Carnarvon Basin Research Articles

Systematics of the spiny trapdoor spider genus Bungulla (Mygalomorphae: Idiopidae): revealing a remarkable radiation of mygalomorph spiders from the Western Australian arid zone

The aganippine spiny trapdoor spiders of the genus Bungulla Rix, Main, Raven & Harvey are revised, and 30 new species are described from Western Australia: B. ajana sp. nov., B. aplini sp. nov., B. banksia sp. nov., B. bella sp. nov., B. bidgemia sp. nov., B. biota sp. nov., B. bringo sp. nov., B. burbidgei sp. nov., B. dipsodes sp. nov., B. disrupta sp. nov., B. ferraria sp. nov., B. fusca sp. nov., B. gibba sp. nov., B. hamelinensis sp. nov., B. harrisonae sp. nov., B. hillyerae sp. nov., B. inermis sp. nov., B. iota sp. nov., B. keigheryi sp. nov., B. keirani sp. nov., B. kendricki sp. nov., B. laevigata sp. nov., B. mckenziei sp. nov., B. oraria sp. nov., B. parva sp. nov., B. quobba sp. nov., B. sampeyae sp. nov., B. weld sp. nov., B. westi sp. nov. and B. yeni sp. nov. The type species, B. bertmaini Rix, Main, Raven & Harvey, 2017, is re-illustrated and re-diagnosed, and B. riparia (Main, 1957) is re-described. Molecular data from seven genes for a subset of taxa are analyzed with Bayesian methods, to complement the morphological descriptions, to help delimit three species known only from female specimens, and to generate a provisional phylogeny of the genus. Species of Bungulla exhibit a remarkable range of genitalic and somatic morphologies, and we here document this diversity, demonstrating that the characteristic loss of the retrolateral tibial apophysis has not been associated with a concomitant loss of genitalic complexity. We further provide a key to all known species and highlight the southern Carnarvon Basin (including the western Yalgoo and northern Geraldton Sandplains) as a hotspot of diversity.

Ancient fluid flow recorded by remarkably long, buried pockmark trains observed in 3D seismic data, Exmouth Plateau, Northern Carnarvon Basin

Abstract We present 3D seismic data from the Exmouth Plateau, offshore northwest Australia shows 275 paleo-pockmarks of latest Jurassic age, organised into three linear NNW-SSE oriented pockmark trains, the longest exceeding 72 km. Spatial statistics indicate a high probability that these linear pockmark arrangements are not random. The trains are parallel to an array of linked normal faults with similar NNW-SSE strikes, with pockmarks underlain by feeder pipes that nucleate along or close to the fault planes. We interpret the faults to have intersected an overpressured unit and acted as a spatial control on the development of the feeder pipes and pockmarks. Our results reveal an important role of faults in controlling laterally extensive pockmark trains, and highlight the value of 3D seismic data in identifying large-scale ancient fluid flow within sedimentary basins.

3D seismic characterization of the polygonal fault systems and its impact on fluid flow migration: An example from the Northern Carnarvon Basin, Australia

We used high-resolution 3D seismic data and applied seismic attribute analysis to geometrically interpret and characterize the polygonal fault systems in the Paleocene-Eocene sequence of the Rankin Platform Sub-basin, Northern Carnarvon Basin, Australia. The polygonal faults were identified in a stratigraphic sequence dominated by calcareous clay with minor calcilutite at its lower section that changes to argillaceous calcilutite and calcareous claystone at its upper section. Seismic sections and time slices, through multi-attribute (coherence, energy ratio similarity, and curvature) volumes display a dense distribution of normal faults with polygonal shape and small throws in the plain view. The polygonal fault systems (PFS) exist in two well-defined tiers, which can act as pathways for the fluid flow migration. The first tier coincides with the upper section (Early-Middle Eocene Wilcox Formation) with a few faults and the second one coincides with the lower section of the sequence (Paleocene Lambert and Dockrell formations) with a greater number of faults. Proper understanding and interpretation of the polygonal fault systems as non-tectonic extensional faults are a critical issue in assessing the migration pathway for hydrocarbon (gas) through the Paleocene-Eocene sequence in the Northern Carnarvon Basin, and elsewhere in the world.

