Northwest Shelf Of Australia Research Articles

A new expression for fluid factor using AVO intercept and gradient: Theory and application on gas sand reservoirs from offshore Australia

Numerous amplitude variation with offset (AVO) fluid indicators have proven to be sensitive to the presence of hydrocarbons. Mathematically, these fluid indicators measure the deviation of seismic responses of hydrocarbon reservoirs from their background in a specific domain. We develop a new expression for the fluid factor commonly used in AVO analysis and interpretation. The expression is a function of the common AVO intercept and gradient, along with a weighting coefficient that suppresses the contribution of lithology and other factors unrelated to fluid content. The coefficient also assists in calibrating the fluid factor to the local geology. We compare the performance of our fluid factor with existing fluid factors. All the fluid factors are tested on a worldwide collection of P- and S-wave velocity and density measurements. The testing is followed by application on synthetic data before applying the attribute on real data from the Northwest Shelf of Australia. In the latter application on the real data, we use two wells for calibration purposes, and the third as a blind well. Our fluid factor demonstrated high capability in detecting the targeted gas zones at the three well locations and identifying new prospective zones.

A study of the effects of early diagenesis on the geotechnical properties of carbonate sediments (North West Shelf, Australia)

Carbonate sediments are often regarded as problematic in geotechnical engineering due to the high variability of their properties. Understanding and quantifying this variability will become increasingly critical in the years ahead, notably with respect to upcoming developments in offshore renewable energy, for which limited in-situ data are typically available to characterise large areas. Here, six intervals from the North West Shelf of Australia, each composed of similar carbonate grains but accumulated in different environments, are investigated to better understand how the post-depositional cementation, alteration and dissolution of sediments, known as diagenesis, impact their geotechnical properties. Intervals are primarily affected by mineralogy-driven meteoric diagenesis, comprising in-situ dissolution of metastable grains and subsequent precipitation of cement that occurred when the shelf was exposed during lower sea-levels, and by marine diagenesis. In both cases, increased diagenesis results in a higher cement-to-solid ratio and compressive strength. However, while marine diagenesis is associated with a reduction in void ratio, this is not initially observed with mineralogy-driven meteoric diagenesis. Additionally, for a similar cement-to-solid ratio, microcrystalline cement results in higher compressive strength than sparite cement. The data further reveal that the level of meteoric cementation and the compressive strength increase as a function of the duration of exposure and of the regional climate, along with a reduction of the specific gravity related to the replacement of aragonite by calcite. However, increased meteoric diagenesis also leads to the formation of macro-scale heterogeneities such as calcrete layers and karsts that can affect the holistic geotechnical behaviour of such deposits.

Study Reviews Multilateral Installation Improvements on the North West Shelf

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 32910,“A Decade of Multilateral Technology Installation Improvements Down Under,” by Stefano Cappiello and Adam Pasicznyk, SPE, Halliburton. The paper has not been peer reviewed. Copyright 2023 Offshore Technology Conference. _ Multilaterals wells on the North West Shelf of Australia have been installed for over a decade. An overview of the well architecture, technology evolution, reliability, and efficiency of these operations is presented in the complete paper. The study is focused on subsea multilateral campaigns, where multiple global and regional first installations took place in mature fields. Introduction Multilateral developments in Australia mainly have been completed off subsea installations, with fewer performed from jack-up rigs. These multilateral completions mostly have been installed with the junction located in the producing reservoir sands. Technology Advancement of Multilaterals (TAML) Level 5 completions have successfully been deployed. The authors classify related systems as follows, with the most attention paid in the synopsis to the final two classes: - Generation One (Field A) - Generation Two (Field B) - Generation Three (Field C) - Generation Four (Field A) General Multilateral Well Design Because of sand production within the reservoir, these applications typically require a TAML Level 5 completion system. These systems deliver hydraulic and mechanical isolation at the junction through the completion. The junction is isolated above and below by production packers and by swell packers in the lateral. The junctions typically are installed horizontally within the reservoir. The original general requirements to construct Level 5 multilateral junctions included the following: - Ability to construct a Level 5 Junction in the 9⅝-in. casing or liner - Ability to run a landing profile or coupling as part of the 9⅝-in. production casing or liner - Ability to run and orient a premilled window at depth - Drill-out 8½-in. openhole section into the mainbore and into the lateral - Junction installed within the reservoir at high angle - Window milling to be performed in one trip - Lateral completion to be installed in one trip - Compressive strength rating capable of running screen sections over 2 km As the operators began to adopt multilateral technology in Australia, the following requirements were incrementally added: - Ability to deliver a Level 5 multilateral system in an existing well - Ability to deliver a trilateral well - Improve efficiency and reduce installation time

