Northern Carnarvon Basin Research Articles

Seismic, petrophysical and petrological constraints on the alteration of igneous rocks in the Northern Carnarvon Basin, Western Australia: implications for petroleum exploration and drilling operations

The Northern Carnarvon Basin (NCB) hosts an extensive record of Jurassic–Cretaceous rift-related igneous activity, manifested by a >45 000 km2 intrusive complex and series of volcanic centres constrained by seismic mapping. However, there are relatively few well penetrations of these igneous rocks (<1% of ~1500 exploration wells) in comparison to other basins that witness extensive magmatism, and thus, their lithological and petrophysical characteristics are poorly understood. Here, we describe the properties of igneous rocks encountered in nine petroleum exploration wells and scientific boreholes in the NCB and evaluate their impacts on exploration and development issues. Igneous rocks in the NCB are characterised by pervasive alteration, with ramifications for seismic imaging and drilling. For example, low acoustic velocities in mafic lavas altered to clays in Toro-1 were mistaken for overpressure, whilst intrusive rocks in Palta-1 were initially unrecorded and only recognised due to subsequent post-drilling thermal history analysis. The alteration of mafic igneous rocks to clays reduces acoustic impedance contrasts relative to sedimentary host rocks, making their identification prior to drilling more challenging. Whilst the preferential emplacement of intrusive rocks in Triassic strata deeper than reservoir targets is primarily responsible for the paucity of well penetrations, our findings of extensive alteration of igneous rocks in the NCB suggests that additional wells may intersect as yet unrecognised intrusive or extrusive sequences.

Three-dimensional seismic analysis of a polygonal fault system (PFS) in the Northern Carnarvon Basin, Australia: Implications for fluid flow migration and gas hydrate system

Polygonal fault system (PFS) is confined to fine-grained strata and have significant potential for hydrocarbon plumbing systems in sedimentary basins from deeper structures to shallow-depth gas hydrate systems. Based on a three-dimensional (3D) seismic reflection dataset from the Exmouth Plateau, Northern Carnarvon Basin, NW Shelf Australia, two different tiers of PFS (PFS–I and PFS-II) are characterized in section as well as in the plan views. The PFS-I is consisted of Early Jurassic-Early Cretaceous shallow marine limestone and marl, while the PFS-II is composed of Late Cretaceous to Paleogene bathyal facies with mudstone, calcareous mudstone, marl and silty mudstone. The polygonal faults of PFS-I show relatively low amplitude reflections, which are difficult to identify in seismic amplitude sections. PFS-I can be effectively depicted via coherency sections and time slices, with a good relationship between the low coherent black anomalies and the polygonal faults. In contrast, the polygonal faults of PFS-II are characterized by high-medium amplitude and low continuity seismic reflections, which can be easily identified in the seismic profiles and coherency time slices. The polygonal faults are represented by small throws (5–15 m) and multidirectional strikes. In addition, a series of gas chimneys, pockmarks, and shallow-depth high-amplitude anomalies of the gas hydrate system have also been identified above the deeper Triassic tectonic faults and uplifts. The polygonal faults might increase the connectivity and permeability of the seal and fine-grained layers, resulting in a certain local reduction of seal integrity and an increase in the possibility of fluid leakage. The typical seismic characteristics of PFS and the shallow depth high-amplitude anomalies of the gas hydrate system indicate a significant relationship among the deep-depth Triassic thermogenic gas, the fluid flow migration, and the development of the generation of gas hydrate system.