Orthorhombic faulting in the Beagle Sub-basin, North West Shelf, Australia

Abstract The Beagle Platform forms a structurally complex outboard boundary to the Mesozoic Beagle Sub-basin in the Northern Carnarvon Basin in 50–1000 m of water c. 250 km offshore on Australia's North West Shelf. North- and NE-trending conjugate fault systems link to form c. 5–10 km-wide rhomboidal horsts bound by c. 5 km-wide graben in complex orthorhombic symmetry. Interpretation of the Canning TQ3D three-dimensional (3D) seismic survey identified four populations of faults comprising: (1) latest Triassic–Early Cretaceous north–south-striking normal faults (Fault Population I); (2) latest Triassic–Early Cretaceous NE–SW-striking normal faults (Fault Population II); (3) Cretaceous polygonal faults (Fault Population III); and (4) Neogene–present-day north–south- and NE–SW-trending en echelon conjugate fault arrays (Fault Population IV). Structural interpretation, comprising fault seismic interpretation, displacement analyses and fault orientation analysis, illustrates that orthorhombic extensional faulting occurred penecontemporaneously. Three-dimensional non-plane strain with non-zero intermediate ( σ 2 ) extension magnitude controls the near-synchronous displacement of fault population I and II conjugate fault sets in orthorhombic symmetry to create the characteristic rhomboidal fault geometry. Neogene–present-day north–south- and NE–SW-striking en echelon conjugate fault arrays (Fault Population IV) form in response to oblique reactivation of these subjacent latest Triassic–Early Cretaceous polymodal faults (Fault Population I and Fault Population II). Fault Population I, II and IV together form a vertically decoupled (soft-linked) pseudo-conjugate fault system partitioned by a Cretaceous interval characterized by polygonal fault systems (Fault Population III) and monoclinic draping. The structural interpretation of the Beagle Platform illustrates a seismic-scale orthorhombic fault symmetry accommodating 3D strain and the insufficiency of plane-strain ‘Andersonian’ conjugate fault theory to resolve complex polymodal faulting evolved penecontemporaneously under a single stress regime. Subsequent oblique extension reactivation of these polymodal fault systems also demonstrates that complex soft-linked en echelon extensional conjugate fault arrays form in response to a single stress regime. This research supports the evolution of complex polymodal faulting under simple states of stress, and has implications for understanding the distribution, linkage and age of extensional faults within the Beagle Sub-basin and other extensional basins.

Petroleum source rocks of Western Australia

Petroleum geochemical analysis of samples from the Canning, Carnarvon, Officer and Perth basins identified several formations with source potential, the: • Triassic Locker Shale and Jurassic Dingo Claystone of the Northern Carnarvon Basin; • Permian Irwin River Coal Measures and Carynginia Formation, Triassic Kockatea Shale and Jurassic Cattamarra Coal Measures of the Perth Basin; • Ordovician Goldwyer and Bongabinni formations, Devonian Gogo Formation and Lower Carboniferous Laurel Formation of the Canning Basin; • Devonian Gneudna Formation of the Gascoyne Platform and the Lower Permian Wooramel and Byro groups of the Merlinleigh Sub-basin of the Southern Carnarvon Basin; and • Neoproterozoic Brown, Hussar, Kanpa and Steptoe formations of the Officer Basin. Burial history and geothermal basin modelling was undertaken using input parameters from geochemical analyses of rock samples, produced oil, organic petrology, apatite fission track analysis (AFTA), heat flows, subsurface temperatures and other exploration data compiled by the Geological Survey of Western Australia (GSWA). Of these basins, the Canning, Carnarvon, and Perth basins are currently producing oil and gas, whereas the Southern Carnarvon and Officer basins have no commercial petroleum discovery yet, but they do have source, reservoir, seal and petroleum shows indicating the presence of petroleum systems. The Carnarvon Basin contains the richest identified petroleum source rocks, followed by the Perth and Canning basins. Production in the Carnarvon Basin is predominantly gas and oil, the Perth Basin is gas-condensate and the Canning Basin is oil dominated, demonstrating the variations in source rock type and maturity across the state. GSWA is continuously adding new data to assess petroleum systems and prospectivity of these and other basins in Western Australia.