Applying integrated Triassic biostratigraphy in Timor-Leste to unlock an under-sampled Gondwanan sector of the Tethys puzzle

The contribution to Tethyan and global Triassic studies from the Australian sector of southeastern Tethys is limited relative to that from the Himalayan sector. Whilst many of the Triassic successions from the Australian sector are located offshore, one succession is exposed on the island of Timor. This marine succession, dismembered during collision between the Australian continent and the volcanic Banda Arc, has been investigated for over 100 years but has also made limited modern contributions to Tethyan and global Triassic studies. To address this a new integrated Triassic biostratigraphic microfossil database (conodonts, palynomorphs and radiolarians) from Timor-Leste is presented here that compliments published foraminiferal data. This integrated data set 1) supports application in Timor of the endemic palynomorph zonal scheme from the North West Shelf of Australia whilst providing additional ties for this to the Geological Time Scale (GTS), 2) provides paleobiogeographic support for accumulation of the Timor-Leste Triassic succession within a southeast Tethyan Gondwanan basin and informs discussions about Tethyan paleobiogeography, 3) calibrates over 300 outcrops to the GTS enabling an improved reconstruction of the dismembered succession that will facilitate enhanced regional and local paleogeographic and tectonostratigraphic assessments, and 4) provides independent GTS calibration for published basinal foraminiferal assemblages which will assist understanding the biostratigraphic utility of these assemblages. This database identifies key outcrops, such as the first outcropping Permian-Triassic Boundary section from this sector of southeastern Tethys, that will facilitate targeted studies addressing a range of Tethyan and global Triassic issues unlocking insights from an under-sampled Gondwanan sector of Tethys.

Petrophysical characteristics of igneous rocks in the Outboard Browse Basin, North West Shelf of Australia: implications for predicting igneous sequences prior to exploration drilling

Abstract The Browse Basin is one of Australia's major hydrocarbon provinces, where significant discoveries have been made in recent decades including the Ichthys and Prelude fields, which accounted for ∼15% of the cumulative Australian liquified natural gas (LNG) production in 2019–20. This rift basin hosts extensive Mesozoic intrusive and extrusive igneous rocks, having been identified from both well and seismic data, and which are recognized as one of the key challenges for exploration and production activities in this region. Their impact on petroleum exploration is demonstrated by the number of wells which encountered unpredicted or thicker than expected igneous rock units both within and adjacent to target sections. This study therefore aims to document the reasons of such unexpectedness, and to develop capability to predict the occurrence of igneous rock units prior to drilling in the Browse Basin and other rift settings that contain igneous rocks. Multiple case studies of uncommercial exploration wells are developed by integrating petrophysical and seismic reflection data, focusing in particular along the outboard part of the basin where igneous rocks are most prevalent. Our study highlights the importance of understanding petrophysical, spatial and chemical heterogeneities of igneous rocks in basins to explain their emplacement and distribution, and thereby predict their occurrence prior to exploration and development activities.

Sources and implications of hydroxylated isoprenoid GDGTs on the northwest shelf of Australia through the Pliocene-Pleistocene era

