Carnarvon Basin Research Articles

Collaboration and Optimization Processes Contribute to Ultra-ERD Offshore Well Success

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 202251, “Transforming the Mindset To Drill Ultra-ERD Wells With High Tortuosity,” by Barry Goodin, SPE, Duane Selman, and Andy Wroth, Vermilion Oil and Gas, 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 The paper has not been peer reviewed. The complete paper describes the extensive integrated engineering collaboration and optimization process that allowed an operator to push the drilling and completion envelope to drill a pair of complex, ultra-extended-reach-drilling (ERD) wells in the mature Wandoo field in the Carnarvon Basin offshore Western Australia. The shallow reservoir depth, extreme ERD profile, and high tortuosity requirement for the wells posed significant challenges. These were overcome with extensive planning; integrated engineering designs; application of new technology; good-quality, real-time data interpretation; and strong execution support from both rig site and town. Introduction The Wandoo field, in 56 m of water off-shore Western Australia, was discovered in 1991 and subsequently developed and placed on production in 1993. The shallow unconsolidated sandstone reservoir consists of a heavily biodegraded oil column overlain by a gas cap and supported by a strong aquifer drive. Field infrastructure consists of a 15-well-slot manned production facility, Wandoo B, and a five-slot monopod, Wandoo A, which is tied back to Wandoo B by subsea in-field pipelines. In late 2018, the operator planned and executed a two-well drilling campaign consisting of two complex, ultra-ERD wells, Wandoo B15 and B16. Both wells were planned to be batch drilled for the top hole and intermediate hole sections, with the production hole sections to be drilled and completed sequentially. The primary objective for the B15 well was to recover unswept oil along the western flank of the field and track the well along the main Wandoo fault to the north to assess the structure and reserves from the northern tip of the field. The B16 well objective was to access unswept reserves through the center and down to the south of the field, essentially twinning the B11ST1 well, another ERD well drilled on an earlier campaign, to its eastern flank. To maximize recovery, both wells needed to be placed approximately 1 m below the top of the reservoir, except where overlain by the gas cap, in which case the wells were to be placed approximately 2 m below the gas/oil contact to avoid gas coning. Drilling Challenges and Solutions The first half of the complete paper presents a detailed discussion of the drilling challenges and solutions, illustrated with schematics, maps, charts, and graphs. Both Wells B15 and B16 were classified as ultra-ERD wells because the shallow true vertical depth (TVD) of the reservoir resulted in extreme stepout ratios and required highly complex well paths to access the remaining reserves. The complete paper lists various specific drilling- and systems-related challenges.

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.

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.

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 >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.

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.

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.

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.

Middle Permian trace fossil assemblages from the Carnarvon Basin of Western Australia: Implications for the evolution of ichnofaunas in wave-dominated siliciclastic shoreface settings across the Permian-Triassic boundary

Guadalupian (middle Permian) ichnoassemblages are described from the Mungadan Sandstone Formation (Kennedy Group), Merlinleigh sub-basin, Carnarvon Basin of Western Australia. In this formation, we recognize five facies (pebbly sandstone, intensely bioturbated sandstone, medium- to coarse-grained cross-stratified sandstone, fine-grained sandstone, and planar laminated siltstone), assigned to two facies associations, interpreted as the deposits of wave-dominated upper shoreface and lower shoreface settings. Ichnoassemblages, which contain 23 ichnospecies in 18 ichnogenera, are assigned to the Skolithos and Cruziana ichnofacies, as well as mixed types, and include representatives of cubichnia, repichnia, pascichnia, fodinichnia, domichnia, and agrichnia, indicating diverse trace-making behaviours. High burrow-density with monospecific occurrences of Skolithos indicate R-selected life strategies and burrow production over a short time period. The anomalous occurrence of Paleodictyon implies that the trace-making organisms of graphoglyptids could inhabit resource-rich shoreface environments as well as resource-limited deep sea niches. To better understand ichnological evolution across the Permian-Triassic (P-Tr) boundary, a database of Permian to Middle Triassic ichnoassemblages was also compiled for wave-dominated shoreface settings (WDSS) globally, and three ichno-ecological measures (ichnodiversity, ethological category, and ichnodisparity) were evaluated. Ichnodiversity shows a stepwise decline in ichnotaxa through the Permian, a pronounced increase in the Early Triassic, and a second phase of decline towards the Middle Triassic. Ethological data exhibit a different trend, with mean and median values dramatically dropping to a late Permian nadir, followed by a significant increase across the P-Tr boundary, culminating in an Early Triassic acme. The globally depauperate nature of late Permian ichnoassemblages may reflect sampling bias or decreased preservation potential resulting from abundant benthos and well-developed mixed layer in the seafloor. The presence of anomalously diverse Early Triassic ichnoassemblages may be due to the WDSS habitat providing a ‘habitable zone’ for trace-making organisms to survive when most marine environments were stressful in the aftermath of the P-Tr mass extinction. We emphasise that caution is needed in interpreting ichnodiversity variations across the P-Tr mass extinction event, as it may reflect shifting of trace-making behaviours of the same animals, or replacement of different animals with the same behaviours. • A new Guadalupian ichnofauna comprising 18 ichnogenera is described; • The Carnarvon ichnofauna represents the Skolithos and Cruziana Ichnofacies; • Paleodictyon occurs in wave-dominated siliciclastic shoreline (WDSS) settings; • The depauperate of ichnotaxa in the late Permian reflect sampling bias or decreased preservation potential of ichnofossils. • The diverse ichnoassemblages in WDSS settings during the Early Triassic results from ‘habitable zone’.