Australian exploration review 2017

Seventy acreage grants were awarded in Australia during 2017, with net acreage increasing in the Canning, Bonaparte and Perth basins. More broadly, the area under licence within Australia has been reducing since 2014 and this trend continued with a reduction of just over 100 000 km2. Both 2D and 3D seismic acquisition levels decreased within Australia during 2017, on an absolute and on a percentage of worldwide basis. An uptick in exploratory drilling occurred in 2017 with the highest levels since 2014 being reached. Exploratory drilling levels in Australia also increased, in percentage terms, compared with those globally. Discovered volumes exhibited a sharp decrease with a total of 22 MMboe recoverable reserves added across 14 fields. In 2018, exploration activity is expected to increase with key wells planned in the North Carnarvon and Roebuck basins.

Unravelling the Triassic Mungaroo Formation within North Carnarvon Basin using Regional Stratal Slice Volumes

Chevron has developed a new method for viewing, rendering and interpreting multiple, proportionally-flattened seismic surveys (US patent). The products of this method are referred to as Regional Stratal Slice Volumes (RSSVs). Within the Northern Carnarvon Basin (NCB), local RSSVs contain a patchwork of 22 3D seismic surveys covering an area of approximately 68 000 km2 and comprising a 4+ km-thick succession of alluvial to shallow-marine deposits of the Late Triassic Mungaroo Formation. Seismic slices for each constituent volume were spliced together, correlated with adjacent volumes and combined with supporting structural, cultural and well-based data. This has created a temporal series of unbroken, regionally-extensive, seismic snapshots which, when viewed successively, capture the evolving geomorphology and palaeogeography of the basin from east of Gorgon and Wheatstone out to the Exmouth Plateau. Through the integration of the RSSVs and well data, the shoreline and marginal marine to non-marine transitions were identified and accurately mapped at an approximately 20 m vertical spacing throughout the Mungaroo Formation. This work resulted in an in-depth understanding of changing depositional environments at a regional scale. Observed, temporally-systematic fluctuations of the shoreline on the RSSVs provide a highly predictive stratigraphic framework for the basin. Additionally, RSSVs have been used to provide insight into regional NCB studies and to support localised prospect and field scale evaluations. Over the past three years, RSSVs combined with automatically generated closures have been used to identify significant additions to Chevron’s prospect portfolio.

Helium in the Australian liquefied natural gas economy

Australia is about to become the premier global exporter of liquefied natural gas (LNG), bringing increased opportunities for helium extraction. Processing of natural gas to LNG necessitates the exclusion and disposal of non-hydrocarbon components, principally carbon dioxide and nitrogen. Minor to trace hydrogen, helium and higher noble gases in the LNG feed-in gas become concentrated with nitrogen in the non-condensable LNG tail gas. Helium is commercially extracted worldwide from this LNG tail gas. Australia has one helium plant in Darwin where gas (containing 0.1% He) from the Bayu-Undan accumulation in the Bonaparte Basin is processed for LNG and the tail gas, enriched in helium (3%), is the feedstock for helium extraction. With current and proposed LNG facilities across Australia, it is timely to determine whether the development of other accumulations offers similar potential. Geoscience Australia has obtained helium contents in ~800 Australian natural gases covering all hydrocarbon-producing sedimentary basins. Additionally, the origin of helium has been investigated using the integration of helium, neon and argon isotopes, as well as the stable carbon (13C/12C) isotopes of carbon dioxide and hydrocarbon gases and isotopes (15N/14N) of nitrogen. With no apparent loss of helium and nitrogen throughout the LNG industrial process, together with the estimated remaining resources of gas accumulations, a helium volumetric seriatim results in the Greater Sunrise (Bonaparte Basin) > Ichthys (Browse Basin) > Goodwyn–North Rankin (Northern Carnarvon Basin) accumulations having considerably more untapped economic value in helium extraction than the commercial Bayu-Undan LNG development.

Drimane-type compounds in source rocks and fluids from fluvial-deltaic depositional settings in the North-West Shelf of Australia

