Offshore Australia Research Articles

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

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

The magma plumbing system of the Northern Carnarvon Basin, offshore Australia: multiscale controls on basin-wide magma emplacement, and implications for petroleum exploration

Abstract The Northern Carnarvon Basin (NCB) located on Australia's North West Shelf hosts an extensive (∼40 000 km 2 ) intrusive igneous complex related to Mesozoic rifting and break-up. Using an extensive suite of modern 3D seismic reflection surveys, we have mapped this intrusive system across the NCB. We identify three predominant intrusion morphologies: stacked sheets of large interconnected sill intrusions (up to ∼170 km long) and smaller (8–30 km long) isolated, strata concordant intrusions, which often interact with normal faults emplaced into deltaic sedimentary rocks, and variably sized (10–40 km long) saucer-shaped intrusions emplaced into marine shales, spread across seven zones (geographically constrained groups of intrusions of a specific morphology). We consider the zones’ margin-parallel orientation, suggesting control by subcrustal extensional processes during rifting, and the variation in intrusion morphology between these zones, suggesting a dominant control by host rock mechanical properties. We integrate previous work with our observations, constraining emplacement to between the Kimmeridgian and Valanginian, coinciding with key phases of margin evolution. Finally, we assess the impact of this intrusive complex on local petroleum systems. There is likely little to no adverse impact on source rock maturation or reservoir contamination by CO 2 , but there is a spatial dissociation between the location of groups of intrusions and the gas fields, particularly in the Exmouth Plateau. This suggests that migrating hydrocarbons may be blocked, baffled and/or redirected by emplaced igneous rocks.

The old and new of fluid indicators: An overview and application

Over the past decades, numerous fluid indicators have been introduced and proven to be sensitive to hydrocarbon presence. Mathematically, fluid indicators measure the deviation of hydrocarbon-saturated rocks from their background in a specific domain or space. The ideal indicator is defined as an attribute that responds to hydrocarbon presence only; at shale- or brine-saturated intervals, it shows weak or no noticeable variation. In this paper, we collect and review several common fluid indicators used in the oil industry. The collection includes common reflectivity-based indicators and impedance-based indicators from the late 1980s to present. We present an overview of these fluid indicators, their past and current uses, their limitations, and their integration using machine learning. We examine these indicators on 35 data samples collected from different world basins and analyze their performance using recent data analysis techniques. We then apply the attributes on real seismic data from offshore Australia and test their ability to detect hydrocarbon-saturated reservoirs. In closing, we invoke machine learning approaches to combine the attributes and produce a fluid probability attribute.

An artificial intelligence workflow for horizon volume generation from 3D seismic data

Horizon-based subsurface stratigraphic model building is a tedious process, especially in geologically complex areas where seismic data are contaminated with noise and thus are of weak and discontinuous reflectors. Seismic interpreters usually use stratal (proportional) slices to approximately inspect 3D seismic data along seismic reflectors yet to be interpreted. We introduce an artificial intelligence workflow consisting of three deep learning steps to provide a conditioned seismic image that is easier to interpret, a stratigraphic model that outlines major formations, and moreover a relative geologic time volume that allows us to automatically extract an infinite number of horizons along any seismic reflectors within a seismic cube. Depending on the availability of interpreters, the proposed workflow can either run fully unsupervised without human inputs or using sparse horizon interpretation as constraints to further improve the quality of extracted horizons, providing flexibility in both efficiency and quality. Starting from only seismic images and a few key horizons interpreted on very sparse seismic lines, we demonstrate the workflow to generate a stack of complete horizons covering the entire seismic volume from offshore Australia.