Multi-attribute geomorphological characterization of the Cenozoic deep-water strata in the Exmouth Plateau, Northern Carnarvon Basin: a contribution to the understanding of deep-water mixed carbonate–siliciclastic environments

Abstract The stratigraphic architecture and relationships to the expression of deep-water architectural elements geomorphology are explored for Cenozoic deposits in the Exmouth Plateau Sub-basin in Northern Carnarvon Basin. We used a high-quality 3D seismic survey and an integrated seismic attribute analysis to provide a guide for seismic interpretation of the mixed carbonate–siliciclastic depositional systems focused on four aspects: morphology, lithology, strata thickness and reflector distortion. Three depositional elements are classified into several categories: mass transport deposits (MTDs), channel systems (channel–levee and braided-like) and lobes, whose deposition is closely related to eustatic changes, and can be explained in a sequence stratigraphic framework. The falling stage system tract is characterized by the initial development of MTDs deposited on the basin floor and small erosive channels on the upper slope. Lobes are deposited in lowstand system tracts while highstand system tracts are characterized by the deposition of MTDs and progradational segments. This mixed carbonate–siliciclastic example proved to be sensitive to other variables such as climate (Middle Miocene Climate Optimum) and plate tectonics (northward Australian Plate drift), which affect geomorphological aspects such as the dimensions and preservation of internal characteristics.

The taxonomy of selected marine microplankton from the Middle and Upper Jurassic (Callovian–Kimmeridgian) of the North West Shelf, Australia

The palynological assemblages of 134 legacy and 212 new palynological preparations from the Callovian–Kimmeridgian of the North West Shelf of Western Australia, were used to identify new species of dinoflagellate cysts with biostratigraphic utility. Samples were derived from cores and sidewall cores from nine wells; four from the Bonaparte Basin, and five from the Northern Carnarvon Basin. This work describes 11 new species of dinoflagellate cyst, one species of acritarch, and emends the descriptions of two genera and five species of dinoflagellate cyst. The new forms include: Batiacasphaera elongata, Cleistosphaeridium? oxfordianum, Egmontodinium twiggii, Evansia? rotundata, Jansonia thormalleus, Lanterna foveolata, Lanterna reticulata, Lanterna spongiosa, Orobodinium tantillum, Oligosphaeridium? varifenestratum, and Sepispinula florida, and the acritarch Triovalium attenuatum. Emendations are proposed for the genera Lanterna and Tringadinium, and the species Broomea fusticula, Egmontodinium toryna, Pyxidiella pandora, Microdinium jurassicum, Jansonia jurassica, and Tringadinium bjaerkei.

The Influence of Tectonics on the Distribution of Exhumation in the Northern Carnarvon Basin, Australia

The Influence of Tectonics on the Distribution of Exhumation in the Northern Carnarvon Basin, Australia

Tilting of the Australian continent: New evidence from the subsidence and deposition history of the Northern Carnarvon Basin

Studies of global sea-level changes, plate kinematics, marine inundation, and morphology of the continental shelves suggest that the Australian continent has been tilting north eastwards since the Late Cretaceous. Here we investigate the direct impact of this phenomenon and the preceding tectonic events on the evolution of the Northern Carnarvon Basin and discuss their implications for hydrocarbon resources. We use backstripping and decompaction techniques to develop subsidence, sedimentation, and porosity evolution models for the basin, which sits on the axis of the tilt. The goal is to highlight the spatial and temporal variation of subsidence and sedimentation rates and assess their impacts on the porosity of the Early Cretaceous reservoirs within the basin, then give insights into the factors that created accommodation space for sediments. The results reveal a north-eastward shift of subsidence and sedimentation rates within the region. Thus, we observe the highest subsidence rates (∼90 m/Ma) in the southwest (Exmouth and Barrow sub-basins) from Early Jurassic to Early Cretaceous. In the Middle Cretaceous, subsidence and sedimentation moved north-east towards the Dampier (∼40 m/Ma) and some parts of the Barrow sub-basin (∼35 m/Ma). In the Cenozoic era, the highest subsidence rates moved further north-east towards the Beagle sub-basin (∼40 m/Ma) and the North Rankin platform. The results show that the Palaeozoic-Mesozoic rifting events produced higher tectonic subsidence (∼3 km) and higher sediment supply in the southwestern Exmouth and Barrow sub-basins, which reduced towards the north-east in the Dampier and Beagle Sub-basins (∼1.5 km). Subsidence plots along a NE-SW transect and the porosity evolution model results also demonstrate the tilting occurring in these sub-basins. We propose that mantle-driven and subduction-driven tilting of the Australian continent caused the observed local tilting of these sub-basins. Additionally, porosity evolution results provide further information for the definition of periods of hydrocarbon migration in the area.