Fluvial-deltaic petroleum systems with gas-prone source rocks are difficult to assess because the released fluids generally lack biomarkers and this limits the implementation of traditional organic geochemistry screening. However, there are a number of compound classes yet to be investigated in order to establish novel parameters for correlation studies in fluvial-deltaic geological settings. In this study we have focused on drimane-skeleton molecules (C15-decalins), particularly the compounds 1,2,2,5,5-pentamethyl-trans-decalin and 1,1,2,5,5-pentamethyl-trans-decalin (termed herein as rearranged drimanes RD1 and RD2, respectively) and drimane (D), in source rock extracts and fluids from fluvial-deltaic petroleum systems in the Carnarvon Basin, North-West Shelf of Australia (NWS). We have calculated the ratio (RD1 + RD2)/D as proposed by Ji et al. (Ji, L., He, C., Zhang, M., Wu, Y., Li, X. [2016]. Bicyclic alkanes in source rocks of the Triassic Yanchang Formation in the Ordos Basin and their inconsistency in oil-source correlation. Marine and Petroleum Geology 72, 359–373) and compared their results with samples from the NWS. Our compilation of rearranged drimane ratios for a variety of lithologies suggests clay-catalysed rearrangement of C15-decalins in siliciclastic source rocks from fluvial-deltaic depositional environments. Therefore, the mineral content seems to be a key factor controlling the isomerisation of decalins. Samples with higher clay content, such as claystone from the Brigadier Formation, have higher (RD1 + RD2)/D ratios (∼6) and higher C29 diasterane/C29 sterane (∼1.1) compared to silty claystone from the Legendre Formation (∼2 and ∼0.8 respectively). The effects exerted by thermal maturity of the source rock and redox conditions of the depositional environment require further investigation, although samples from the Yanchang Fomation and La Luna Formation, where deposition occurred under more reducing conditions, exhibit lower rearranged drimane ratios (< 2).

Variations in Zircon Provenance Constrain Age and Geometry of an Early Paleozoic Rift in the Pinjarra Orogen, East Gondwana

AbstractThe Tumblagooda Sandstone in Western Australia documents Early Paleozoic rifting in East Gondwana and provides the earliest evidence for terrestrial activity of multicellular animals on Earth. We constrain the provenance of this sequence using 737 concordant U–Pb ages of detrital zircons from two stratigraphic wells in the northern Perth Basin and Southern Carnarvon Basin and outcrops of the type sections near Kalbarri. Detrital zircon age signatures are linked to infrared spectral data and stratigraphic logs. These ages span 3,312–466 Ma, including major Precambrian age peaks at 1,079–1,544 Ma, 1,695–2,403 Ma, and 2,640–2,879 Ma, consistent with igneous sources in the West Australian Craton. Significant Early Paleozoic age peaks at 499–541 Ma suggest a North Indian Orogen source. The maximum depositional age is constrained by the youngest detrital zircon, which yields an age of 466 ± 8 Ma. Our age constraints imply that terrestrial activity of multicellular animals on Earth may not be older than Middle Ordovician (Darriwilian). Rifting resulted in the exposure of the Yilgarn Craton and the segmentation of the Pinjarra Orogen. The Northampton Complex segment of the Pinjarra Orogen constituted a basement high that separated subbasins during the onset of rifting. Discordant Archean zircons provide a consistent record of radiogenic Pb loss at ~470 Ma, which we interpret as being related to the denudation of the Yilgarn Craton. The Pb loss event suggests that intracratonic rifting in the Pinjarra Orogen was initiated in the Middle Ordovician, after the Kuunga Orogeny completed the final amalgamation of Gondwana.

Permian (Kungurian) Foraminifera from Western Australia described by Walter Parr in 1942: reassessment and additions