Syn‐rift tectono‐stratigraphic development of the Thebe‐0 fault system, Exmouth Plateau, offshore NW Australia: The role of fault‐scarp degradation

AbstractThe syn‐rift architecture of extensional basins records deposition from and interactions between footwall‐, hangingwall‐, and axially‐derived systems. However, the exact controls on their relative contributions and the overall variable depositional architecture, and how their sediment volume varies through time, remains understudied. We undertook a quantitative approach to determine temporal and spatial changes in the contribution of fault‐scarp degradation to the syn‐rift tectono‐stratigraphic development of the Thebe‐0 fault system on the Exmouth Plateau (NW Shelf, offshore Australia), using high‐quality 3D seismic reflection and boreholes data. The magnitude of footwall erosion was measured in terms of vertical (VE) and headward (HE) erosion by calculating the volume of eroded material along the footwall scarp. A detailed seismic‐stratigraphic and facies analysis allowed us to constrain the architectural variability of the hangingwall depositional systems and the types of resulting deposits (i.e., fault‐controlled base‐of‐scarp, settling from suspension, and hangingwall‐derived). After addressing the syn‐rift tectono‐stratigraphic framework, we suggest that periods of significant erosion along the Thebe‐0 fault scarp are related to the accumulation of fault‐controlled base‐of‐scarp deposits characterised by comprising a lower wedge with chaotic to low‐continuity reflections. Footwall‐derived deposits characterised by an upward decrease in stratigraphic dip are interpreted as related to periods of reduced fault activity and sustained sediment delivery sourced from the footwall scarp and systems beyond it (e.g., antecedent systems). We then analysed the tectono‐stratigraphic framework and the volumetric comparison between material eroded from the fault‐scarp and accumulated in the basin, aiming to estimate the contribution of fault‐scarp degradation to the hangingwall syn‐rift fill. Our results suggest periods of enhanced fault activity control fault‐scarp degradation variability through time, and we agree with that described by previous researchers—fault throw variability along‐strike regulates the variability in the magnitude of erosion. However, we propose that fault‐scarp degradation timing and its spatial variability are also influenced by the interaction and linkage with adjacent normal faults and by sea level variations. Lastly, we determine broader similarities and differences with a system located in the same fault array (i.e., Thebe‐2 fault system), aiming to give insights into the tectono‐stratigraphic evolution of a broader area and the spatial variability in fault‐scarp degradation.

Re-evaluation of the Hydrocarbon Potential of the Scarborough Field, Offshore Northern Carnarvon Basin, Australia.

This study examines the detailed basin analysis and re-evaluation of the hydrocarbon potential of the Scarborough Field in the Northern Carnarvon Basin, offshore Australia, aiming to enhance energy reserves. The research integrates extensive datasets, including a 3-D seismic reflection volume, fifty-two 2-D seismic lines, well logs from six wells, checkshot surveys, and surface geologic maps. The Northern Carnarvon Basin, a significant hydrocarbon reserve in Australia, has experienced a multi-phase extensional history and exhibits a complex stratigraphy linked to six tectonic phases. The pressing need for increased energy reserves necessitates revisiting old hydrocarbon fields to boost their production capacity using integrated methods. Well-log interpretation and correlation have revealed the lateral extents of petroleum system elements. The stratigraphic analysis identified three sequences and associated system tracts using the Depositional Sequence Model IV. Structural framework interpretation highlighted faults as migration pathways and hydrocarbon traps. Time and depth structure maps were produced to identify leads and prospects. Four leads and three prospects were pinpointed using RMS amplitude surfaces. Porosity values range from 20% to 29%, with hydrocarbon saturation between 51% and 65%. Volumetric assessment indicates that NAU Deep-1 contains 10.6 bcf, NAU Deep-2 holds 4.24 tcf, and NAU Deep-3 has 6.22 tcf of gas. These significant gas volumes highlight potential for future exploration and development in the Scarborough sedimentary infill.

