Northern Carnarvon Basin Research Articles

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.

The prospectivity of the Late Triassic intervals in the outboard Exmouth Plateau, Western Australia

The Exmouth Plateau is a deep-water plateau on the Australian continental margin underlain by 10–15 km of flat-lying, tilted and block-faulted Paleozoic–Mesozoic sedimentary rocks, which were deposited predominantly during periods of extension before continental break-up commenced in the middle Jurassic. The recent acquisition of the Mawson MC3D broadband seismic survey provides a modern, high-quality dataset located ~230 km to the north-west of Dampier, in the outboard Exmouth Plateau, in a relatively under-explored portion of the Northern Carnarvon Basin. The Late Triassic Mungaroo Formation is the primary reservoir target for the majority of the exploration in the Exmouth Plateau to date. A detailed investigation into the reservoir potential and prospectivity of the Mungaroo Formation is possible by utilising this modern dataset. The high-quality data presents a high-resolution view of the Rhaetian carbonate reef platforms and pinnacle reef complexes that are present within the Mawson survey footprint, revealing an under-explored play type within the region. The development of a detailed stratigraphic framework through the Late Triassic–Jurassic, combined with the use of attribute analyses and amplitude versus offset products can help de-risk identified prospects and highlight further prospectivity during the exploration process.

Kerogen associations and paleoenvironmental interpretation of the Upper Triassic Mungaroo Formation in the Gorgon Area, Northern Carnarvon Basin, Western Australia

Kerogen slides were made from 92 core samples selected from the Upper Triassic Mungaroo Formation in four wells in the Gorgon area of the Northern Carnarvon Basin. The slides were examined to inves...

The Impact and Causes of Subsidence in the Exmouth Sub-basin, Northern Carnarvon Basin

Understanding the kinematic processes around the formation of sedimentary basins is central to quantifying their resource potential. Building accurate kinematic models is a subjective and time-cons...

Regional Stratal Slice Imaging of the Northern Carnarvon Basin, Western Australia

Chevron has developed a method for merging, viewing, and interpreting multiple 3D seismic surveys in stratal slice domain. The products of this method are referred to as Regional Stratal Slice Volu...

New insights into the Exmouth Sub-basin from basin and petroleum system modelling

The Exmouth Sub-basin forms part of the intra-cratonic rift system of the Northern Carnarvon Basin, Australia. It has undergone a complex tectonic history with multiple phases of uplift, erosion, i...

The Western Australia Modeling project — Part 1: Geomodel building

A key goal in industry and academic seismic research is overcoming long-standing imaging, inversion, and interpretation challenges. One way to address these challenges is to develop a realistic 3D geomodel constrained by local-to-regional geologic, petrophysical, and seismic data. Such a geomodel can serve as a benchmark for numerical experiments that help users to better understand the key factors underlying — and devise novel solutions to — these exploration and development challenges. We have developed a two-part case study on the Western Australia (WA) Modeling (WAMo) project, which discusses the development and validation of a detailed large-scale geomodel of part of the Northern Carnarvon Basin (NCB) located on WA’s North West Shelf. Based on the existing regional geologic, petrophysical, and 3D seismic data, we (1) develop the 3D geomodel’s tectonostratigraphic surfaces, (2) populate the intervening volumes with representative geologic facies, lithologies, and layering as well as complex modular 3D geobodies, and (3) generate petrophysical realizations that are well-matched to borehole observations point-wise and in terms of vertical and lateral trends. The resulting 3D WAMo geomodel is geologically and petrophysically realistic, representative of short- and long-wavefield features commonly observed in the NCB, and leads to an upscaled viscoelastic model well-suited for high-resolution 3D seismic modeling studies. In the companion paper, we study WAMo seismic modeling results that demonstrate the quality of the WAMo geomodel for generating shot gathers and migration images that are highly realistic and directly comparable with those observed in NCB field data.