Tectonic and stratigraphic evolution of the central Exmouth Plateau, NW Shelf of Australia

A detailed tectono-stratigraphic analysis of the Mesozoic through Present-day structures and sedimentary sequences has been carried-out on a very large merged 3-D seismic survey that covers the great part of the central Exmouth Plateau, northern Carnarvon Basin, NW Shelf of Australia. Four main megasequences bounded by regional unconformities have been delineated. They are; (1) Megasequence-I - Early-Late Triassic pre-extension, (2) Megasequence-II - Latest Triassic (Rhaetian)-earliest Late Jurassic (Oxfordian) syn-extension; (3) Megasequence-III - Late Jurassic-Early Cretaceous post-extension; and (4) Megasequence-IV - Early Cretaceous-Present-day passive margin strata. Three populations of extensional faults and a previously unrecognised compressional phase of folding have been identified. The main Mesozoic (Rhaetian-Oxfordian) WNW-directed extension produced domino-style planar extensional faults (Fault System 1) with two principle fault sets; (i) large N–S to NNE-SSW-trending, west-dipping faults and (ii) smaller-scale, largely antithetic east-dipping faults. The large faults are possibly reactivated pre-existing deeper and older faults (possibly related to listric growth faults of the large Lower-to Mid-Triassic Mungaroo delta) as well as completely new faults developed in the upper part of the Triassic Mungaroo Formation. Synclinal hangingwall basins of this system are strongly segmented and underfilled with little (<450 m) syn-extensional sedimentation.Fault System 1 was reactivated during the Early Cretaceous-Neogene post-extension stage and strongly influenced the development of the segmented NNE-SSW-trending Fault System 2 in the Cenozoic strata. Two stacked polygonal non-tectonic fault tiers (Fault System 3) separated by little deformed stratigraphy occur within the Berriasian to Pliocene post-extension sequences. In the early post-extension phase, Late Jurassic to Early Cretaceous NNE-SSW-directed minor contraction event produced broad WNW-ESE-striking folds in the Triassic-Early Cretaceous strata. This study documents the development of the extensional fault systems and proposes new 4-D tectono-stratigraphic evolutionary models for the central Exmouth Plateau. The results of this study have wider implications for the tectono-stratigraphy of the northern Carnarvon Basin and future exploration.

Thin-section detrital zircon geochronology mitigates bias in provenance investigations

Detrital zircon U–Pb geochronology has enabled advances in the understanding of sediment provenance, transportation pathways, and the depositional age of sedimentary packages. However, sample selection and processing can result in biasing of detrital zircon age spectra. This paper presents a novel approach using in situ detrital zircon U–Pb measurements on thin-sections to provide greater confidence in maximum depositional ages and provenance interpretations. New U–Pb age data of 310 detrital zircon grains from 16 thin-sections of the Triassic Mungaroo Formation from two wells in the Northern Carnarvon Basin, Australia, are presented. Whilst detrital zircon age modes are consistent with previous work, there are some differences in the relative proportions of age modes, which are partly attributed to a lack of small grains in hand-picked grain mounts. The relative sample bias is quantified via grain size comparison of dated zircon (in thin-sections or hand-picked mounts) relative to all zircons identified in bulk-mounts and thin-sections. The youngest age mode ( c. 320–195 Ma) is consistent with an active margin to the north. The dated zircons reveal a single grain defined maximum depositional age of 197 Ma for the upper part of the Mungaroo Formation, suggesting deposition may have continued into the Early Jurassic. Supplementary material: Additional figures, tables, datasets and programming scripts are available at https://doi.org/10.6084/m9.figshare.c.5628911

Early Production Life of Wheatstone Project Offshore Australia Yields Key Lessons

