The Caswell Sub-basin, situated within the Browse Basin in the North West Shelf constitutes one of Australia’s primary hydrocarbon producing regions, with notable gas-condensate producing fields including Ichthys and Prelude. Jurassic syn-rift sandstones are extensively distributed across the basin and serve as one of the major reservoirs. However, reservoir sequences are typically intensely faulted and exhibit heterogeneity in thickness and lithofacies, with some areas experiencing localised erosion on uplifted fault blocks (e.g. Northern Caswell Sub-basin). Hence, understanding the nature of the 3D fault patterns and their growth history is crucial for evaluating the reservoir characteristics for field development and additional exploration activities. This study, therefore, aims to evaluate the structural framework and the tectonic evolution of the Caswell Sub-basin. Detailed structural interpretation of Paleozoic and younger sequences was conducted using multiple 3D seismic datasets extending over Ichthys, Prelude, Lasseter, and Crown fields. Our study focuses mainly on Mesozoic faulting patterns and kinematics evaluated from interpreted structural maps, thickness changes of each stratigraphic interval and fault throw profiles of major bounding faults. The extensional phase during the Early–Middle Jurassic triggered the development of NE–SW trending faults and the deepening of the Brewster Graben. While, from the Late Jurassic to the Early Cretaceous, the development of E–W trending faults in the north of the Crown and Lasseter fields indicate a shift in the regional stress regime. We highlight the importance of evaluating the structural linkage from basement to cover sequences to achieve a comprehensive understanding of reservoirs and associated petroleum systems.
Carbon capture and storage (CCS) is a critical component of proposed pathways to limit global warming, though considerable upscaling is required to meet emissions reduction targets. Quantifying and managing the risks of fault reactivation is a leading barrier to scaling global CCS projects from current levels of ~40 million tonnes of carbon dioxide(CO2) per year (to target levels of several gigatonnes of CO2 per year), because CO2 injection into reservoirs can result in increased pore-fluid pressure and temperature changes, which can reduce the strength of rocks and faults and induce brittle failure. This can result in induced seismicity, whilst hydraulic fracturing of seals could provide pathways for CO2 leakage. Consequently, identifying favourable geomechanical conditions (typically determined through data on pre-injection rock stress, mechanical and elastic properties, and pore-fluid pressures) to minimise deformation of reservoirs and seals represents a key challenge in the selection of safe and effective sites for CCS projects. Critically, however, such geomechanical data are typically spatially limited (i.e. restricted to wells) and mainly consist of pre-injection crustal stress orientation measurements, rather than a full 3D description of the stress tensor and related geomechanical properties. This paper reviews some key geomechanical issues and knowledge gaps (particularly those associated with data availability and limitations) that need to be understood to enable successful reservoir and seal management for CCS projects. We also highlight recent advances in multi-scale and dimensional geomechanical modelling approaches that can be used to assess sites for the secure storage of CO2 as well as other gases, including hydrogen.
Summary The North West Continental Margin of Australia is a volcanic rifted margin. Jurassic-Cretaceous breakup of Gondwana resulted in the intrusion of significant volumes of igneous rock into sediments of the Carnarvon Basin. The last review of the regional distribution of this intrusive system was published 20+ years ago. Since, there have been major advances in the understanding of intrusions in sedimentary basins, and step changes in the quality of seismic reflection data available in the Carnarvon basin. We present preliminary findings of a new study that investigates the spatial and temporal distribution of intrusions in the basin and the impact of this magmatism on petroleum systems. Using modern 3D and reprocessed 2D seismic reflection data, we have mapped plumbing systems of intruded igneous complexes of the Carnarvon Basin in unprecedented detail. We see: typical ‘saucer shaped’, and ‘stepped’ intrusion geometries; intrusions exploiting faults and cross-cutting stratigraphy as they rise to the surface; interconnected intrusions, forming continuous networks >50 km in length; intrusions mainly present within Triassic sediments and possibly sourced from a hotspot which, during breakup, was located beneath the shelf edge to the west of the Exmouth Sub-basin. Finally, we consider impacts of this intrusive system on petroleum exploration.
This paper reports the first evidence for significant overpressures in the Otway Basin, southern Australia, where most previous studies have assumed near-hydrostatic pore pressures. Overpressures are observed in the Upper Cretaceous Shipwreck supersequence in several wells in the Voluta Trough, such as Bridgewater Bay–1, Normanby–1 and Callister–1. One of these wells penetrated successions of Pliocene-Recent marine clastic sediments nearly 700 m thick that were deposited rapidly in submarine channels and that were probably carved during the late-Miocene to early-Pliocene. Wireline and drilling data suggest that overpressures present in Upper Cretaceous shales and sandstones in the Belfast Mudstone and Flaxman and Waarre formations developed either due to disequilibrium compaction—where there is no evidence of hydrocarbon generation and thick Pliocene stratigraphy is present—or due to fluid expansion where there is evidence of hydrocarbon generation and the Pliocene stratigraphy is thin to absent. The two key factors that may indicate abnormal pore pressures in Upper Cretaceous sediments in the central Otway Basin are the thickness of Pliocene stratigraphy and whether or not hydrocarbons are actively generating from source rocks.