Full-volume 3D seismic interpretation methods: A new step towards high-resolution seismic stratigraphy

Following decades of technological innovation, geologists now have access to extensive 3D seismic surveys across sedimentary basins. Using these voluminous data sets to better understand subsurface complexity relies on developing seismic stratigraphic workflows that allow very high-resolution interpretation within a cost-effective timeframe. We have developed an innovative 3D seismic interpretation workflow that combines full-volume and semi-automated horizon tracking with high-resolution 3D seismic stratigraphic analysis. The workflow consists of converting data from seismic (two-way traveltime) to a relative geological time (RGT) volume, in which a relative geological age is assigned to each point of the volume. The generation of a horizon stack is used to extract an unlimited number of chronostratigraphic surfaces (i.e., seismic horizons). Integrated stratigraphic tools may be used to navigate throughout the 3D seismic data to pick seismic unconformities using standard seismic stratigraphic principles in combination with geometric attributes. Here, we applied this workflow to a high-quality 3D seismic data set located in the Northern Carnarvon Basin (North West Shelf, Australia) and provided an example of high-resolution seismic stratigraphic interpretation from an Early Cretaceous shelf-margin system (Lower Barrow Group). This approach is used to identify 73 seismic sequences (i.e., clinothems) bounded by 74 seismic unconformities. Each clinothem presents an average duration of approximately 63,000 years (fifth stratigraphic order), which represents an unprecedented scale of observation for a Cretaceous depositional system on seismic data. This level of interpretation has a variety of applications, including high-resolution paleogeographical reconstructions and quantitative analysis of subsurface data. This innovative workflow constitutes a new step in seismic stratigraphy because it enables interpreters to map seismic sequences in a true 3D environment by taking into account the full variability of depositional systems at high frequency through time and space.

Tectono-stratigraphy of the Dampier Sub-basin, North West Shelf of Australia

Abstract The Dampier Sub-basin, an inboard rift system of the Northern Carnarvon Basin in the North West Shelf, Australia, underwent two major phases of continental rifting in the Late Paleozoic and in the Latest Triassic to Late Jurassic. Six tectono-stratigraphic megasequences separated by regional unconformities have been identified: (1) Pre-Late Carboniferous Pre-rift 1; (2) Late Carboniferous to Late Permian Syn-rift 1; (3) Early to Latest Triassic Post-rift 1; (4) Latest Triassic to Early Late Jurassic Syn-rift 2; (5) Late Jurassic to Early Cretaceous Post-rift 2; and (6) Early Cretaceous to Present-day passive margin megasequences. The Late Paleozoic rifting produced a series of planar extensional faults on the eastern flank, some of which were later rotated by the Latest Triassic to Late Jurassic WNW–ESE extension to low-angles ( c. 30°). This Mesozoic extension was localized above a NE–SW-trending basement structure, resulting in en échelon inboard rift basins and overlapping boundary fault systems. The study outlines an updated basin tectono-stratigraphic model for the Dampier Sub-basin and provides new insights for structural evolution associated with the development of the North West Shelf passive continental margin of Australia.

From quantitative 3D seismic stratigraphy to sequence stratigraphy: Insights into the vertical and lateral variability of shelf-margin depositional systems at different stratigraphic orders