Hydroxylated glycerol dialkyl glycerol tetraethers (OH-GDGTs) have been recently recognized as alternative biomarkers for sea surface temperature (SST) estimations in oceanic environments. Yet, it is still unclear whether OH-GDGTs are applicable on the northwest shelf of Australia. Here we present results of OH-GDGTs from sedimentary cores at IODP Site U1461 covering the Pliocene-Pleistocene period. We discussed potential sources and applications for OH-GDGTs on the northwest shelf of Australia by comparison with previously reconstructed records of surface and subsurface temperatures, terrigenous inputs, water depth changes, and phytoplankton composition. The historical variations in concentrations and distribution patterns of OH-GDGTs suggest that besides oceanic Thaumarchaea production, OH-GDGTs could be also contributed from terrigenous input to some extent. The relative abundance of OH-GDGTs over isoprenoid GDGTs (%OH-GDGTs) might track the variation in water temperature of subsurface layers or the whole water column as the isoGDGT-based TEX86 proxy does, while the ring indices of OH-GDGTs (RI-OH and RI-OH’) have some difficulties in tracking temperature variation on the northwest shelf of Australia through the whole Pliocene-Pleistocene period. However, when the samples were separated into two sections bounded at ∼1.3 Ma, the RI-OH and RI-OH’ show strong positive correlations with TEX86 values. Therefore, at ∼1.3 Ma, ∼1000 m lowering of water depth as well as the exposure of continental paleo-soils/sediments that previously formed in warmer periods potentially induced additional OH-GDGT-2 contributions to oceanic OH-GDGTs, eventually resulting in relatively higher RI-OH, RI-OH’ and %OH-GDGTs values on the northwest shelf of Australia in the mid- and late Pleistocene. If source of terrigenous OH-GDGTs and water depth are relatively stable, these OH-GDGT-based proxies are capable of tracking water temperature of subsurface layers or whole water column.

New records, one new genus and 21 new species of Callianassidae (Crustacea, Axiidea) from the Indo-West Pacific

Poore, G.C.B. 2023. New records, one new genus and 21 new species of Callianassidae (Crustacea, Axiidea) from the Indo-West Pacific. Memoirs of Museum Victoria 82: 167–255. The collections of ghost shrimps from the Indo-West Pacific that contributed to recent molecular phylogeny and resulting reclassification of Callianassidae (Poore et al., 2019; Robles et al., 2020) provide opportunities to review 11 genera and describe one new genus, redescribe existing species and describe 21 new species. Aqaballianassa aqabaensis (Dworschak, 2003), A. brevirostris (Sakai, 2002), A. lewtonae (Ngoc-Ho, 1994) are rediagnosed and Aqaballianassa papua sp. nov. from Papua New Guinea and Aqaballianassa seychellensis sp. nov. from the Seychelles are described as new. Three new species of Caviallianassa Poore, Dworschak, Robles, Mantelatto and Felder, 2019 are compared in a key with Caviallianassa cavifrons (Komai and Fujiwara, 2012): Caviallianassa arafura sp. nov. from the Arafura Sea, Caviallianassa riwo sp. nov. from Papua New Guinea and Caviallianassa moorea sp. nov. from French Polynesia. Callianassa thailandica Sakai, 2005 is treated as species inquirenda. No species of Cheramus Bate, 1888 occurs in the Indo-West Pacific despite the genus name having been widely used. Five species of Coriollianassa Poore, Dworschak, Robles, Mantelatto and Felder, 2019 are recognised and figured: C. sibogae (De Man, 1905), C. coriolisae (Ngoc-Ho, 2014) and new species, Coriollianassa mainbazae sp. nov. and Coriollianassa maputo sp. nov. from the Mozambique Channel, and Coriollianassa nyinggulu sp. nov. from the North West Australian slope. Darryllianassa felderi gen. et sp. nov. is described from Papua New Guinea. Necallianassa nosybeensis sp. nov. is described as a new species from Madagascar. Praedatrypaea Poore, Dworschak, Robles, Mantelatto and Felder, 2019 is rediagnosed. Thirteen species are recognised and most diagnosed; four species are described as new: Praedatrypaea jangamo sp. nov. and Praedatrypaea mozambiquensis sp. nov. from Mozambique, and Praedatrypaea mandu sp. nov. and Praedatrypaea ningaloo sp. nov. from the North West Shelf of Australia. Callianassa malaccaensis Sakai, 2002 is treated as a junior synonym of P. orientalis (Bate, 1888). A key to separate ten species of Praedatrypaea is presented. Pugnatrypaea Poore, Dworschak, Robles, Mantelatto and Felder, 2019 is rediagnosed and confined to three species. Rayllianassa Komai and Tachikawa, 2008 is rediagnosed and its species discussed. Rayllianassa amboinensis (De Man, 1888), a variable species, is rediagnosed; Callianassa ngochoae Sakai, 1999 and Callianassa sahul Poore, 2008 are treated as synonyms. Rayllianassa aurora sp. nov., Rayllianassa bifida sp. nov. and Rayllianassa huonensis sp. nov. are described as new species; the first two are associated with submerged wood at depths greater than 100 m. Rudisullianassa Poore, Dworschak, Robles, Mantelatto and Felder, 2019 is rediagnosed. Rudisullianassa rudisulcus Komai, Fujita and Maenosono, 2014 is discussed and partially illustrated; Rudisullianassa pandan sp. nov. is described as a new species from submerged wood. Spinicallianassa Poore, Dworschak, Robles, Mantelatto and Felder, 2019 is reviewed and a key is provided to five species; three species are described as new: Spinicallianassa bilbili sp. nov. from Papua New Guinea; Spinicallianassa papetoai sp. nov. from French Polynesia; and Spinicallianassa westralia sp. nov. from Western Australia. Callianassa parvula Sakai, 1988 is treated as a possible synonym of S. spinicauda (Komai, Maenosono and Fujita, 2014). Trypaea Dana, 1852 contains only T. australiensis Dana, 1852 and does not occur in the Indo-West Pacific as sometimes reported.