Transformation of dense shelf water cascade into turbidity currents: Insights from high-resolution geophysical datasets

Dense shelf water cascade (DSWC) is a common oceanographic phenomenon on many continental shelves. Previous studies indicate that the DSWC could shape seabed physiography and carry seawater, sediment, and organic carbon a long distance from the continental shelf to the basin floor. However, it remains enigmatic how these DSWC's interact with seabed geomorphology and travel long distances from the shallow to deep marine environments. In this study, we employed high-resolution multibeam bathymetry, 2D and 3D seismic reflection, core description, and sediment grain size data from the Gippsland Basin, southeast offshore Australia. The continental shelf of the central Gippsland Basin stores sediment supplied by the along-shelf transported DSWC. By calculating the sediment motion threshold, we demonstrate that the DSWC is capable of entraining sediment from, and forming dense bottom nepheloid layers above, the seabed. Seismic reflection data reveal that cyclic steps are common on the shelf and slope, indicating a downslope-transported, supercritical current-dominated environment. Core observation and grain size analyses reveal that coarse-grained, Ta-typed turbidites are the major facies, indicating the presence of high-intensity downslope-traversing turbidity currents. Thus, supercritical turbidity currents are the dominant sedimentary process in the central Gippsland Basin. We illuminate that DSWC can interact with pre-existing seabed bathymetry created by a buried submarine landslide, resuspending sediment and igniting downslope-transported turbidity currents. The presence of numerous cyclic steps indicates that the turbidity current can evolve into a supercritical regime upon ignition, leaving complex seabed geomorphology and allowing the forming currents to travel across the shelf and extend more than 80 km down the lower slope. As revealed by our literature review, we imply that the transformation of DSWC into turbidity currents should be a common sedimentary process on outer continental shelves globally, significantly sculpting the seabed morphology and facilitating sediment and other marine particles transportation from shallow to deep sea.

Comments: Global CCS Projects’ CO2 Capture Capacity Grows Nearly 50% in 2022

CCS projects “accelerated” in 2022 with the CO2 capture capacity of all CCS facilities under development growing 44% over the past 12 months, bringing the total capacity of those projects to 244 mtpa of CO2. In a report released in mid-October, the Global CCS Institute said 61 new facilities were added to the project pipeline in 2022 for a current tally of 30 projects in operation, 11 under construction, and 153 in development. The Americas, especially North America, lead the world in CCS deployment. Recent US and Canadian governmental incentives were cited by the Institute in a regional overview. The US Inflation Reduction Act of 2022 includes enhancements to Internal Revenue Service Section 45Q and $369 billion in funding for climate and energy. The legislation extends the start of construction timing to the end of 2032; lowers capture thresholds, including direct pay; and expands transferability. The US Infrastructure Investment and Jobs Act includes more than $12 billion to be spent on CCS over the next 5 years. Canada’s 2022 federal budget includes an investment tax credit: the credit rate is 60% for direct air capture projects, 50% for all other carbon capture projects, and 37.5% for transportation, storage, and use from 2022 to 2030. From 2031 to 2040, the tax rates drop to 30%, 25%, and 18.75%.The boost in activity is reflected in recent CCS‑related updates reported in JPT: - ExxonMobil joined CF Industries and EnLink in a blue ammonia project in Louisiana that could capture and store 2 million metric tons of CO2 starting in 2025. - Technip Energies signed a letter of intent to design and build a large-scale floating storage and injection hub offshore Australia. It would be the world’s first, since to date, offshore carbon capture and storage projects use pipelines to transport CO2 to injection sites. - Equinor and Wintershall Dea have agreed to develop a comprehensive CCS supply chain system connecting Germany with CSS storage on the Norwegian Continental Shelf. - Texas and Louisiana are stepping up efforts to assume regulatory authority for an emerging wave of CCS projects.- In October, Canada released draft guidelines on how new oil and gas projects should demonstrate “best-in-class” greenhouse gas emissions performance. SPE’s CO2 Storage Resources Committee, under the SPE Carbon Dioxide Capture, Utilization, and Storage Technical Section, published Storage Resources Management System (SRMS) Guidelines to support the commercialization of CO2 storage. Released in September, the guidelines include suggestions for the application of the SRMS with the intent of including details of the processes of quantification, categorization, and classification of storable quantities so that the subjective nature of subsurface assessments can be consistent between storage resource assessors. The role of petroleum engineers in achieving technically sound results in energy transition projects of all kinds was highlighted during a presentation at the SPE Annual Technical Conference and Exhibition by Josh Etkind, global upstream deepwater digital transformation manager for Shell, and Rita Esuru Okoroafor, assistant professor, Texas A&M University, Harold Vance Department of Petroleum Engineering. In their presentation in the SPE Pavilion, “Transferable Skills: Petroleum Engineering and Geoscience Skills Are Shaping the Low-Emission Energy Transition,” they shared a chart showing the core oil and gas-related technical skill sets required for low-emission energy technologies. Etkind and Okoroafor emphasized the opportunities offered by the technologies shown in the chart below for upstream petroleum engineers and young engineers entering the industry. Looking only at CCS, the Global CCS Institute’s call for the growth of global CO2 storage to “billions of tons per year to meet climate targets” from the current 40 mtpa also points to a growing need for the relevant skills and technical knowledge.