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 202246, “Wheatstone: What We Have Learned in Early Production Life,” by John Pescod, SPE, Paul Connell, SPE, and Zhi Xia, Chevron, et al., prepared for the 2020 SPE Asia Pacific Oil and Gas Conference and Exhibition, originally scheduled to be held in Perth, Australia, 20–22 October. The paper has not been peer reviewed. Wheatstone and Iago gas fields, part of the larger Wheatstone project, commenced production in June 2017. The foundation subsea system includes nine Wheatstone and Iago development wells tied back to a central Wheatstone platform (WP) for processing. Hydrocarbons then flow through an export pipeline to an onshore processing facility that includes two liquefied-natural-gas (LNG) trains and a domestic gas facility. The complete paper highlights some of the key learnings in well and reservoir surveillance analysis and optimization (SA&amp;O) developed using data from early production. Asset Overview Chevron Australia’s Wheatstone project is in the North West Shelf region offshore Australia (Fig. 1). Two gas fields, Wheatstone and Iago (along with a field operated by a different company), currently tie into the WP in the Northern Carnarvon Basin. These two gas fields are in water depths between 150 and 400 m. The platform processes gas and condensate through dehydration and compression facilities before export by a 220-km, 44-in., trunkline to two 4.45-million-tonnes/year LNG trains and a 200 tera-joule/day domestic gas plant. A Wheatstone/Iago subsea system consisting of two main corridors delivers production from north and south of the Wheatstone and Iago fields to the WP. Currently, the subsea system consists of nine subsea foundation development wells, three subsea production manifolds, two subsea 24-in. production flowlines, and two subsea 14-in. utility lines. The nine foundation development wells feed the subsea manifolds at rates of up to 250 MMscf/D. These wells have openhole gravel-pack completions for active sand control and permanent downhole gauges situated approximately 1000-m true vertical depth above the top porosity of multi-Darcy reservoir intervals for pressure and temperature monitoring. All wells deviate between 45 and 60° through the reservoir with stepout lengths of up to 2.5 km. The two subsea 24-in. production flowlines carry production fluids from the subsea manifolds to two separation trains on the WP. Each platform inlet production separator can handle up to 800 MMscf/D. The two 14-in. utility flowlines installed to the subsea manifolds allow routing of a single well to the platform multiuse header, which can direct flow into the multiuse separator (MUS) or other production separators at a rate of 250 MMscf/D.

Upper Triassic carbonate-platform facies, Timor-Leste: Foraminiferal indices and regional tectonostratigraphic association

Timor is part of the non-volcanic Outer Banda Arc with chaotic geology in the late Neogene collision zone between Sundaland (Southeast Eurasia) and the Australian continent. Confusion in the distinction between Triassic, Jurassic and Cenozoic shallow-marine carbonate units has led to a lack of appreciation of the widespread extent of Triassic carbonate-platform facies, at least in Timor-Leste. It also resulted in misunderstanding the different tectonostratigraphic/palaeogeographic affinities of the shallow-water limestones. Foraminiferal assemblages are critical in stratigraphic discrimination of these units. This study records for the first time the foraminiferal microfauna of Upper Triassic shallow-water carbonate deposits in Timor. Many of the foraminiferal species are known from North Africa and Europe and date these units to the Carnian–Rhaetian. The Upper Triassic Bandeira Group represents extensive sheet-like carbonate-platform deposits (possibly on horst plateaus as well as shelf areas adjacent land in the intracratonic sea). Contacts between the hard limestone, forming fatus (limestone peaks) and ridges, and friable mud-dominated surrounding units are often obscured under deep tropical soil cover. At all localities the Bandeira Group is closely associated in outcrop with coeval basinal mud-dominated deposits (Babulu and Aitutu groups). With these it forms part of the East Gondwana Interior Rift Association (EGIRA), similar to the classical Dachstein Platform–Hallstatt Basin facies associations found in the European Alps. The basinal units contain turbiditic and debris-slide deposits that include clasts derived from the carbonate platform. Shallow-water carbonate facies attributed to EGIRA, are present throughout the East Gondwana Interior Rift from the Papuan Basin in the north to Exmouth Plateau, part of the Northern Carnarvon Basin, in the south. These were deposited before Gondwanan breakup along this rift system. The Triassic carbonate-platform deposits have a different dominant biogenic mineralogy (aragonite rather than calcite) and a different architecture to the bryozoan-crinoidal carbonate mounds present in the Permian of EGIRA in Timor-Leste. No Lower and Middle Triassic shallow-water carbonate-platform deposits have been recognized in Timor-Leste. The Bandeira Group, confined to the Upper Triassic, belongs to a stratigraphic association (EGIRA; autochthonous) different both in constituent units and outcrop coverage from that of the allochthonous Overthrust Terrane Association to which the Lower Jurassic carbonate-platform deposits (viz. Perdido Group) belong. The Bandeira Group has been confused not only with the Perdido Group but also with other shallow-water limestone units of the Permian and Miocene in some former studies of Timor. The distinction between these units solves a major tectonostratigraphic problem in Timor, and may apply elsewhere along the Outer Banda Arc.