Abstract A major challenge in sequence stratigraphy is objectively identifying stratigraphic surfaces and sequences across multiple scales of observation. Identification is commonly dependent on the resolution of the data used (i.e., seismic vs. well data), its dimension (i.e., 1D vs. 2D vs. 3D) or the criteria chosen to select sequence boundaries. Through shelf-edge trajectory analysis, the clinothem (i.e., highest order seismic sequence identified on seismic data) can constitute the elementary building block of an observation-based and data-driven quantitative workflow to develop sequence stratigraphic frameworks across different orders and ranks of hierarchy. Here, we use high-quality 3D seismic data to interpret a Late Tithonian–Early Cretaceous shelf margin, the Lower Barrow Group (LBG), developed in the Northern Carnarvon Basin on the North West Shelf of Australia. Based on full-volume seismic interpretation techniques that integrate the 3D variability of the data when identifying seismic unconformities, a high-resolution seismic stratigraphic framework was built (73 interpreted clinothems with an average time duration of ~63,000 yrs). The computation of high-frequency shelf-edge trajectory angle (Tse) curves on selected seismic cross-sections is used to objectively pick sequence stratigraphic surfaces based on the accommodation succession method, thereby highlighting small changes in trajectory and proposing a method reproducible by interpreters based on the same quantitative data. Within the D. lobispinosum interval (142.3–140.9 Ma), the definition of stratigraphic sequences and composite stratigraphic sequences through this workflow is used to discriminate the controls at high and low temporal frequency on the vertical and lateral variability (which is here quantified) of this shelf-slope-basin system. The results show that the high-frequency interplay between short-term glacio-eustasy (i.e., Milankovitch eccentricity cycles of ~100,000 yrs) and sediment supply (locus of fluvial input along the margin) impacted the three-dimensional stratigraphic architecture of the LBG. In contrast, tectonic subsidence had a significant impact on the stratigraphic architecture of the LBG within the main depocentre at lower temporal frequency by overprinting the eustatic signal and accelerating/decelerating the rates of accommodation creation. However, identification of long-term glacio-eustatic Milankovitch cycles (~400,000 yrs) outside the main depocentre, where the rates of accommodation creation due to rift-related subsidence are moderate, also suggests low-frequency eustatic control. Therefore, the vertical and lateral variability of the LBG results from variations in sediment supply and subsidence regime under local (i.e., process regime, currents), regional (i.e., tectonics) and global (i.e., eustasy, climate) forcing parameters interplaying across timescales. In contrast to standard sequence stratigraphic workflows that are based on model-dependent choices to select sequence boundaries, quantitative 3D seismic stratigraphy constitutes an improved method to interpret 3D seismic data in shelf-margin depositional systems within a sequence stratigraphic framework, which provides an observation-based and model-independent tool allowing the definition of stratigraphic sequences with results that are reproducible across multiple stratigraphers. This work highlights the need for developing new sequence stratigraphic tools and methods that integrate the 4D variability of depositional systems and moves beyond the two-dimensionality inherent to current sequence stratigraphic methods. Quantitative 3D seismic stratigraphy represents a first step towards the creation of 3D sequence stratigraphic workflows that could improve the prediction of stratigraphic patterns and facies relationships (source, reservoir, seal distribution) across shelf margins.

Isotope heterogeneity in ethyltoluenes from Australian condensates, and their stable carbon site-specific isotope analysis

Low-molecular-weight (LMW) aromatic compounds from petroleum fluids have not been widely studied for fluid-source correlations due to their volatility and their relatively low abundances in source rocks. However, LMW aromatics are important components in fluids, including condensates which lack biomarkers (molecular fossils typically used for correlation studies). Here, we have investigated the distribution of ethyltoluenes (o-ET, m-ET and p-ET; ortho, meta and para, respectively) in fluvial-deltaic condensates which contain relatively high abundances of the meta-isomer. The meta-selectivity found in these petroleum fluids is consistent with a mineral catalytic effect on the molecular distribution of these compounds, as it occurs during the clay-catalyzed synthesis of ethyltoluenes. Isomers differ up to 6‰ in δ13C values. Condensates from the Northern Carnarvon Basin, North West Shelf of Australia (NWS), have been analyzed by compound specific isotope analysis (CSIA) by GC-ir-MS, and site-specific isotope analysis (SSIA) using a Q-Exactive-GC Orbitrap™-based mass spectrometer. The SSIA revealed a 13C enrichment at the methyl end of the ethyl branch of m-ET, following a normal kinetic isotope effect during thermal maturation (cleavage). Continuous development of this first SSIA application will make possible high resolution analysis of light aromatics to outline the evolution of the organic matter to hydrocarbons in petroleum systems.

Assessing shale mineral composition: From lab to seismic scale

Shales have always been a difficult target for drilling of deviated and horizontal wells. In the presence of azimuthal stress fields, inclined boreholes in smectite-rich shales exhibit geomechanical instabilities and can result in borehole failure. The complex geology of the major gas fields in the Northern Carnarvon Basin on the North West Shelf of Australia makes it necessary to drill deviated wells through the smectite-rich shale seal extending more than 1 km in thickness. Predicting the mineralogical composition of shales in the area is therefore crucial for the success of drilling operations related to hydrocarbon exploration and production. Here we introduce a novel workflow that combines seismic data, well logs, and laboratory measurements to rapidly infer smectite content in shale. The workflow is applied to the Duyfken 3D seismic survey in the central part of the Northern Carnarvon Basin. The results of our quantitative interpretation are verified against the laboratory X-ray diffraction measurements from the test well that was not used for interpretation, and they match the test data well within the determined uncertainty bounds.