A Simple Breakage Model for Calcareous Sand and Its FE Implementation

This paper reports the development of a particle breakage model within a finite-element (FE) framework based on the modified Mohr–Coulomb model (MMC). A modified form of the strength-dilatancy relationship is used to incorporate additional energy consumed due to particle breakage. A new particle breakage index is introduced and constitutively linked to the mobilized friction angle. The model is verified against carefully conducted drained triaxial tests on medium-dense calcareous sand obtained from offshore Australia as well as other published data. The model is implemented in the FE program ABAQUS for both small strain and large deformation FE analyses in practical field problems. Several important microscopic observations can be made from the proposed model that are not limited to identifying soil regions where particle breakage is maximized. Due to its inherent simplicity, the newly developed model can provide reasonable predictions for soil-structure interaction problems in calcareous sands.

E&P Notes (August 2022)

E&P Notes (August 2022)

Concurrent 9. Presentation for: Managing offshore Australia and its resources

Presented on Wednesday 18 May: Session 9 The Australian Government manages over 10 million km2 of ocean, one of the largest marine jurisdictions in the world. Australia’s marine estate is a significant and growing source of wealth for all Australians. The area is used by a range of industries, government and persons for various purposes including shipping and navigation, tourism, conservation, culture and heritage, commercial and recreational fishing, oil and gas exploration and production, and defence activities. Looking ahead, activities such as carbon capture and storage (CCS) and offshore renewable energy will be active in the offshore. This paper explores the regulatory frameworks which govern interaction and co-existence of CCS projects, petroleum exploration and development and offshore wind proposals in Commonwealth waters. To access the presentation click the link on the right. To read the full paper click here

Managing offshore Australia and its resources

The Australian Government manages over 10 million km2 of ocean, one of the largest marine jurisdictions in the world. Australia’s marine estate is a significant and growing source of wealth for all Australians. The area is used by a range of industries, government and persons for various purposes including shipping and navigation, tourism, conservation, culture and heritage, commercial and recreational fishing, oil and gas exploration and production, and defence activities. Looking ahead, activities such as carbon capture and storage (CCS) and offshore renewable energy will be active in the offshore. This paper explores the regulatory frameworks which govern interaction and co-existence of CCS projects, petroleum exploration and development and offshore wind proposals in Commonwealth waters.