3D seismic classification of fluid escape pipes in the western Exmouth Plateau, North West Shelf of Australia

Fluid escape pipes are vertical pathways of focused flow venting from a variety of deep overpressure sources. These geological features are typical of many sedimentary basins, including proven petroliferous provinces worldwide, such as the North Sea and the Exmouth Plateau in the Northern Carnarvon Basin, NW Australia. High-quality three-dimensional (3D) seismic reflection data from the western Exmouth Plateau revealed the occurrence of exceptionally well-imaged fluid escape pipes affecting the Jurassic strata and the Triassic Mungaroo Formation, a key reservoir unit in the basin. A total of 171 fluid escape pipes, including blowout, seepage and hydrothermal pipes, were mapped, and their geomorphological characteristics were analysed. In the study area, these features form prominent vertical columns up to 4.5 km long disrupting continuous reflections of the Triassic to Jurassic section. Numerous fluid escape pipes terminate with palaeo-pockmarks affecting the Upper Jurassic syn-extension strata, providing evidence for pipe genesis during the early stages of the Late Jurassic rifting in the Exmouth Plateau. Fluid escape pipes were found rooting from different stratigraphic levels, suggesting multiple fluid sources within the Triassic sediments. Several fluid flow structures nucleated along or near rift-related fault planes within the Mungaroo Formation, providing further evidence of rifting as a main triggering factor of important fluid flow in the basin. In the study area, the presence of fluid escape pipes represents a significant risk for the preservation of potential hydrocarbon accumulations, as, when these features form, vertical fluid venting breaches through stratigraphy, compromising the integrity of seal units. This seems supported by the lack of significant discoveries within the area covered by seismic survey analysed in this study.

Controls on the preservation of Jurassic volcanism in the Northern Carnarvon Basin

The Northern Carnarvon Basin (NCB) forms part of the North West Australian margin. This ‘volcanic’ rifted margin formed as Greater India rifted from the Australian continent through the Jurassic, culminating in breakup in the Early Cretaceous. Late Jurassic to Early Cretaceous syn-rift intrusive magmatism spans 45000km2 of the western Exmouth Plateau and the Exmouth Sub-basin; however, there is little evidence of associated contemporaneous volcanic activity, with isolated late Jurassic volcanic centres present in the central Exmouth Sub-basin. The scarcity of observed volcanic centres is not typical of the extrusive components expected in such igneous provinces, where intrusive:extrusive ratios are typically 2–3:1. To address this, we have investigated the processes that led to the preservation of a volcanic centre near the Pyrenees field and the Toro Volcanic Centre (TVC). The volcanic centre near the Pyrenees field appears to have been preserved from erosion associated with the basin-wide KV unconformity by fault-related downthrow. However, the TVC, which was also affected by faulting, is located closer to the focus of regional early Cretaceous uplift along the Ningaloo Arch to the south and was partly eroded. With erosion of up to 2.6km estimated across the Ningaloo Arch, which, in places, removed all Jurassic strata, we propose that the ‘Exmouth Volcanic Province’ was originally much larger, extending south from the TVC into the southern Exmouth Sub-basin prior to regional uplift and erosion, accounting for the ‘missing’ volume of extrusive igneous material in the NCB.