Estimating smectite content from seismic data

Predicting the mineralogical composition of shales is crucial for drilling operations related to hydrocarbon exploration/production as well as for the assessment of their sealing capacity as hydrocarbon or CO2 barriers. For example, hydrocarbon exploration in the Northern Carnarvon Basin, North-West Shelf, Australia is hindered by the presence of a thick (up to 1 km) smectite-rich shale seal that spreads regionally. Complex structures of the channelised oil and gas fields in the area make it necessary to drill deviated wells through that seal. The maximum deviation angle at which successful drilling is possible depends strongly on the clay mineralogy and, in particular, on the smectite content in the shale. Here, we introduce a novel workflow combining seismic data, well logs and laboratory measurements to infer shale composition at the reservoir scale. It is applied to the Duyfken 3D seismic survey in the central part of the Northern Carnarvon Basin. Interpretation results are verified against the laboratory X-ray diffraction measurements from the test well that was not used for the interpretation. The results match the test data well within the determined uncertainty bounds.

Recalibrating Australian Triassic Palynostratigraphy to the International Geologic Timescale Using High Resolution CA-IDTIMS Dating

The Triassic is an important interval for Australian petroleum exploration, with Middle to Upper Triassic Mungaroo Formation reservoirs in the Northern Carnarvon Basin, and recent Lower Triassic discoveries in the Roebuck Basin. The chronostratigraphic understanding of Triassic petroleum systems is underpinned by biostratigraphic dating using palynological zonations. The numerical ages of these zones are usually assigned through inference and interpolation, often via tenuous correlations to the international geologic timescale using scattered marine biota, (primarily foraminifera, and rare ammonites, conodonts and/or dinoflagellates). In contrast, we tie Australian biozones to the timescale through Chemical Abrasion-Isotope Dilution Thermal Ionisation Mass Spectrometry (CA-IDTIMS) dating of interbedded volcanic tuffs. Such ashfalls are reasonably common in Australian basins, and can provide high-precision CA-IDTIMS ages if they contain magmatic zircons. We recently recalibrated Australian middle and late Permian palynozones using this approach and preliminary results suggest that Triassic biozone ages are likewise in need of considerable revision.We have targeted Triassic tuffs across Queensland, (Tarong beds, Brisbane Tuff, Moolayember Formation, Rewan Group), New South Wales (Garie Formation, Coal Cliff Sandstone, Milligan Road Formation), and Tasmania (upper Triassic coal measures) to provide numerical ages for palynozones. Additional dates in New Zealand (Murihiku Supergroup) and Timor-Leste (Wailuli Formation) will allow international correlation of dinocyst and spore-pollen zones. Numerical constraints for Triassic biozone boundaries facilitate correlation of Australian biozones with the international geologic timescale. This can impact burial history models used in petroleum exploration anywhere these biozones are used, often far beyond the basins from which the samples were collected.

Multiphase Deformation of the Northern Carnarvon Basin

The Northern Carnarvon Basin has experienced multiple phase of deformation, most likely starting in the Lower Palaeozoic and continuing to the present day. The widespread availability of public dom...

Effect of finely-layered stiff carbonates on a seismic response. Northern Carnarvon basin synthetic study

Fine layering is known to cause both seismic attenuation and VTI-anisotropy. In typical geological environments, the contrast in elastic properties of adjacent layers rarely exceeds 30% so that the layer-induced effects are negligible. However, it’s not true for the overburden of Northern Carnarvon basin (Northwest shelf of Western Australia) that is characterized by very stiff carbonates alternated with more porous and softer rocks. In this paper, we present a workflow for preliminary analysis of seismic wavefield and, in particular, effects of layer-induced scattering attenuation and anisotropy in the target area. The workflow is based on the walk-away VSP full-wave modelling (5-100 Hz) for the flat-layered elastic models that are constructed using logs of four wells, namely, Dampier 1, Parker 1, Wilcox 2 and Withnell 1. We show that particular sequences with carbonates produce significant amplitude loss and degradation of spectrum of a transient seismic pulse. Maximum attenuation is observed for Withnell 1 borehole and is characterized by the drop of 30% in the centroid frequency in the 200 m interval. Anellipticity parameter η is estimated by fitting of the moveout curves and varies from 0.1 to 1.5. In addition, the modelling reveals a very complex wavetrain with energetic reflected and converted waves at large offsets. All these effects should be taken into account in seismic processing and imaging.