Haig, D.W., October 2017. Permian (Kungurian) Foraminifera from Western Australia described by Walter Parr in 1942: reassessment and additions. Alcheringa 42, 37–66. ISSN 0311-5518.Exceptionally well-preserved siliceous agglutinated Foraminifera originally recorded by Walter Parr in 1942 are redescribed and illustrated by rendered multifocal reflected-light images. Significant new observations are made on wall texture and apertural morphology. The specimens are from the Quinnanie Shale and lower Wandagee Formation in the Merlinleigh Sub-basin of the Southern Carnarvon Basin, a marginal rift that splayed from the East Gondwana interior rift. During the Early Permian, a restricted shallow sea inundated the rift. The formations are part of sequence III of the Byro Group and belong within the Kungurian Stage (Cisuralian, Lower Permian). Of the 14 agglutinated species described by Parr, six are retained under their original names, viz., Hyperammina coleyi Parr, 1942, H. rudis Parr, 1942, Ammodiscus nitidus Parr, 1942, A. wandageeensis Parr, 1942, Tolypammina undulata Parr, 1942 and Reophax tricameratus Parr, 1942; one is transferred to a different species, viz., Thurammina texana Cushman & Waters, 1928a; six are placed with other genera, viz., Thuramminoides pusilla (Parr, 1942), Teichertina teicherti (Parr, 1942), Sansabaina acicula (Parr, 1942), Tolypammina? adhaerens (Parr, 1942), Kunklerina subasper (Parr, 1942), Trochamminopsis subobtusa (Parr, 1942); and a species of Ammobaculites Cushman, 1910 identified by Parr is now left in open nomenclature. From Parr's material, eight additional species are described: two new species, viz., Hyperammina parri sp. nov. and Gaudryinopsis raggatti sp. nov.; rare representatives of Aaptotoichus quinnaniensis Haig, 2003; and very rare species of Lagenammina Rhumbler, 1911, Giraliarella Crespin, 1958, Glomospira Rzehak, 1885, Hormosinella Shchedrina, 1969, and Reophax Denys de Montfort, 1808, all of which are left in open nomenclature. Hyperammina rudis is the type species of Hyperamminita Crespin, 1958, a genus now considered a junior subjective synonym of Hyperammina Brady, 1878. Thuramminoides pusilla is considered a senior subjective synonym of T. sphaeroidalis Plummer, 1945, the type species of Thuramminoides Plummer, 1945. Imagery is presented confirming that the simple cylindrical canals through the wall of Teichertia teicherti differ from the branching canals in Crithionina rotundata Cushman, 1910, type species of Oryctoderma Loeblich & Tappan, 1961. The collection contains some of the earliest representatives of the revised family Verneuilinoididae Suleymanov, 1973, herein elevated from subfamily rank, and considered to include Pennsylvanian–Cisuralian representatives of Mooreinella Cushman & Waters, 1928a, Aaptotoichus Loeblich & Tappan, 1982, Digitina Crespin & Parr, 1941, Gaudryinopsis Podobina, 1975, Caronia Brönnimann, Whittaker & Zaninetti, 1992 (=Palustrella Brönnimann, Whittaker & Zaninetti, 1992) and Verneuilinoides Loeblich & Tappan, 1949.David W. Haig [david.haig@uwa.edu.au] Centre for Energy Geoscience, School of Earth Sciences, University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia.

Origin and tectonic evolution of upper Triassic Turbidites in the Indo-Burman ranges, West Myanmar

The Pane Chaung Formation is exposed in the Indo-Burman Ranges, and has been involved in collision between the Indian Plate and West Burma Block. However, controversies exist over the provenance and paleogeographic reconstruction of the Pane Chaung Formation. This study presents new petrographical and detrital zircon UPb ages and Hf isotopic data from the Pane Chaung Formation in Rakhine Yoma and Chin Hills, west Myanmar. The depositional age of the Pane Chaung Formation is Late Triassic, indicated by the Carnian-Norian Halobia fossils and maximum depositional ages between 233.0±2.5Ma and 206.2±1.8Ma. Upper Triassic sandstones contain 290–200Ma detrital zircons, εHf(t) values of −6 to 11 and TDMC of 1.6 to 0.6Ga, interpreted to be derived from West Papua region. The most abundant zircon age population of 750–450Ma is derived from Pan-African orogenic belts in Australia. Zircons of 1250–900Ma age were derived from the Grenvillian orogen in Australia. Archean zircons are interpreted to be derived from the Yilgarn and Pilbara cratons in Western Australia. Detrital zircon ages of the Pane Chaung Formation are distinct from similar aged strata in Indochina and Sibumasu, but comparable to NW Australia (Carnarvon Basin) and Greater India (Langjiexue Formation). It is suggested that the Pane Chaung Formation was deposited in a Late Triassic submarine fan along the northern margin of Australia.

Characterizing carbonate facies using high-definition frequency decomposition: Case study from North West Australia

Carbonate facies identification is difficult using conventional seismic attributes due to subtle lithologic changes that cannot be easily recognized. Therefore, there is a need to develop new methodologies to study their evolution and their associated sedimentary processes, which will eventually lead to better prediction for reservoir-quality rocks. New insights into the Cenozoic carbonates in North West Australia have been captured with the application of a high-definition seismic attribute workflow. The workflow starts with conditioning of the seismic volume using structurally oriented noise attenuation filters to remove any random and coherent noise from the input data. It also benefits from a high-definition frequency decomposition that matches the original seismic resolution without smearing interfaces using a “matching pursuit” algorithm. A color blend of multigeometric attributes, such as semblance and conformance, has also been used in the workflow to define edges and discontinuities present in the data within carbonate deposits that are attributed to depositional geometries, such as barrier reefs. Our workflow has been developed to investigate the geomorphology and the sedimentary processes affecting Cenozoic successions in the Northern Carnarvon Basin in North West Australia. Geomorphological and sedimentological observations have been documented such as an Eocene rounded carbonate ramp with evidence of slump blocks and scarps, Middle Miocene accretions generated due to longshore drift, and the presence of Pliocene-Pleistocene patch and barrier reefs. These observations were extracted as geobodies to allow for visualization, and they can be used in an automated seismically based facies classification scheme. The new appreciations are not only useful for understanding the carbonate evolution but can also be used to identify geohazards such as slumps ahead of future drilling.