Integration of new interpretation, attribute analysis and inversion results for the Dampier Sub-basin, North West Shelf of Australia

The Davros-Typhon Multi-Client 3D surveys are located approximately 70km from the north-west coast of Australia, largely covering the NE trending Dampier Sub-basin and straddling the Rankin Trend within the Northern Carnarvon Basin. The basins within the North West Shelf formed as a result of seafloor spreading, associated with the breakup of the North West margin of East Gondwana. The combined, contiguous Davros-Typhon survey areas cover a number of significant discoveries and producing fields, which include both oil and gas accumulations. The key objective of the survey was to enhance the imaging of Triassic to Lower Cretaceous reservoir units and to develop a new interpretation framework, made possible by the modern broadband acquisition parameters and advanced processing techniques. Challenges associated with imaging and interpretation include the effects of high velocity carbonate overburden, steeply dipping structures, fault shadow and structural complexity at depth, which is critical for evaluation of reservoir targets. A major reprocessing effort was undertaken to further mitigate these issues, which included Davros and a number of adjacent existing 3D surveys, resulting in the Typhon Multi-Client 3D. CGG Multi-client and New Ventures geoscientists, in collaboration with CGG Seismic Imaging, have undertaken new interpretation and amplitude versus offset (AVO) inversion analysis using subsets of the Typhon 3D. The resulting volume-based attribute analysis and integration of new AVO inversion results demonstrates enhanced attribute quality for the reprocessed data and provides a platform for quantitative analysis over a large area of the Northern Carnarvon Basin.

Permian source rocks of the onshore and nearshore Carnarvon Basin

A new sampling program of Permian potential source rocks was undertaken to improve knowledge of the Permian petroleum prospectivity in new parts of the Southern Carnarvon and inboard Northern Carnarvon Basins. Presented here are new Rock-Eval data from previously unsampled wells, drillholes and outcrop and new infill sampling between existing data points. Kerogen assemblages of selected intervals were also determined from palynofacies analysis or organic petrography, which suggests the good Permian source rocks are generally dominated by gas-prone kerogens. Possibly terrestrial-derived oil-prone kerogen can also be frequently found in thin intervals of the upper Permian and more rarely in lower Permian in the onshore northern Carnarvon Basin.

Geological overview of the 2021 offshore acreage release areas

In 2021, a total of 21 areas were released for offshore petroleum exploration. They are located in the Bonaparte Basin, Browse Basin, Northern Carnarvon Basin, Otway Basin, Sorell Basin and Gippsland Basin. Despite COVID-19 negatively impacting the industry, participation in the acreage release nomination process was again robust. However, as has been the case in recent years, industry interest is focussed on those areas that are close to existing discoveries and related infrastructure. In tune with the Australian government’s resource development strategy, the areas being offered for exploration are likely to supply extra volumes of natural gas, both for export to Southeast Asian markets and domestically to meet the forecasted shortage in supply to eastern Australia. According to the 2019 implementation of a modified release process, only one period for work program bidding has been scheduled. The closing date for all submissions is Thursday, 3 March 2022. Geoscience Australia continues to support industry activities by acquiring, interpreting and integrating pre-competitive datasets that are made freely available in the context of the agency’s regional petroleum geological studies. As part of a multidisciplinary study, new data, including regional seismic and petroleum systems modelling, for the Otway Basin are now available. Also, a stratigraphic/sedimentological review of the upper Permian to Early Triassic succession in the southern Bonaparte Basin has been completed, the results of which are being presented at this APPEA conference. Large seismic and well data sets, submitted under the Offshore Petroleum and Greenhouse Gas Storage Act 2006 (OPGSSA), are made available through the National Offshore Petroleum Information Management System (NOPIMS). Additional data and petroleum-related information can be accessed through Geoscience Australia’s data repository.