Seismic characterization of a blocky mass-transport deposit in the Trealla Limestone Formation, North Carnarvon Basin, Australia

The deepwater Cenozoic strata in the North Carnarvon Basin, Australia, represent an interval of interest for stratigraphic studies in passive margins settings of mixed siliciclastic-carbonate environments. We have explored the geomorphological characteristics of a mass-transport deposit (MTD) within the Trealla Limestone Formation to describe in detail the differences among the blocks. To characterize the individual geometry and structural configuration of the blocks within the MTD, we used geometric seismic attributes such as coherence, curvature, dip azimuth, and dip magnitude using horizon slices and vertical profiles. The evaluation finds two types of blocks: remnant and glide (or rafted) blocks. Remnant blocks are in situ and stratigraphically continuous fragments with the underlying strata. This type of block is frequently fault-bounded and displays low deformation evidence. Glide blocks are part of the transported material detached from a paleoslope. These blocks are deformed and occasionally appear as “floating” fragments embedded within a chaotic matrix in the MTD. Glide blocks are used as kinematic indicators of the direction of deposition of MTDs. We evaluate these elements in a modern continental analog that resembles a similar setting for a better understanding of the slide occurrence. Geological feature: Glide blocks, North Carnarvon Basin, Australia Seismic appearance: Discrete angular blocks with internal reflectors Alternative interpretations: Differential dissolution in a mixed siliciclastic-carbonate environment Features with a similar appearance: Carbonate buildups, differential dissolution blocks Formation: Trealla Limestone Formation, North Carnarvon Basin Age: Early-Middle Miocene Location: Offshore Northwest Australia, North Carnarvon Basin Seismic data: Obtained from Western Australian Petroleum and Geothermal Information Management System, Draeck 3D seismic data set Analysis tools: Visualization software (Petrel 2019) and attribute performance software (AASPI 6.0)

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.

The influence of mantle flow on intracontinental basins: Three examples from Australia

AbstractDuring the Paleozoic, sedimentary basins developed within Gondwana without evolving to diverging plate boundaries. Such intracontinental basins present long subsidence histories with multiple phases of accelerated subsidence that are not always easily explained by far‐field tectonic forces, and may be driven by processes other than rifting and thermal subsidence. Here we investigate the subsidence of Paleozoic Australian intracontinental basins by comparing one‐dimensional backstripped tectonic subsidence histories from the western Australian Canning and Southern Carnarvon Basins and the central Australian Cooper Basin to forward subsidence models for pure shear lithospheric thinning. We make the hypothesis that differences between observed and model subsidence may be explained by mantle‐flow driven topography, in addition to tectonic forces. To test this hypothesis, we compute dynamic topography from the first geodynamic models of mantle flow spanning the entire Phanerozoic Eon, and we analyse the relationship between dynamic topography and anomalous basin subsidence to dynamic topography and mantle flow. Although reconstructions of mantle flow in deep geological times are uncertain, our results suggest that long‐wavelength dynamic topography could explain aspects of the complex tectonic histories intracontinental basins. In the presented reconstruction of mantle flow, topographic rebound following the sinking of a Cambrian aged slab resulted in a minor phase of dynamic uplift in the Cooper Basin in middle Permian times. Throughout Carboniferous‐Triassic times Australia was positioned above a mantle upwelling driven by a hot structure at the base of the mantle. Structural uplift in the Canning and Southern Carnarvon basins during the Triassic‐Jurassic interval was augmented by dynamic uplift produced by that large‐scale upwelling, and possibly augmented by a focused active mantle plume during the Permo‐Triassic. In Late Jurassic‐Cretaceous times, Australia drifted east away from the mantle upwelling, resulting in a period of subsidence in the Canning and Southern Carnarvon basins. During the Cretaceous the Cooper Basin moved over a downwelling produced by long‐lived subduction along the east Australian margin, resulting in a period of accelerated subsidence.

