Full-Waveform Inversion and Least-Squares Migration to Improve Images in Northern Carnarvon Basin

Northern Carnarvon Basin is located in North West Shelf of Western Australia. The basin has over 10km sediments and owns both oil-prone and gas-prone sediments and is the current largest oil and gas producing basin in Australia. A geological section through this basin is shown in Figure 1, the complex geological settings from shallow to deep leads to significant processing challenges. In the vintage processing, the seismic image at reservoir level is deteriorated due to the presence of following geological complexities: 1) rugose water bottom, 2) shallow frequent canyons or channel systems, 3) shallow spatial-variant Tertiary carbonates, and 4) shallow gas chimneys and other geo-bodies. These complex overburdens plus limited small-angle coverage of primary reflections from narrow azimuth (NAZ) streamer surveys make it very difficult for ray-based reflection tomography to resolve the shallow velocity. As a result, the target image suffers from large well mis-ties, low signal-to-noise ratio (S/N) and severe event undulations. In addition, shallow fast-velocity layers cause severe illumination issues for deep targets which are compounded by limited offsets of NAZ surveys. Furthermore, localised absorption effects from gas pockets lead to dimming amplitudes for events beneath them. To deal with these issues, we propose to use time-lag full wave-form inversion (TLFWI) to resolve the velocity of complex overburdens and least-squares Q prestack depth migration (LS Q-PSDM) to compensate for illumination issues and absorption effects for the latest reprocessing. In the following sections, application procedure and results of these two technologies will be discussed. Seismic inversion was also conducted to assist the processing and analysis of the final result.

Multiphase oblique extension on the North West Shelf of Australia

<p>The North West Shelf of Australia has experienced numerous rift events during its prolonged evolution that most likely started in the Lower Palaeozoic and continued through to the formation of the present day passive margin in the Lower Cretaceous.  Carboniferous and Permian is associated with rifting of the Lhasa terrane, a phase extension in the Lower and Middle Jurassic associated with the separation of the Argo terrane Upper Jurassic to Lower Cretaceous extension culminated in the separation of Greater India and Australia.  Investigations based on interpretation of extensive, public domain seismic data, combined with numerical mechanical modelling, demonstrate that crustal structure, rheology and structural fabrics inherited from older events exert a significant control on the architecture of younger rifts.</p><p>Defining the older, more deeply buried rift episodes is challenging, but with seismic data that now images deeper structures more effectively, it is clear that NE-SW oriented Carboniferous to Permian aged rift structures control the overall geometry of the margin.  Variations in the timing, distribution and intensity of that rift may account for some of the complexity that governs the Triassic – a failed arm of the rift system might account for the accumulation of thick sequences of fluvio-delatic sediments in an apparent post-rift setting, while active deformation and igneous activity continued elsewhere on the margin.</p><p>A renewed phase of extension began in the latest Triassic in the western part of the Northern Carnarvon Basin, but became progressively younger to the NE.  High-resolution mechanical numerical experiments show that the dual mode of extension that characterises the Northern Carnarvon Basin, where both distributed and localised deformation occurs at the same time, is best explained by necking and boudinage of strong lower crust, inherited form the Permian rift event, proximal to the continental margin, and a subdued extensional strain rate across the distal extended margin.  A very clear and consistent pattern of ENE oriented extension, which interacts obliquely with the older NE-SW oriented Permian aged structures, is apparent across the whole of the Northern Carnarvon Basin and extends north east into the Roebuck and Browse Basins.  This is at odds with the NW-SE oriented extension predicted by the separation of the Argo terrane which occurs at this time.  This may be explained by the detached style of deformation that characterises the Mesozoic interval.  Alternatively, the separation of Greater India may have exerted a stronger influence on the evolution of the margin during the Jurassic than hitherto recognised.</p>

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

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

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

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

Permian source rocks of the onshore and nearshore Carnarvon Basin

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

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)