Unsupervised Machine Learning, Multi-Attribute Analysis for Identifying Low Saturation Gas Reservoirs within the Deepwater Gulf of Mexico, and Offshore Australia

An effective method of identifying and discriminating undersaturated gas accumulations remains unresolved, resulting in uncertainty in hydrocarbon exploration. To address this problem, an unsupervised machine learning multi-attribute analysis is performed on 3D post-stack seismic data over several blocks within the deepwater Gulf of Mexico and within the Carnarvon Basin, offshore Australia. Results reveal that low-saturation gas (LSG) reservoirs can be discriminated from high-saturation gas (HSG) reservoirs by using a combination of instantaneous attributes that are sensitive to small amplitude, frequency, and phase anomalies with self-organizing maps (SOMs). This methodology shows promise for de-risking prospects, even if it is not quantitative, particularly in frontier and exploration basins where wells may not exist or be very limited. However, this method only proved to be successful within the Gulf of Mexico and yielded limited results in the Carnarvon Basin. This difference is most likely due to the Carnarvon Basin having a different amplitude response resulting from a different burial history and fluid saturations when compared to the Gulf of Mexico. Therefore, this method is non-transferrable, and a different combination of attributes may be needed in other LSG-prone basins.

Characterization of tropical cyclones to identify the response based design metocean conditions for an FPSO mooring system

Design metocean conditions (DMC) for a single-point moored weathervaning vessel exposed to the cyclonic environment are obtained by characterising the tropical cyclones on the basis of extreme responses induced in its mooring system. Determination of design metocean conditions, as combinations of wave, wind and current associated with the extreme response, is a multivariate problem that does not have a unique solution. Its probabilistic nature dictates that the design metocean condition should be a short-term sea state which is most probable to occur while at the same time inducing the extreme N-year response of the system. This study aims at the development of a methodology for obtaining the DMC for a mooring system of a weathervaning vessel by characterizing the tropical cyclonic environment based on the mooring responses. First, a method for identifying the DMCs within the metocean history is described, along with its limitations. Then, the storm characterisation approach is introduced, based upon the maximum responses of the system rather than the extreme sea states in a particular event. The metocean conditions at a specific location during the passage of a cyclone are described by a set of dimensionless shape functions, with their main parameters linked to the most probable mooring tension experienced during each cyclonic event. It is further determined that for a weathervaning floating system, cyclones should be separated into two distinct types according to the relative wave-wind direction highly affecting the mooring response. The cyclone shape functions are generalized to describe the characteristic cyclones at the N-year return period response, which are determined by considering a limited group of storms inducing the mooring responses close to the N-year response. By averaging the dimensionless shape functions for each of the two cyclone types, the characteristic cyclones can be synthesized to describe two types of events, where the N-year mooring response is most likely to occur. The robustness of the method is checked through a case study, in which the characteristic 5-year, 10-year and 100-year DMCs are produced for a mooring system of a floating production, storage and offloading vessel (FPSO) permanently moored offshore Australia. By using a 44-year hindcast time history of cyclonic metocean conditions, the DMCs were developed from the response-based synthesized N-year cyclones and compared with the specific short-term conditions retrieved from the same time history. An alternative averaging method for the cyclone shape functions is also considered and results are discussed.

Lithology, porosity and saturation joint prediction using stochastic rock physics modelling and litho-petro-elastic inversion

In this paper we present the application of a single-loop litho-petro-elastic (LPE) inversion, which is a data assimilation algorithm that uses nonlinear Zoeppritz reflectivity operators with sequential filtering. It integrates rock physics models with seismic amplitude variation with offset (AVO) inversion and Bayesian inversion to define lithology, elastic, and petrophysical properties in a single loop, thus, combining several steps of the conventional reservoir characterization workflow. In the conventional multistep approach, the lithology and petrophysical properties are generated sequentially from elastic properties after AVO inversion is performed, which can add prediction uncertainty at each step and produce results that are not correlated with each other. The LPE inversion ensures that the predicted properties maintain the relationships defined by the rock physics model. The LPE inversion was tested on data from offshore Australia with three wells, for a faulted reservoir zone containing oil with a gas cap. It provided robust predictions for lithology, porosity, and water saturation, which matched acceptably at the three wells. The algorithm also accounted for subsurface uncertainties as it produced prediction probabilities of facies, porosity, and water saturation using multidimensional probability density functions. This approach can be effectively used to classify reservoir properties in a single-loop workflow.

