australia. The present-day structural?stratigraphic framework is dominated by salt deformation and associated features. In the central western Officer Basin, approximately 6500 km of 2D seismic data were recently reinterpreted and integrated with potential field regional datasets. This has led to improved understanding of the halokinetic evolution and the petroleum potential of the region, summarised herein. Compressional processes, associated with tectonism in the adjacent Paterson Orogen, are probably the key mechanism initiating multi-phase mobilisation of a thick halite-dominated sequence, the Browne Formation, low in the succession. Significant salt redistribution resulted in thickness variations of the Browne Formation and the overlying section, and a variety of halokinetic structures. Prospective structures caused by salt tectonics include drape folds with a potential for multiple pay, thrustrelated anticlinal features, combined traps at diapir flanks and enhanced porosity traps. Most of these traps were in place before the main phases of hydrocarbon generation and could form attractive petroleum exploration targets.

Discussion on mantle plume uplift in the sedimentary record: origin of kilometre-deep canyons within late Neoproterozoic successions, South Australia

Scientific editing by Nick Rogers. Nicholas Christie-Blick writes: Kilometre-deep buried canyons in the upper Neoproterozoic successions of South Australia are unusual by any standard, and any viable explanation for their origin will probably involve an unusual set of circumstances. ⇓Williams & Gostin (2000) review published geological constraints and competing hypotheses, and suggest a role for uplift and crustal deformation associated with the passage of a mantle plume. On the basis of available evidence, I continue to favour an alternative explanation—that they are due to the short-lived evaporative drawdown of sea level in a marine embayment that became temporarily isolated from the global ocean (⇓von der Borch et al. 1989; ⇓Christie-Blick et al. 1990; ⇓Christie-Blick 1998). ### Spatial and temporal distribution of erosion Canyons more than 1 km deep in the Wonoka Formation of the Adelaide fold belt and up to 700 m deep in the subsurface of the eastern Officer basin are reasonably interpreted as coeval and of common origin, with a length scale of order 1000 km. Comparable erosional features are found nowhere else in Australia, even in the Kimberley region and western Officer basin (⇓Coats & Preiss 1980; ⇓Walter et al. 1994, ⇓1995), where flood basalts of possible plume origin are widespread (⇓Bultitude 1976; ⇓Jackson & van de Graaff 1981). ⇓Bender (2000) independently noted the distribution of late Neoproterozoic to early Palaeozoic magmatism in Western Australia, and analyzed the possible role of plume-related uplift and erosional thinning of the crust in early Palaeozoic subsidence of the Canning basin. However, that basin is some 2000 km to the northwest of the Adelaide fold belt, where the canyons are best known. A rather thornier problem is that in the Adelaide fold belt prominent erosion surfaces are present at three other horizons above the level of Marinoan glaciation and below Cambrian strata. …

Mid-Neoproterozoic (∼830–750 Ma) isotope stratigraphy of Australia and global correlation

Carbon and strontium isotope studies of numerous drillcore and outcrop sections within individual Australian Neoproterozoic basins, in a tectonic and sedimentologic framework, have allowed the construction of isotopic records for the interval 830–750 Ma. This, combined with the discovery of a restricted stratigraphic range for the acritarch Cerebrosphaera buickii in mid-Neoproterozoic strata of Australia, makes possible correlation with successions in Canada, Namibia and Spitsbergen. That part of the mid-Neoproterozoic succession in Spitsbergen which contains C. buickii was previously considered younger than the Sturtian glacial epoch, but this study has indicated that the Akademikerbreen Group is older than that. A correlation scheme is proposed along five tie lines, and is based on a correlation between Australia and Canada at ∼830 Ma, and Australia, Spitsbergen and Namibia at ∼760 Ma. The lowest ever recorded seawater 87Sr/ 86Sr ratios in the upper Shaler Supergroup of Canada (0.70561–0.70670) compare with ratios in evaporites of the ∼830 Ma Bitter Springs Formation, Amadeus Basin, Australia (0.70568–0.70720). Comparable δ 13C carb records support a correlation. At about 760 Ma, a correlation is made between the heaviest mid-Neoproterozoic δ 13C carb values of 7.2‰ (upper Burra Group, Adelaide Rift Complex) and 8.1‰ (Kanpa Formation, western Officer Basin) in Australia, 8.5‰ in Spitsbergen (Backlundtoppen Formation, Akademikerbreen Group), and 8.5‰ in Namibia (Ombombo Subgroup, Congo Craton). The stratigraphic range of C. buickii in Australia and Spitsbergen supports this correlation. A compilation of δ 13C carb and 87Sr/ 86Sr records between about 830 and 750 Ma from Australia, Canada, Namibia and Spitsbergen reveals numerous negative excursions, which are not associated with preserved glacial deposits. It is necessary, therefore, to consider additional models to the ‘Snowball Earth’ to explain the large and rapid carbon isotopic excursions.