Lithofacies prediction from seismic data using deep learning: A case study from North West Shelf Australia

A fully convolutional neural network was applied to prestack 3D seismic data to predict lithofacies. A key challenge was to segregate volcanics and shale, which have similar elastic properties. We achieved reasonable lithofacies prediction by using pseudowells as labeled data, which could incorporate geologic information into the model (i.e., vertical distribution patterns). In particular, the addition of stratigraphic information as input to the neural network enabled effective learning. In addition, multiple networks were separately trained to evaluate prediction uncertainty. Even in such cases, the number of pseudowells required for the robust training was much smaller than the number of traces in the 3D seismic data.

Automatic click detector for sperm and beaked whale distribution monitoring

Passive acoustic monitoring (PAM) is an effective method for monitoring marine fauna. Automatic detection, identification, and quantification of echolocation clicks by toothed whales is commonly computationally inefficient. We present an autoregression and correlation-based algorithm for detection and classification of odontocete clicks. The detection process consists of three steps: (1) detection and extraction of impulsive signals from PAM recordings, (2) cross-correlation of the extracted sounds with template signals, and (3) binary classification of detected clicks. The extraction of impulsive signals from background noise is based on automatic detection of outliers in an autoregression model of noise. The model settings can be adjusted to detect signals of certain length. The cross-correlation analysis involves the use of template signals of high signal-to-noise ratio, which aremanually identified and extracted from PAM data. Using the cross-correlationcoefficient as a criterion allows distinguishing echolocation clicks by sperm or beaked whales from other impulsive sounds. Finally, binary classification utilizes thresholds determined through the receiver operating characteristic analysis of an independent subset of manually labeled, extracted signals. The approach was tested for detection of sperm and beaked whales in PAM datasets recorded on the Northwest Shelf of Australia and demonstrated low false positive rates of 3%–5%.

CHANNEL WAVE REFRACTION EFFECT ON MOORED LNG CARRIERS

The Karratha Gas Plant (KGP), operated by Woodside, is located in Mermaid Sound on the North West Shelf of Australia and contains three loading jetties in open water. Mermaid Sound is protected from swells from the Southern Indian Ocean by several islands to the west. As a result, swells enter the Sound from the north. The bathymetry in the Sound is relatively flat but waves refract along the slopes of the navigation channel to the KGP terminal. A single uni-directional swell spectrum close to the alignment of the channel was previously assumed to assess the moorings and agreed well with measured mooring line forces (Van der Molen et al., 2003). However, after the Pluto LNG Plant was developed to the south-west of KGP and operated with a dedicated channel, more westerly waves were observed by local pilots. Such waves were not consistently captured at the wave buoy that is permanently deployed near the terminal and so were not easily characterised. The objective of the study described in this paper was to determine the origin of these westerly waves and the effect on the vessel’s response at berth.