Technical Solution Improves Safety, Efficiency of Well Construction Offshore Australia

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 202353, “Drilling-Performance and Risk-Management Optimization Offshore Australia: Improving Overall Safety and Efficiency of the Well-Construction Process,” by Chandrasekhar Kirthi Singam, Farshid Hafezi, and Clyde Rebello, Schlumberger, 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 emergence of real-time well construction performance-monitoring centers has improved the service delivery for operators across numerous offshore oil fields in Australia significantly. The complete paper details new technologies and work flows implemented for three Australian offshore wells, with the primary objective of improving drilling efficiency while managing associated risks. Additional objectives included optimizing daily operational performance, thus delivering time savings for the operator and highlighting areas of possible improvements. Introduction The paper describes a successful drilling campaign in a challenging field in the Timor Sea. It describes how data analysis, risk evaluation, and real-time performance monitoring can be influential in saving rig time and project success. As part of this project, a major operator in Australia decided to perform an infill drilling campaign involving three high-angle directional wells (J type) in a saturated, complex field. The campaign design stage was initiated in 2017 with a main objective of delivering the project within authority-for-expenditure (AFE) budget and with planning for all potential challenges. Technical Overview The technical solution (Fig. 1) was deployed using drilling-interpretation software and executed its work flows to achieve the required objectives.

Uncertainty and interpretability analysis of encoder-decoder architecture for channel detection

We have adopted a method to understand uncertainty and interpretability of a Bayesian convolutional neural network for detecting 3D channel geobodies in seismic volumes. We measure heteroscedastic aleatoric uncertainty and epistemic uncertainty. Epistemic uncertainty captures the uncertainty of the network parameters, whereas heteroscedastic aleatoric uncertainty accounts for noise in the seismic volumes. We train a network modified from U-Net architecture on 3D synthetic seismic volumes, and then we apply it to field data. Tests on 3D field data sets from the Browse Basin, offshore Australia, and from Parihaka in New Zealand prove that uncertainty volumes are related to geologic uncertainty, model mispicks, and input noise. We analyze model interpretability on these data sets by creating saliency volumes with gradient-weighted class activation mapping. We find that the model takes a global-to-local approach to localize channel geobodies as well as the importance of different model components in overall strategy. Using channel probability, uncertainty, and saliency volumes, interpreters can accurately identify channel geobodies in 3D seismic volumes and also understand the model predictions.

Normal Fault Kinematics and the Role of Lateral Tip Retreat: An Example From Offshore NW Australia

AbstractUnderstanding how normal faults grow is key to determining the tectono‐stratigraphic evolution of rifts. According to recent studies, normal faults tend to grow in two temporally distinct stages: a lengthening stage, followed by a throw/displacement accumulation stage. However, this model is still debated and not widely supported by many additional studies. Relatively few studies have investigated what happens to a fault as it becomes inactive, that is, does it abruptly die, or does the at‐surface trace‐length progressively shorten by so‐called tip retreat? We, here, use a 3D seismic reflection data set from the Exmouth Plateau, offshore Australia, to develop a three‐stage fault growth model for seven normal faults of various sizes and to show how the throw‐length scaling relationship changes as a fault dies. We show that during the lengthening stage, which lasted <30% of the faults’ lives, faults reached their near‐maximum lengths, yet accumulated only 10%–20% of their total throw. During the throw/displacement accumulation stage, which accounts for c. 30%–75% of the faults’ lives, throw continued to accumulate along the entire length of the faults. All of the studied faults also underwent a stage of lateral tip‐retreat (last c. 25% of the faults’ lives), where the active at‐surface trace‐length decreased by up to 25%. The results of our study may have broader implications for fault growth models, slip rate variability during fault growth, and the way in which faults die, in particular the role of lateral tip‐retreat.