Prolonged co-existence of ‘archaic’ and ‘modern’ Palaeozoic ophiuroids – evidence from the early Permian, Southern Carnarvon Basin, Western Australia

The discovery of a very large ophiuroid (disk diameter of 80 mm) in the early Permian (Kungurian) Cundlego Formation in the Southern Carnarvon Basin of Western Australia extends the stratigraphical range of ‘archaic’ ophiuroids unequivocally into the Permian, unlocking a lost fossil record of this group. Hitherto such ophiuroids have been discovered preserved articulated from strata no younger than the late Carboniferous. Herein we describe the new ophiuroid as Teleosaster creasyi gen. et sp. nov. Although existing temporally with Permian ophiuroids with a ‘modern’ morphological architecture, Teleosaster was biogeographically separated. This ‘archaic’ ophiuroid persisted in high latitude seas, suggesting such ‘archaic’ forms were displaced from the shallow-water, low latitude niches by the ‘modern’ ophiuroids. In modern oceans, ophiuroid gigantism similar to that in Teleosaster creasyi is typically only expressed in cold, high latitude oceans. It has been argued that the frequent occurrence of gigantism in such environments is due to the much lower levels of predation pressure. Unlike other echinoderm classes, the morphological and ecological transformation that resulted in the evolution of ‘modern’ ophiuroids had already taken place well before the events of the Permo–Triassic mass extinction. With the increase in diversity of durophagous predators in low latitude shallow-water communities during the mid-Palaeozoic Marine Revolution, we argue that ‘archaic’ ophiuroids were more susceptible to these higher levels of predation than the ‘modern’ forms and were displaced into regimes of lower predation pressure in high latitude oceans.http://zoobank.org/urn:lsid:zoobank.org:pub:E4FD9264-127D-4995-AD12-C9721F212FCD

Oligocene and Miocene Carbonate Platform Development In the Browse Basin, Australian Northwest Shelf

Abstract: Extensive 2D and 3D seismic-reflection data calibrated by wells provide new insights into the lateral extent, morphology, internal architecture, and age of carbonate platforms in the subsurface of the Browse Basin, Australian Northwest Shelf (NWS). The oldest carbonate build-ups are interpreted as a giant bryozoan build-up complex of Oligocene age (34.03–27.8 Ma). In the late Burdigalian, tropical reef growth began, and numerous reef-rimmed carbonate platforms progressively coalesced into an extensive barrier reef. From the mid-Langhian to the early Tortonian, the Browse Basin contained an elongate (along-margin) barrier-reef system over 500 km long. This barrier reef possibly extended in the southwest into the Northern Carnarvon Basin. After the early Tortonian, reefs on top or landward of this major barrier-reef system were smaller and less connected. This significant reduction in area covered by reef-rimmed platforms likely resulted from cooling following the Mid-Miocene Climate Optimum and relatively high subsidence rates. The reef-rimmed carbonate platforms that formed between the late Burdigalian and the early Tortonian are very similar throughout the Browse Basin. Platform development after the early Tortonian was regionally diverse. High subsidence rates in the northern Browse Basin are interpreted to have promoted the development of thick, aggrading platforms. Lower subsidence in the south led to thinner, more widespread platforms, recurrently shifting location in response to eustatic change. The final phase of reef decline around 6 Ma coincides with an increase in current-driven drift sedimentation. Current-related winnowing of sediments from the lagoons and slopes of the remaining carbonate platforms likely contributed to their final drowning.