Advantages of model averaging of species sensitivity distributions used for regulating produced water discharges.

Produced water (PW) generated by Australian offshore oil and gas activities is typically discharged to the ocean after treatment. These complex mixtures of organic and inorganic compounds can pose significant environmental risk to receiving waters, if not managed appropriately. Oil and gas operators in Australia are required to demonstrate that environmental impacts of their activity are managed to levels that are as low as reasonably practicable, for example, through risk assessments comparing predicted no-effect concentrations (PNECs) with predicted environmental concentrations of PW. Probabilistic species sensitivity distribution (SSD) approaches are increasingly being used to derive PW PNECs and subsequently calculating dilutions of PW (termed "safe" dilutions) required to protect a nominated percentage of species in the receiving environment (e.g., 95% and 99% or PC95 and PC99, respectively). Limitations associated with SSDs include fitting a single model to small (six to eight species) data sets, resulting in large uncertainty (very wide 95% confidence limits) in the region associated with PC99 and PC95 results. Recent advances in SSD methodology, in the form of model averaging, claim to overcome some of these limitations by applying the average model fit of multiple models to a data set. We assessed the advantages and limitations of four different SSD software packages for determining PNECs for five PWs from a gas and condensate platform off the North West Shelf of Australia. Model averaging reduced occurrences of extreme uncertainty around PC95 and PC99 values compared with single model fitting and was less prone to the derivation of overly conservative PC99 and PC95 values that resulted from lack of fit to single models. Our results support the use of model averaging for improved robustness in derived PNEC and subsequent "safe" dilution values for PW discharge management and risk assessment. In addition, we present and discuss the toxicity of PW considering the paucity of such information in peer-reviewed literature. Integr Environ Assess Manag 2024;20:498-517. © 2023 Commonwealth Scientific and Industrial Research Organisation. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Mesophotic benthic communities associated with a submerged palaeoshoreline in Western Australia.

Key ecological features (KEFs) are elements of Australia's Commonwealth marine environment considered to be important for biodiversity or ecosystem function, yet many KEFs are poorly researched, which can impede effective decision-making about future development and conservation. This study investigates a KEF positioned over the Last Glacial Maximum (LGM) shoreline on the northwest shelf of Australia (known as the 'Ancient Coastline at ~125m depth contour'; AC125). Seafloor bathymetry, sedimentology and benthic habitats were characterised within five study areas using multibeam sonar, sediment samples and towed video imagery. Direct evidence for the existence of a palaeoshoreline formed during the LGM was not found, however candidate areas to find palaeoshoreline material at or just below the modern seabed were discovered. Approximately 98% of the seabed surveyed was comprised of unconsolidated soft sediment habitat (mud/sand/silt) supporting negligible epibenthic biota. The prevalence of soft sediment suggests that post-glacial sediments have infilled parts of the palaeoshoreline, with cross-shelf, probably tidal currents in the northern section of the study area responsible for some of the sediment mobilisation and southern study areas more influenced by oceanic conditions. Within study areas, total biotic cover ranged from 0.02% to 1.07%. Of the biota encountered, most comprised filter feeder organisms (including gorgonians, sponges, and whip corals) whose distribution was associated with pockets of consolidated hard substrate. Benthic community composition varied with both study area and position in relation to the predicted AC125. In general, consolidated substrate was proportionally higher in water shallower than the AC125 compared to on the AC125 or deeper than the AC125. Spatially continuous maps of predicted benthic habitat classes (pre-determined benthic communities) in each study area were developed to characterise biodiversity. Spatial modelling corroborated depth and large-scale structural complexity of the seafloor as surrogates for predicting likely habitat class. This study provides an important assessment of the AC125 and shows that if a distinct coastline exists in the areas we surveyed, it is now largely buried and as such does not provide a unique hard substrate habitat. However, much work remains to fully locate and map the ancient coastline within the vast region of the AC125 and additional surveys in shallow waters adjacent to the AC125 may identify whether some sections lie outside the currently defined KEF.