Basin-to-prospect-scale subsurface characterisation: new insights into the Exmouth Sub-basin, North West Shelf, Australia

The Exmouth Sub-basin represents part of the intracratonic rift system of the northern Carnarvon Basin, Australia. Hydrocarbon exploration has resulted in the discovery of a variety of oil and gas accumulations, mainly in Upper Triassic, Upper Jurassic and Lower Cretaceous intervals. Recent 3D petroleum systems modelling aided in understanding the interaction of the complex basin evolution and hydrocarbon charge history, shedding light on the variety and distribution of hydrocarbon types encountered, whilst also highlighting future remaining potential in both proven and untested plays. As a result of this modelling, the Exmouth Subsurface Characterisation Study was commissioned to further leverage &amp;gt;12000km2of recently acquired and processed seismic data and integrate data from specifically conditioned wells from across the Exmouth Sub-basin. The primary study objective was to better understand the distribution of lithologies across the basin, with focus upon the reservoir presence and properties over proven and potential deeper sections. Furthermore, given the variety of hydrocarbon types encountered, this study set out to understand the amplitude behaviour of these types within the different reservoirs. Collectively, these results have aided in identifying analogous hydrocarbon amplitude responses across the basin, derisking identified plays, prospects and existing discoveries and fields whilst also identifying new plays and leads.

Application of conditional generative model for sonic log estimation considering measurement uncertainty

Well-log data is a cost-effective means to characterize the petrophysical properties of a geological formation. Among the data, compressional- and shear-slowness (DTC and DTS, respectively) are the most reliable and have been widely applied in the interpretations. However, the availability of DTS data tends to be limited because of its high acquisition cost. This study proposes a method to reproduce or reconstruct the DTS data using other well-log data, such as gamma ray, neutron porosity, bulk density, and DTC. The developed method is based on the conditional variational autoencoder (CVAE) and effectively considers uncertainty associated with the variability of the measured data. The performance of this developed method is validated by applying the well-log data acquired from Satyr-5 and Callihoe-1 wells in the Northern Carnarvon Basin, Western Australia, and the prediction accuracy of the developed method is compared to recently developed data-driven methods (i.e., long short-term memory (LSTM) and bidirectional LSTM (bi-LSTM)). The results reveal that the developed method produces a better DTS estimation than LSTM and bi-LSTM. Furthermore, the effectiveness of the proposed method remains unaltered regardless of whether the data contain a specific trend over the depth or amount of training data are insufficient. As a further application of the developed method, an uncertainty relative to DTS estimation is quantitatively obtained from Monte-Carlo estimation, which uses a trained probability model of the developed method. Sensitivity analysis reveals the high effectiveness of DTC in improving the performance of the CVAE method. From our results, we can conclude that the proposed CVAE-based method is an effective tool for improving the efficiency and accuracy of DTS estimation.

Understanding the gravity response variability of sedimentary basins using forward stratigraphic modelling

It is accepted that gravity inversions are non-unique and that multiple models will fit the data equally well. Over sedimentary basins, this implies that gravity anomalies can be equally well explained by sedimentary strata with constant densities over a highly heterogeneous basement, or by highly heterogeneous sedimentary strata over a constant density basement. While both scenarios minimise the difference between the predicted and observed gravity signal, neither are geologically plausible. In this work, we explore stratigraphic modelling as a new parameterisation for gravity models. By using a process-based numerical simulation to generate the sedimentary basins' density models we seek to focus only on geologically plausible models. We then study the variability of the sedimentary strata's gravity response (i.e. the range of possible geophysical signatures) to stratigraphic parameter changes. Results from a study of a 2D section in the Northern Carnarvon basin (the northwest shelf of Australia) emphasize that even when the stratigraphic models are trialled using a wide range of input parameters, the simulated gravity response still has a limited variability when compared to model results generated using other parameterisations. Lastly, our results provide insights into the global sensitivity of the gravity response, highlighting the sedimentary and stratigraphic parameters that most affect the geophysical signal, allowing for a discussion of the geologic and stratigraphic controls of the sedimentary basin's gravity response.