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.

Intramolecular 13C isotope distributions of butane from natural gases

A single-step method allowing the measurement of 13C intramolecular distribution of butane isomers (n- and i-butane) from natural gas samples was developed. The method builds up on the one that has been developed for propane, using on-line pyrolysis followed by isotope analysis of the pyrolytic fragments. The isotopic filiation between n-butane and its pyrolytic fragments was elucidated using samples spiked with 13C on terminal positions. The isotopic filiation for i-butane could not be shown experimentally due to the lack of commercially available enriched i-butane. We thus used a recent model named Reaction Mechanism Generator (RMG) to assess the origin of the fragments arising from i-butane pyrolysis. Butanes from natural gas samples from the Carnarvon Basin (Australia) and the Arkoma Basin (USA) were analyzed. The results are consistent with a model for thermogenic natural gas generation and suggest that combined intramolecular isotope composition of propane and n-butane can be related to the gas maturity. In addition, the method allows the detection and shed light on the mechanism of n-butane and i-butane biodegradation in natural gas reservoirs.

Characterization of fluvio-deltaic gas reservoirs through AVA deterministic, stochastic, and wave-equation-based seismic inversion: A case study from the Carnarvon Basin, Western Australia

Multiple state-of-the-art inversion methods have been implemented to integrate 3D seismic amplitude data, well logs, geologic information, and spatial variability to produce models of the subsurface. Amplitude variation with angle (AVA) deterministic, stochastic, and wave-equation-based amplitude variation with offset (WEB-AVO) inversion algorithms are used to describe Intra-Triassic Mungaroo gas reservoirs located in the Carnarvon Basin, Western Australia. The interpretation of inverted elastic properties in terms of lithology- and fluid-sensitive attributes from AVA deterministic inversion provides quantitative information about the geomorphology of fluvio-deltaic sediments as well as the delineation of gas reservoirs. AVA stochastic inversion delivers higher resolution realizations than those obtained from standard deterministic methods and allows for uncertainty analysis. Additionally, the cosimulation of petrophysical parameters from elastic properties provides precise 3D models of reservoir properties, such as volume of shale and water saturation, which can be used as part of the static model building process. Internal multiple scattering, transmission effects, and mode conversion (considered as noise in conventional linear inversion) become useful signals in WEB-AVO inversion. WEB-AVO compressibility shows increased sensitivity to residual/live gas discrimination compared to fluid-sensitive attributes obtained with conventional inversions.

Controls On Deep-Water Sand Delivery Beyond the Shelf Edge: Accommodation, Sediment Supply, and Deltaic Process Regime