The passive southern margin of the Australian continent contains a rich record of late Miocene–Pliocene neotectonic deformation and uplift that continues to the present day as witnessed by unusually high levels of seismicity for a so-called 'stable continental region.' To date, however, few studies have sought to estimate the magnitude of exhumation triggered by this deformation and uplift. Here we combine apatite fission track analysis (AFTA), apatite (U–Th)/He dating and vitrinite reflectance (VR) data from the Nerita-1 well in the Torquay sub-basin with seismic reflection data from this basin and the adjoining Otway Ranges to constrain the magnitude and driving mechanisms of exhumation in this part of the southern Australian margin. The Cenozoic succession in this basin has been deformed by folding and reverse faulting and contains a major, low-angle mid-Miocene unconformity that can be traced for distances of ∼1500 km along the margin. Paleothermal data from Nerita-1 show that the sub-mid-Miocene succession has been more deeply buried by ∼1 km of now missing section, and indicate that exhumation began between 10 and 5 Ma, in excellent agreement with stratigraphic constraints. Our estimates of removed section and higher than previous estimates based on extrapolation of seismic reflectors, but are corroborated by AFTA results from nearby wells. Seismic data show that late Miocene-onwards intraplate deformation in the Torquay-sub-basin and Otway Ranges has been accomplished by reverse-reactivation of normal faults within Cretaceous–early Paleogene syn-rift successions, resulting in folding of overlying post-rift late Paleogene–Neogene sediments. The probable cause of this deformation and uplift is increased levels of intraplate stress induced by enhanced coupling of the Indo-Australian and Pacific plates ∼10 Myr ago, and our results thus demonstrate the key role that plate boundary-controlled stress fields play in intraplate uplift and deformation.
The Middle Jurassic Rattray Volcanic Province is located at the triple junction of the North Sea continental rift system. It has previously been thought to be sourced from three large central volcanoes: the Glenn, Fisher Bank and Ivanhoe volcanic centres. Re-interpretation using 3D seismic and well data shows that no volcanic centres are present and the Rattray Volcanics were instead sourced in fissure eruptions from linear vents, including the Buchan–Glenn Fissure System, a c. 25 km long zone of WSW–ENE-striking linear fissure vents and associated small volcanic edifices across the Buchan–Glenn Horst. The orientation of the fissures is broadly parallel to the Highland Boundary Fault, which intersects the Rattray Volcanics at the Buchan–Glenn Fissure System, implying that Mid-Jurassic magmatism exploited pre-existing crustal structural anisotropies established during the Caledonian Orogeny. The lack of large intrusive complexes beneath the Rattray Volcanics indicates that the pre-Middle Jurassic sedimentary sequences (e.g. the Devonian–Carboniferous Old Red Sandstone Group, the Permian Rotliegend and Zechstein groups and the Triassic Skagerrak Formation) extend further than previously supposed and therefore the presence of possible subvolcanic reservoir and source rock units within the triple junction of the Central North Sea may have been overlooked.
Abstract The UK Rockall Basin is one of the most underexplored areas of the UK Continental Shelf (UKCS), with only 12 exploration wells drilled since 1980. With only one discovery made in 2000 (Benbecula (154/1-1) gas discovery), the general view of the basin from an exploration viewpoint is not positive. However, over the last 15 years, our knowledge of the petroleum systems of the Atlantic Margin has substantially increased. With the recent acquisition of new seismic data by the UK Government as part of the OGA's Frontiers Basin Research Programme, it is a pertinent time to re-examine the prospectivity of the UK Rockall Basin. This paper presents a history of exploration within the UK Rockall Basin, from the first well drilled in the basin in 1980, to the last well, drilled in 2006. We then present new insights into the lack of success during exploration within the basin, in particular by focusing on the extensive Early Cenozoic volcanic rocks within Rockall, to illustrate the wide range of potential interactions with the petroleum system. We also present evidence that points to the potential of a viable intra-basaltic (Rosebank) type play along the eastern flank of the Rockall Basin.
Summary The Faroe–Shetland Basin (FSB) is one of the only significant exploration frontiers remaining on the UKCS and will continue to contribute to indigenous oil and gas supply throughout the next 40 years. Over half of the reserves and resources discovered within the FSB are situated on the Corona and Rona Ridge. However, in contrast to the Rona Ridge, as of January 2023, none of the discoveries made along the Corona Ridge have been sanctioned for development. Throughout the last 20 years, exploration success along the Corona Ridge has been largely limited to the Paleocene–Eocene-aged reservoirs around the Rosebank Field. Subsequently, published literature focused on the geology of the central-northern Corona Ridge is scarce. This study reveals new insights into the geology of the central-northern Corona Ridge, which lies within an area in need of a substantial discovery, through the interpretation of 3D seismic data integrated with the analysis and reinterpretation of hydrocarbon exploration wells. We interpret that no Triassic-age strata are present along the Corona Ridge, challenging the previous understanding of Triassic distribution within the FSB. We also extend the Colsay Member intra- and sub-basaltic play concept from the Rosebank Field north-east into 213/23-1.
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