Regional seismic stratigraphic framework and depositional history of the post-Valanginian passive margin sequences in the Northern Carnarvon Basin, North West Shelf of Australia

The post-rift succession of the Northern Carnarvon Basin (north-western Australia) has commonly been interpreted as a relatively simple and uniform sequence that records a transition from siliciclastic to carbonate sedimentation deposited on a passive margin. However, given the significant vertical and lateral variation in seismic facies visible on seismic data, this interpretation likely oversimplifies the depositional history of the margin. Regional composite seismic lines that cross most of the basin, integrated with lithological and biostratigraphic information from exploration wells, provide the context for a better understanding of the depositional processes and environments that characterize the post-rift continental margin succession. We show that the sedimentary sequences deposited above the Valanginian breakup unconformity contain a wide variety of seismic facies that can be linked to a number of different marine depositional environments, with the greatest lateral variation occurring in the Turonian – Rupelian and in the Tortonian – Present. The former interval consists of three dominant seismic facies, namely polygonally faulted, incised, and parallel bedded, which we interpret to indicate a lateral transition from an environment primarily dominated by fine-grained pelagic/hemipelagic deposition to one dominated by energetic bottom currents that created both depositional and erosional features, such as contourite drifts and associated moats. The latter interval is expressed by sigmoidal and continuous reflections which pass laterally into more chaotic reflection packages, which we interpret as clinoforms and mass-transport complexes (MTCs). The development of bottom currents may be connected to changes in circulation associated with the opening of oceans adjacent to the northwest margin of Australia, while the MTCs may indicate increased regional seismic activity and slope instability resulting from the development of collisional plate boundaries. Definition of these sequences highlights the significant changes that have occurred in the sedimentary processes that operated on the margin, and their potential link to its tectonic evolution.

Using shallow hydroacoustic data to image seafloor mass transport deposits on the North West Shelf of Australia: links to neotectonics

Abstract Mass transport deposits have long been known on the Exmouth Plateau, offshore NW Australia, identified in 2D and 3D industry seismic lines. The expedition SO257 in 2017 collected 30 high-resolution, shallow seismic lines along targeted transects on the northern Australian margin. Many of these imaged mass transport deposits, with the top 700–800 m of the section captured in detail not available with industry seismic data. We present nine new high-resolution seismic lines from three separate areas of the North West Shelf. Slides in the Roebuck Basin show complex anastomosing ductile extensional mechanisms, with multiple slip surfaces and no headscarps or adjacent faults. Slides on the Exmouth Plateau have fault control, with surface fault offsets of up to 300 m, indicating seismicity as a likely triggering mechanism. Slumps along the western margin of Western Australia are more limited in extent, associated with surface notches, with indications of previous activity at depth. All areas show a repeated history of mass transport deposits. The area of the active landslide province offshore of NW Australia is far larger than the individual slides recognized on the Exmouth Plateau.

Production, composition and preservation of phytoplankton on the northwest shelf of Australia through the Pliocene-Pleistocene period

In this study, we applied steroid records at IODP Site U1461 to reconstruct long-term variations of production and composition of phytoplankton, and the microbial degradation on phytoplankton-derived organic carbon (OC) on the northwest shelf of Australia through the Pliocene-Pleistocene period. We aim to probe whether and how these behaviors are impacted by strengths of the Indonesian Throughflow (ITF) and the Leeuwin Current (LC), sea level variation, and continental material input. The results indicate that production of phytoplankton generally increased during 6–1.2 Ma, and decreased after ∼1.2 Ma, due to combination effect from intensified upwelling strength by pronounced Pliocene-Pleistocene cooling and low-salinity, tropical nutrient-deficient surface water input by the ITF and LC. The results also demonstrate increasing contributions of diatoms at ∼5.2 Ma, relatively higher contributions of dinoflagellates during 3.8–1.7 Ma, and another increasing contribution of diatoms after ∼1.7 Ma. We suggest that stronger ITF during ∼6.0–3.8 Ma and stronger LC during ∼1.2–0 Ma both induced higher contributions of diatoms to total phytoplankton, while increased terrestrial soil input in the mid-Pleistocene might be the potential reason why the timing of increasing contribution of diatoms (∼1.7 Ma) is slightly ahead of the intensified LC condition (∼1.2 Ma). Compared with the Pliocene, the microbial degradation on phytoplankton-derived organic carbon was more active in the mid-late Pleistocene, which could be related to terrestrial soil input, subsurface currents, and sea level variation. With synchronously lowered bioproductivity, phytoplankton-derived organic carbon burial capacity on the northwestern shelf of Australia lowered since ∼1.7 Ma.