COMPOSITION AND OCCURRENCE OF THE GRANDISPORA MACULOSA ZONAL ASSEMBLAGE (MISSISSIPPIAN) IN THE SUBSURFACE OF THE CARNARVON BASIN AND THE COOLCALALAYA SUB-BASIN OF WESTERN AUSTRALIA, AND ITS GONDWANAN DISTRIBUTION

The Grandispora maculosa miospore assemblage – initially described in 1968 from Middle-Late Mississippian strata of New South Wales (eastern Australia) – is well represented in samples examined herein from 10 Western Australian subsurface sections located in the northern Perth Basin (Coolcalalaya Sub-basin) and, to its immediate north, in several sub-basins of the southern and northern sectors of the Carnarvon Basin. Of particular stratigraphic-correlative importance is the presence of the eponymous G. maculosa together with, inter alia, Reticulatisporites magnidictyus, Verrucosisporites quasigobbettii, V. gregatus, Apiculiretusispora tersa, Raistrickia accinta, R. radiosa, Foveosporites pellucidus , and Cordylosporites asperidictyus . Four species are newly described herein: Apiculatasporites spiculatus, Dibolisporites sejunctus, Raistrickia corymbiata, and Vallatisporites valentulus . Published accounts from elsewhere in Gondwana collectively signify the widespread dissemination of the G. maculosa palynoflora, particularly through northern and western regions of the supercontinent, thus affording an effective means of intra-Gondwanan stratal correlation. Limited absolute dating and stratigraphic-successional considerations across Gondwana indicate that the age of the G. maculosa Assemblage can be bracketed within the middle Visean-early Serpukhovian of the Middle-Late Mississippian. This age is supported by the complete absence of bilaterally symmetrical, non-striate, saccate pollen grains, produced by walchian conifers, which were introduced globally (including in Australia) and near-synchronously late in the Serpukhovian. Cryptogamic land plants (ferns, articulates, lycophytes) are the inferred source of the palynoflora.

Bottom currents and sediment waves on a shallow carbonate shelf, Northern Carnarvon Basin, Australia

The modern seafloor of the Australian Northwest Shelf between Exmouth and Dampier was analyzed for large scale sedimentary bedforms on 3D seismic reflection data. The Carnarvon MegaSurvey of Petroleum Geo-Services (PGS), a merged dataset of multiple industrial 3D seismic reflection surveys with a total size of 49,717km2, offers an extensive view of the continental shelf, slope and rise of the Northern Carnarvon Basin. Over the shelf two fields of large scale sediment waves were observed in water depths between 55–130m, where the seafloor may be influenced by different processes including internal waves, tides and storms. Based on the dimensions and orientations of the sediment waves the dominant direction and approximate strength of local bottom currents could be estimated. Information on local sediment grain-size distribution was provided by the auSEABED database allowing a classification of the observed sediment waves into sand- or mudwaves. The first sediment wave field is positioned northwest of the Montebello Islands where the shelf is comparatively narrow and local sediment is mainly sand-sized. It most likely formed by increased bottom currents induced by the diversion of tidal flows around the islands. The second sediment wave field is located north of the Serrurier and Bessieres Islands within a local seafloor depression. Local sediments are poorly sorted, containing significant amounts of mud and gravel in addition to the mainly sand-sized grains. The coarser sediment fraction could have been reworked to sandwaves by cyclone-induced bottom currents. Alternatively, the finer sediment fraction could form mudwaves shaped by less energetic along-slope oriented currents in the topographic depression. The sediment waves consist partially of carbonate grains such as ooids and peloids that formed in shallow water during initial stages of the post glacial sea-level rise. These stranded carbonate grains thus formed in a different environment than the sediment waves in which they were redeposited. In fossil examples of similar high-energy ramp systems this possible out-of-equilibrium relationship between grains and bedforms has to be taken into account for the interpretation of the depositional environment.

δ13C of polycyclic aromatic hydrocarbons to establish the facies variations in a fluvial deltaic Triassic record (Dampier sub-Basin, Western Australia)