Interpretation of a Permian conjugate basin margin preserved on the outer Northwest Shelf of Australia

The Northwest Shelf (NWS) of Australia is characterized as a series of northeast-southwest trending Mesozoic offshore depocenters which both juxtapose and partially overprint a series of onshore, northwest-southeast trending Palaeozoic basins. An integrated interpretation of well bore data, regional seismic data and plate tectonic models suggests that the Palaeozoic section is also present below the Mesozoic depocenter. Referred to as the East Gondwana Interior Rift, the primary rift axis is oriented in a (present day) NE-SW direction, with orthogonal marginal rift basins such as the onshore Canning and Southern Carnarvon basins. While precise age dating for the formation and stratigraphy of the axial rift is speculative, our integrated interpretation suggests that a significant portion of the pre-existing rift was modified by a Mid-Permian extensional event, forming the Northern Carnarvon basin. Interpretation of recently acquired 3D reflection seismic data suggests that the conjugate basin margin from this Permian rifting event is preserved, and is visible below the Mesozoic section. A series of back-stepping, Late Permian carbonate ramps and banks is interpreted to form on a thermally subsiding rift flank. Our interpretation of these carbonate banks is based primarily on seismic geometries, and is supported by area well control and regional paleogeographic models. This interpretation suggests that a deep marine intra-continental basin bisected the NWS in the Late Permian. Shallow marine conditions then persisted across the conjugate margin through the Triassic and into the Jurassic. After Late Jurassic rifting associated with Gondwanan break-up, the region subsided into deep water.

Marginal fault systems of the Northern Carnarvon Basin: Evidence for multiple Palaeozoic extension events, North-West Shelf, Australia

Palaeozoic structures along the North-West margin of Australia have long been recognised as fundamental events responsible for the formation of the offshore basins that comprise this prolific hydrocarbon producing region. However, the tectonic setting in which this rifting occurred remains unclear. Detailed mapping of the geometry and tectono-stratigraphy of Permian and Carboniferous structures of the inboard Barrow, Dampier and Beagle sub-basins of the Northern Carnarvon Basin is possible using regional scale interpretation of publicly available 2D and 3D seismic data. Through seismic interpretation and mapping, we show two distinct orientations of structures that provide evidence for a poly-phase rift history of the North-West margin during the Palaeozoic. NNE trending faults of the Candace Terrace (Barrow sub-basin) were initiated in the Carboniferous or Devonian but were underfilled, resulting in erosion of the fault block crest and filling of the remnant rift-related topography by conformable sequences of later Permian and Triassic sediments. By contrast, NE-SW oriented faults of the Mermaid Nose (Dampier sub-basin) experienced a distinct phase of Permian activity and are unconformably overlain by Triassic sediments. In the Beagle sub-basin, both orientations of structures are present. Throughout the marginal fault systems of the Northern Carnarvon Basin, complex fault geometries and significant deformation of hanging wall strata are associated with reactivation of the eroded fault block crests and overprinting of faults during subsequent Mesozoic extension. This has implications for understanding the geodynamic evolution of a poly-phase extensional continental margin.

Controls on Mesozoic rift-related uplift and syn-extensional sedimentation in the Exmouth Plateau

The Exmouth Plateau, part of the Northern Carnarvon Basin, has experienced a multi-phase extensional history, which is associated with regional-scale uplift, as well the uplift and erosion of individual footwall blocks. Detailed interpretation of 3D seismic surveys over the area shows that fault activity began in in the latest Triassic, mainly on NNE-SSW and NE-SW trending faults. Rotation of Triassic fault blocks initiated in the Upper Triassic and continued during the Jurassic. Erosion of pre-rift Triassic sediment occurred during the latest Triassic and the Jurassic. In the latest Jurassic deposition infilled half-grabens and deposition onto highs was limited in the west as the area was starved of sediment. A significant change in sediment supply in the early Cretaceous associated with progradation of the Barrow Delta resulted in the infilling of previously starved half-grabens. Fault activity had decreased by the mid-Cretaceous, with limited activity confined to major faults. Later Cretaceous sediment distribution in the study area was largely controlled by remnant topography. High-quality 3D seismic data allows a detailed examination of the way in which rift-related fault activity affects sediment distribution. In addition to creating fault block traps in pre-rift Triassic sediments, understanding syn-extensional sediment patterns and fault reactivation has implications for syn-rift plays and seal integrity.