Drilling 1100-km-long seafloor ridges reveals how palaeoshorelines control carbonate shelf morphologies (North West Shelf, Australia)

The Rowley Shelf, the southern half of the tropical, carbonate-dominated, North West Shelf of Australia, is covered with linear ridges that can be tracked parallel to the coast over 1100 km between the modern coast and the 140 m isobath. Here, we investigate the origin and nature of these ridges based on the integration of extensive borehole data, high-resolution geophysical data, age dating and compaction analysis.Our investigation reveals that each ridge consists of now-submerged relict coastal deposits that were formed over the last 200,000 years through wave, tidal, fluvial and aeolian processes. These features were dominantly preserved through early diagenesis and illustrate the longest continuous submerged palaeoshorelines reported to date.The distribution of relict coastal features, and therefore early diagenesis, controls the morphology of the continental shelf. First, relict coastal features tend to be stacked on top of each other, resulting in the formation of composite diachronous coastal deposits that form distinctive steps on the seabed, up to 10s m high. Second, relict coastal features form a hard substrate that controls the location of coral reefs along the shelf. Reefs, including a 20-m-thick MIS 3 drowned coral reef described here for the first time, as well as modern reefs of the Muiron Islands, Montebello Islands and Dampier Archipelago, are all developed on top of relict coastal features.The distribution, size and mineralogy of relict coastal features highlight climatic changes along the Rowley Shelf. High relative sea levels (RSL) are associated with low carbonate production and up to 50% of terrigenous grains, while low RSL deposits exhibit increased carbonate production and reduced terrigenous grain content. This asymmetry is interpreted to reflect the onset of the monsoon associated with increased fluvial runoffs during interglacial periods.Lastly, our work shows that compaction analyses are critical for palaeoenvironment and RSL reconstructions. Indeed, the accumulation of 50 m of sediment on a RSL indicator can result in an offset of the measurement by 12.7 m.

Petrophysical properties of igneous rocks in continental rift basins – a case study from the Mesozoic sequence of the Browse Basin, North West Shelf of Australia

Capturing the ubiquitous distribution and properties of igneous rocks is necessary to succeed in hydrocarbon exploration, field development and underground gas storage when dealing with magma-rich rift margins. In recent years, numerous researches have revealed detailed morphologies of intrusive and extrusive rock units embedded within sedimentary basins owing to advances in 3D seismic data. Outcrop studies have also provided deeper understandings of their occurrence and geometries. However, there has been comparatively little focus on the rock physics of igneous bodies and their relationships to seismic expression. To bridge this gap between the outcrop and the seismic information, we undertook a well-log-based petrophysical study of igneous rocks using subsurface dataset from the Browse Basin located in the Australian North West Shelf. In this contribution, we describe a classification of volcanic facies (e.g. lava flows, volcaniclastics, intrusive sills) based on well log motifs and textures apparent in borehole image data. Statistics of the petrophysical properties of each volcanic facies are also analysed in order to examine their correlation to seismic patterns. Our study implies the importance of detailed characterisation of igneous petrophysical properties of igneous rock units where concentrated within a sedimentary basin, which will help reconcile interpretations of seismic data.

A VARIETY OF TRANSFORMATIONS OF NONLINEAR INTERNAL TIDAL WAVES ON THE NORTH-WEST AUSTRALIAN SHELF

Highly non-linear internal waves at the continental margin on the northwest shelf of Australia are discussed using detailed velocity and temperature time series recorded from moored current meters. The region has a vigorous semidiurnal inland tide that propagates towards the shore along the shelf. In this area, there is an increase in the internal tide and the formation of bore-like formations, often accompanied by internal solitary waves. In this paper, the emphasis is on the analysis of the shape and properties of the internal tide. The frequency spectrum of internal waves is presented, calculated from the records of the current component directed along the normal to the coast. Five different types of waves are distinguished, which consist of burrs on the front or back side of the wave, “square” waveforms with a bores on both the front and back sides of the wave at the same time, as well as linear waves of small and large amplitude. The statistics of transformations of the types of internal tides during their propagation along the section of the route from the slope to the shelf is given. The main types of waves passing without transformation and waves with a short lifetime, which include “square” tidal waves and waves with bores on the back slope of the wave, are revealed. In general, the analysis showed a real picture of the ongoing nonlinear transformations of tidal internal waves occurring on the north-west shelf of Australia.