Mid-Neoproterozoic biostratigraphy and isotope stratigraphy in Australia

Palynomorph and stromatolite assemblages from mid-Neoproterozoic strata of Australia appear highly significant when collated with results obtained from isotope stratigraphy, and allow correlation across the Centralian Superbasin and Adelaide Rift Complex. The majority of palynomorphs from the lower mid-Neoproterozoic (∼830 Ma) have limited biostratigraphic value, although a few apparently have restricted ranges. Carbonates at this stratigraphic level contain the distinctive stromatolite Acaciella australica and other taxa of the Acaciella australica Stromatolite Assemblage. These carbonates have δ 13C carb values of between −4 and 0‰, and associated anhydrite has 87Sr/ 86Sr ratios of 0.7069. These depleted δ 13C carb values are immediately preceded by values of 6.1‰ in the western Officer and Amadeus Basins of the Centralian Superbasin. Similar C and Sr isotopic features are seen in the similarly aged upper Shaler Supergroup of Canada. Upper mid-Neoproterozoic (∼760 Ma) carbonates contain the distinctive stromatolite Baicalia burra and other stromatolites of the Baicalia burra Stromatolite Assemblage. Siliciclastic horizons in the same interval contain the acritarch Cerebrosphaera buickii, and have δ 13C carb values of between 0 and 8‰. The highest mid-Neoproterozoic δ 13C carb values of between 6.7 and 8.1‰ occur within the stratigraphic range of C. buickii in Australia. In the western Officer Basin and Adelaide Rift Complex the mid-Neoproterozoic δ 13C carb peaks are preceded by values of ca. 0‰. The apparently restricted range of C. buickii in Australia suggests the need for reinterpretation of the stratigraphic position of its occurrence in the Svanbergfjellet Formation, Akademikerbreen Group, Spitsbergen, which was previously considered a post-Sturtian-glacial deposit.

Microfossils from Neoproterozoic Supersequence 1 of the Officer Basin, Western Australia

The discovery of a rich assemblage of microfossils from the Neoproterozoic western Officer Basin (Centralian Superbasin) provides a more complete understanding of the biostratigraphy of this Basin. The microfossils are found in Supersequence 1 (∼800 Ma) in Western Australia. The assemblages are comprised of acritarchs and cyanobacteria isolated by acid maceration from siliciclastics of the Browne (Madley), Hussar, Kanpa and Steptoe Formations. The distinctive acritarchs Cerebrosphaera buickii, Satka colonialica, Stictosphaeridium sinapticuliferum and Pterospermopsimorpha insolita are of particular interest in the Neoproterozoic. These taxa are found in similar depositional environments in Spitsbergen, Arizona, Canada and Siberia. This evidence, together with lithostratographic correlations, isotope chemostratigraphy, and sequence analysis contributes to the continuing development of Neoproterozoic stratigraphy.

Neoproterozoic microfossils from the Officer Basin, Western Australia

Microfossil assemblages are described from the early Neoproterozoic Madley and Browne Formations, western Officer Basin. One chert and eleven siliciclastic samples yielded microfossils. Myxococcoides cantabrigiensis occurs as pustular mats in the chert sample and Eomicrocystis malgica, Pterospermopsimorpha granulata, Skiagia sp. cf. S. pusilla, and undetermined species of Obruchevella, Heliconema, and Trachystrichosphaera are present in acid macerated samples. Leiosphaeridia spp. and Siphonophycus spp. are also found in fine-grained siliciclastic samples, with clusters of Synsphaeridium sp. in some samples. These findings enable a more substantial reconstruction of the palaeoenvironment of Supersequence 1 in the western Centralian Superbasin. The acanthomorph acritarchs are considered to be planktonic eucaryotes washed into environments which ranged from coastal sabkha through to tidal flats, which may be the source of the prokaryotic, benthic, matforming cyanobacteria.

THE GEOLOGY AND HYDROCARBON POTENTIAL OF THE NORTH-WESTERN OFFICER BASIN

Recent petroleum exploration in EP 186 and EP 187 in the north-western Officer Basin has greatly increased knowledge of the regional stratigraphy, structure and petroleum prospectivity of the region. This exploration programme has involved the drilling of two deep stratigraphic wells (Dragoon 1 and Hussar 1) and the acquisition of 1438 km of seismic data. Integration of regional gravity and aeromagnetic data with regional seismic and well data reveals that the Gibson Sub-basin primarily contains a Proterozoic evaporitic sequence. In contrast, the Herbert Sub-basin contains a Late Proterozoic to Cambrian clastic and carbonate sequence above the evaporites. This sequence, which was intersected in Hussar 1, is identified as the primary exploration target in the Western Officer Basin. The sequence contains excellent reservoir and seal rocks in association with mature source rocks. Major structuring of the basin has also been caused by compressive movements associated with the Alice Springs Orogeny. The northwestern Officer Basin thus has all of the ingredients for the discovery of commercial hydrocarbons.