Eustatic, Climatic, and Oceanographic Influences on Geomorphology and Architecture of Isolated Carbonate Platforms: Miocene, Northwest Shelf, Australia

Abstract The Miocene represents an interval of marked global change, and this evolution is reflected in carbonate platforms from this epoch. Seismic stratigraphic characterization of high-resolution (ca 60 Hz) 3D seismic data from the Browse Basin, offshore Australia, reveals a middle to upper Miocene three-part seismic stratigraphic subdivision. Each unit consists of several seismic sequence sets and their component sequences. Seismic stratal geometries and seismic facies define a prograding shelf (Langhian and older), a barrier-reef complex with scattered platforms (upper Langhian–early Tortonian), and aggrading and prograding isolated platforms (early Tortonian–Messinian). The data permit description and interpretation of high-fidelity stratigraphic details of the initiation, expansion, termination, and geomorphology of over 100 platforms in this interval. The results reveal that the isolated platforms initiated following the Middle Miocene Climatic Optimum. The succession includes major seismic stratigraphic boundaries and overall patterns of platform growth and demise that correspond roughly with periods of pronounced eustatic change associated with initiation of eastern Antarctic ice sheets. Although invoking a eustatic control for coarse trends may be tempting, mismatch between the numbers and ages of sequences, as well as the variable stacking patterns among contemporaneous platforms regionally, precludes such an interpretation; conversely, some globally recognized eustatic changes do not have a pronounced manifestation in this area. Thus, it appears that the eustatic signal combined with dynamic physical regional processes such as waves, currents, and variable subsidence creates the complex architecture and geomorphology of platforms. These results illustrate how global changes can interact with local controls to create diverse patterns of birth, growth, and demise of carbonate platforms and drive local stratal heterogeneity.

Quantitative analysis of a footwall‐scarp degradation complex and syn‐rift stratigraphic architecture, Exmouth Plateau, NW Shelf, offshore Australia

AbstractInteractions between footwall‐, hangingwall‐ and axial‐derived depositional systems make syn‐rift stratigraphic architecture difficult to predict, and preservation of net‐erosional source landscapes is limited. Distinguishing between deposits derived from fault‐scarp degradation (consequent systems) and those derived from long‐lived catchments beyond the fault block crest (antecedent systems) is also challenging, but important for hydrocarbon reservoir prospecting. We undertake geometric and volumetric analysis of a fault‐scarp degradation complex and adjacent hangingwall‐fill associated with the Thebe‐2 fault block on the Exmouth Plateau, NW Shelf, offshore Australia, using high resolution 3D seismic data. Vertical and headward erosion of the complex and fault throw are measured. Seismic‐stratigraphic and seismic facies mapping allow us to constrain the spatial and architectural variability of depositional systems in the hangingwall. Footwall‐derived systems interacted with hangingwall‐ and axial‐derived systems, through diversion around topography, interfingering or successive onlap. We calculate the volume of footwall‐sourced hangingwall fans (VHW) for nine quadrants along the fault block, and compare this to the volume of material eroded from the immediately up‐dip fault‐scarp (VFW). This analysis highlights areas of sediment bypass (VFW > VHW) and areas fed by sediment sources beyond the degraded fault scarp (VHW > VFW). Exposure of the border fault footwall and adjacent fault terraces produced small catchments located beyond the fault block crest that fed the hangingwall basin. One source persisted throughout the main syn‐rift episode, and its location coincided with: (a) an intra‐basin topographic high; (b) a local fault throw minimum; (c) increased vertical and headward erosion within the fault‐scarp degradation complex; and (d) sustained clinoform development in the immediate hangingwall. Our novel quantitative volumetric approach to identify through‐going sediment input points could be applied to other rift basin‐fills. We highlight implications for hydrocarbon exploration and emphasize the need to incorporate interaction of multiple sediment sources and their resultant architecture in tectono‐stratigraphic models for rift basins.