The Facies Analysis, Evolution, and Coal-Bearing Source Rock Features of the Middle–Late Triassic Shallow-Water Delta in the North Carnarvon Basin, Northwest Shelf of Australia

The sedimentary facies, architecture, and depositional mechanism of deltaic systems have been one of the global research hotspots in recent decades; however, the detailed distribution, sedimentary evolution, source rock potential, and major control factors of the coal-bearing shallow-water delta are still unclear. A typical shallow-water braided delta with coal-bearing source rocks developed in the Middle–Late Triassic Mungaroo Formation of the North Carnarvon Basin, which can be a good study area for an analysis of coal-bearing source rocks. In this study, the sedimentary facies, distribution and evolution, and coal-bearing source rock features of the Triassic strata were analyzed based on the integrated study of wireline logs, drilled cores, thin sections, seismic facies and attributes, and geochemical data. A range of shallow-water delta sedimentary facies was identified, including the proximal delta plain channel/interdistributary bay, distal delta plain channel/interdistributary bay, and the delta front. The coal-bearing shallow-water delta system of the Middle–Late Triassic Mungaroo Formation was characterized by the largest scale delta system with relatively broad proximal and distal delta plains and relatively narrow delta front subfacies. The scale of the delta system showed a trend of increasing from the Early Triassic Locker Shale to the Middle–Late Triassic Mungaroo Formation and then decreasing to the Late Triassic Brigadier Formation. The distal delta plain subfacies of the Mungaroo Formation should have the highest potential coal-bearing source rock, and the proximal delta plain also can be a favorable target for source rock evaluation. The major control factors of the coal-bearing source rocks of the Mungaroo shallow-water delta mainly included the Triassic megamonsoon climate, the topographic features, eustatic changes, and provenance supply. The proximal and distal delta plains of the shallow-water delta system with thin coal seams, carbonaceous mudstone, and dark mudstone lithologies’ association could be a favorable source rock exploration facies for the next stage of natural gas field exploration.

Calculating the large leakage flux of a breached hydrocarbon trap using geophysical interpretation of a paleo-gas-water contact

Integration of three-dimensional seismic and well data from the Northern Carnarvon Basin on the North West Shelf of Australia was used to assess the evidence for top seal breach of a paleo-gas accumulation. Several seismic indicators of vertical hydrocarbon leakage from the crest of the stratigraphic Mungaroo Trap point to a significant flux of gas within the past few hundred thousand years. Mapping of the top reservoir within the Triassic aged Mungaroo Formation revealed strong evidence for a paleo-gas-water contact (GWC), approximately 100 m down flank from the erosional crest of the trap. This contact conforms to structure and delimits an original volume of 1.1 trillion cubic feet (Tcf), based on reservoir property calibration from nearby wells. Mapping also revealed a present-day gas-water contact with a closed volume of 0.2 Tcf, 30 m down flank from the crest. This contact displays a discordant geometry indicative of a dynamic contact.The leakage zone is located directly above the crest of the structure. It is seismically well imaged and comprises seafloor pockmarks, shallow gas anomalies and gas hydrate anomalies. The presence of a leakage zone distributed vertically above a reservoir containing a present and paleo-GWC provides compelling evidence this trap has leaked by breaching of the top seal. Volumetric calculations using the two GWCs indicate that the Mungaroo Trap has been depleted by 0.9–1.1 Tcf. A dynamic-GWC indicates the leakage event within the trap has occurred recently. Analysis of dating of horizons correlated to the seafloor pockmarks places the leakage event at sometime within the last 300 Ka.