Late Triassic dinoflagellate cysts from the Northern Carnarvon Basin, Western Australia

The Late Triassic radiation of cyst-forming dinoflagellates in the Southern Hemisphere is investigated in the Northern Carnarvon Basin, Western Australia. This major depocentre, situated on the southern margin of the Tethys Ocean, accumulated extensive deltaic and shallow marine successions at this time, that frequently host early dinoflagellate cyst assemblages. Numerous petroleum exploration wells in the basin have penetrated the fluvially dominated Mungaroo Formation and shallow marine Brigadier Formation, of Anisian–Norian and Rhaetian ages, respectively. Consequently, huge numbers of cuttings and sidewall core samples from these northwest prograding deltaic systems are available for study. Many of the dinoflagellate cysts from the Mungaroo and Brigadier formations have not been taxonomically formalised, including many forms that are used in open nomenclature within the oil and gas industry. This study formally documents these occasionally abundant and diverse dinoflagellate cyst assemblages with the aim of providing a consistent taxonomic framework for future work on the Upper Triassic successions of the Northern Carnarvon Basin. This will aid the recognition of individual flooding events via their characteristic palynomorph signatures and help to build on significant recent advances in regional sequence stratigraphy. One new genus, 14 new dinoflagellate cyst species and one new subspecies are described from the most diverse Late Triassic dinoflagellate assemblage yet published. A further nine genera and 15 dinoflagellate species are also recorded from the Carnian–Rhaetian R. wigginsii, W. listeri, H. balmei, R. rhaetica and D. priscum dinoflagellate zones. The documented assemblages are not only significant biostratigraphically, but it is also postulated that high diversity Triassic dinoflagellate cyst associations were paleoclimatically controlled, and were likely confined to the temperate and cool temperate paleolatitudes.

Depositional settings and palynofacies assemblages of the Upper Triassic fluvio-deltaic Mungaroo Formation, northern Carnarvon Basin, Western Australia

ABSTRACTPalynofacies analysis was carried out on 92 core samples from the fluvio-deltaic Middle to Upper Triassic Mungaroo Formation, Northern Carnarvon Basin, Western Australia. The analyses demonstrate that each depositional environment (“depofacies”) sampled has a characteristic palynofacies assemblage reflecting the varied origins, transport, sorting, and preservation histories of organic particles in sediments. The sampling covered a wide range of depofacies identified in fluvial channel, floodplain, crevasse splay, distributary channel, and tidal zone paleoenvironments and included laminated to massive mudstones and siltstones, cross-bedded sandstones, immature pedogenically altered paleosols, and coals. Although each depofacies has a characteristic palynofacies association, there is a high degree of variability within and overlap between preparations. Black-opaque particles were the dominant component in active fluvial, crevasse, and distributary channels. In contrast, palynomorphs, brown wood particles, and cuticle were more common in abandoned channels, floodplain lakes, and other lower-energy environments. The composition of palynomorphs also varies greatly between depofacies due to factors including the bioproductivity of the surrounding vegetation source area, water-table levels, preservation potential, and the fluid dynamic properties of organic particles. The depofacies were grouped into five “process regimes” (active channels, abandoned channels, lakes and periodically flooded areas, paleosols and swamps, tidal mudflats) based on their dominant depositional process. Depofacies in the same process regime tended to have similar palynofacies associations. Active channels yielded similar assemblages irrespective of whether they were fluvial, crevasse, or distributary channels because their dominant characteristic is high flow energy, which encourages the bypass of finer-grained particles, enhances the mechanical degradation of plant debris, and may inhibit local vegetation growth. Organic particles found in lower-energy environments (e.g., floodplain lakes) are on average larger, more elongate, and better preserved than particles found in high-energy environments (e.g., active channels). Although this study was restricted to samples from the upper Samaropollenites speciosus and lower Minutosaccus crenulatus biostratigraphic zones in a geographically limited area, its results are not influenced by the specific taxonomic composition of the vegetation but by the physiographic structure of surrounding plant communities; this suggests that palynofacies analysis could be used to distinguish depositional environments in deltaic settings from other stratigraphic intervals.