The δ13C of polycyclic aromatic hydrocarbons (PAHs) was measured in a series of Triassic and Jurassic source rocks from the Dampier sub-Basin, Carnarvon Basin, North West Shelf of Australia. These sequences correspond to fluvial deltaic systems which integrate the Mungaroo, Brigadier and Legendre Formations, and the organic matter has been assigned as mixed terrigenous/marine material. The relative abundance of the PAHs was plotted against the difference between δ13C values of 1,3,6,7-tetramethylnaphthalene (TeMN) and each PAH. We demonstrated here that samples plotting below 0 for Δ13C (Combustion Marker-1,3,6,7 TeMN) are related to delta-plain/front-deltaic facies or coaly fluvial deltaic facies (e.g. samples from the Mungaroo Formation - North Rankin-5, Goodwyn-9 and Goodwyn-5 wells) whereas the positive region for Δ13C (Combustion Marker-1,3,6,7 TeMN) is assigned to transitional facies deposited under an estuarine/lagoonal environment with a high sapropelic contribution (Brigadier Formation) and marginal marine environment (Legendre Formation). A similar separation for the samples was obtained with phenanthrene and other PAHs. The δ13C of PAHs in combination with their distributions can be used to establish the facies type i.e. delta-plain/front-deltaic from estuarine/lagoonal in a dominantly fluvial deltaic record. Our research provides new tools to take into account for further applications in fluid-source rock correlation studies which represent the main challenge in assessing petroleum systems in the sub-Basin.

3D Close-the-Loop Method Based on Probabilistic Seismic Inversion

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 18913, “3D Close the Loop Using Probabilistic Seismic Inversion for a Gas Field in the Carnarvon Basin, Australia,” by Shilpi Srivastava, Jeroen Goudswaard, Sito Busman, and Justin Ugbo, Shell, prepared for the 2016 International Petroleum Technology Conference, Bangkok, Thailand, 14–16 November. The paper has not been peer reviewed. Copyright 2016 International Petroleum Technology Conference. Reproduced by permission. The paper discusses an approach for predicting the lateral variation of net to gross (NTG) by use of 3D probabilistic seismic inversion. The goal is to define and understand the distribution of sands and shales on the basis of seismic reflection data. The modeling and inversion are supported by the good quality of seismic data. This study underpins the benefits of seismically constrained reservoir modeling. The use of probabilistic inversion to map geological features is a new insight in the applicability of this methodology. Introduction The study field is located in the Carnarvon Basin offshore western Australia. The field was appraised with one well, which has added incremental volumes to existing nearby discoveries. Seismically constrained reservoir-model building creates models that are constrained by, and matched to, geological concepts and seismic amplitudes and travel times. The complete paper shows an approach using 3D probabilistic seismic inversion to predict the lateral variation in NTG over a proven discovery. This, in turn, can be used to update the static-model properties, improving on the first-pass static-model build. Additionally, the inversion will aid in the understanding of some poorly understood seismic expressions, especially in the south of the field. In this area, the authors noticed dim amplitudes on the amplitude maps extracted from the seismic data, where the static model predicts the presence of the reservoir. Field The field is located in the Carnarvon Basin. The discovery was appraised by one well, which penetrated a thin sand layer of interdistributary bay reservoir facies (referred to as Sand 1) and a thicker distributary channel facies (referred to as Sand 2). The overburden comprises basinal shales and marls. The main steps in the 3D close-the-loop work flow are The rock-property-trend models are derived using the available vertical wells in or near the field. A “rock model” is a set of equations that honors well data and links the petrophysical properties such as NTG and porosity with rock acoustic properties. A qualitative 3D check-the-loop (CTL) step is carried out. This involves the initial fit of the static model with the seismic data by forward modeling the synthetic seismic computed from the initial static model before inversion. The quality-control step that follows aims to detect mismatches between seismic and synthetic data. Finally, constrain the reservoir model by use of the seismic data by carrying out 3D probabilistic inversion of the key uncertain rock property.

A re-evaluation of the Lower to Middle Triassic on the Candace Terrace, Northern Carnarvon Basin

The Lower to Middle Triassic mixed carbonate–clastic system in the Northern Carnarvon Basin is poorly understood relative to the stratigraphically younger Jurassic play systems. Few well penetrations and a lack of quality seismic data have deterred exploration of this interval for many years. In recent times, the Lower to Middle Triassic source potential has been comprehensively de-risked within the Roebuck Basin, with subsequent implications across the entire North West Shelf of Australia, opening up the possibility of an entirely new regional play fairway. This paper focuses on the Candace Terrace, on the southern flank of the Carnarvon Basin, where seismic observations and interpretations of Lower to Middle Triassic submarine canyon systems have been made. The stratigraphic elements of this play interval can now be more clearly observed with the aid of 3D seismic data. Amplitude extractions show the internal geometries of these highly erosive systems are sinuous, compensating flows. The aims of this paper are to postulate the stratigraphy of the Lower to Middle Triassic on the Candace Terrace, highlight the tectonic cause of the canyon systems and discuss the prospectivity of the observed turbidite features.