ABSTRACT Stratigraphic models typically predict accumulation of deep-water sands where coeval shelf-edge deltas are developed in reduced-accommodation and/or high-sediment-supply settings. On seismic data, these relationships are commonly investigated on a small number of clinothems, with a limited control on their lateral variability. Advanced full-volume seismic interpretation methods now offer the opportunity to identify high-order (i.e., 4th to 5th) seismic sequences (i.e., clinothems) and to evaluate the controls on shelf-to-basin sediment transfer mechanisms and deep-water sand accumulation at these high-frequency scales. This study focuses on the Lower Barrow Group (LBG), a shelf margin that prograded in the Northern Carnarvon Basin (North West Shelf, Australia) during the Early Cretaceous. Thanks to high-resolution 3D seismic data, 30 clinothems (average time span of ∼ 47,000 years) from the D. lobispinosum interval (142.3–140.9 Ma) are used to establish quantitative and statistical relationships between the shelf-margin architecture, paleoshoreline processes, and deep-water system types (i.e., quantitative 3D seismic stratigraphy). The results confirm that low values of rate of accommodation/rate of sediment supply (δA/δS) conditions on the shelf are associated with sediment bypass, whereas high δA/δS conditions are linked to increasing sediment storage on the shelf. However, coastal process regimes at the shelf edge play a more important role in the behavior of deep-water sand delivery. Fluvial-dominated coastlines are typically associated with steep slope gradients and more mature, longer run-out turbidite systems. In contrast, wave-dominated shorelines are linked to gentle slope gradients, with limited development of turbidite systems (except rare sheet sands and mass-transport deposits), where longshore drift currents contributed to shelf-margin accretion through the formation of extensive strandplains. In this context, reduced volumes of sand were transported offshore and mud belts were accumulated locally. This study highlights that variations from fluvial- to wave-dominated systems can result in significant lateral changes in shelf-margin architecture (i.e., slope gradient) and impact the coeval development of deep-water systems (i.e., architectural maturity). By integrating advanced tools in seismic interpretation, quantitative 3D seismic stratigraphy represents a novel approach in assessing at high resolution the controls on deep-water sand delivery, and potentially predicting the type and location of reservoirs in deep water based on the shelf-margin architecture and depositional process regime.

Comprehensive basin-wide 3D petroleum systems modelling providing new insights into proven petroleum systems and remaining prospectivity in the Exmouth Sub-basin, Australia

The Exmouth Sub-basin is part of the Northern Carnarvon Basin, offshore north-west Australia, and has undergone a complex tectonic history. Hydrocarbon exploration resulted in the discovery of a variety of oil and gas accumulations; however, their distribution and charge history from different petroleum systems is still poorly understood due to limited knowledge of the deeper basin architecture. The basin-wide, long-offset, broadband 2017 Exmouth 3D multiclient seismic dataset allowed a seamless interpretation into this deeper section. This work revealed new insights on the tectono-stratigraphic evolution of the Exmouth Sub-basin. Mesozoic extension, that was restricted to the latest Triassic, was followed by a sag phase with homogeneous, shale-dominated deposition, resulting in source rock potential for the entire Jurassic section. These findings, together with potential field modelling, were integrated into this first basin-wide 3D petroleum system model to better constrain the thermal history and petroleum systems. The model improved our understanding of the complex charge history of hydrocarbon fields. It predicts that hydrocarbon expulsion from Late Jurassic source rocks continued into the Late Cretaceous, a period when the regional Early Cretaceous Muderong Formation was an efficient seal rock. This implies that, in addition to long-distance, sub-Muderong migration, vertical, short-distance migration may have contributed significant petroleum charge to the discovered accumulations in the southern Exmouth Sub-basin. The model also predicts additional prospective areas: fault-seal structures within Early Cretaceous intervals north of the Novara Arch, intra-formational Late Jurassic sandstones north of the current fields (with low biodegradation risk) and Triassic reservoirs along the basin margins and north of the Jurassic depocentre.

PESA year in review 2019 – development and production

Australia’s production has been steadily increasing since 2013 with the main contributors being the large liquefied natural gas (LNG) projects. The North Carnarvon Basin accounted for over half of Australian production in 2019, dominated by North West Shelf LNG, Gorgon, Wheatstone and Pluto. Just under a quarter of production was from the Bowen-Surat Basin, with the highest producing project being the Condabri, Talinga and Orana cluster of coal seam assets. The next most prolific basin was the Browse Basin at just over 10%, with Prelude and Ichthys, followed by the Gippsland at 7%. During the year, the Greater Enfield Project, in the North Carnarvon Basin, was brought onstream, which involved a 30-km tie-in of the Laverda and Cimatti fields to the Ngujima-Yin floating production, storage and offloading vessel at the Vincent Field via sub-sea pipelines. Also brought into production during 2019 was the Roma North and Project Atlas, Bowen-Surat Basin, coal bed methane projects. Gas from Roma North is exclusively contracted to the Gladstone LNG consortium while Project Atlas gas will be supplied to domestic customers.