Rift-related magmatism resulting in widespread igneous intrusions has been documented in various basins, including the Faroe Shetland Basin (UK), the Voring and Møre basins (Norway), and along the NW Shelf of Australia. Seismic mapping, combined with fieldwork, has resulted in greater understanding of subsurface intrusive plumbing systems but knowledge of emplacement style and the mechanisms by which intrusions propagate is limited. The interpretation of a 3D seismic dataset from the Exmouth Sub-basin, NW Shelf of Australia, has identified numerous igneous intrusions where a close relationship between intrusions and normal faults is observed. These faults influence intrusion morphology but also form pathways by which intrusions have propagated up through the basin stratigraphy. The steep nature of the faults has resulted in the intrusions exploiting them and thus manifesting as fault-concordant, inclined dykes; whereas in the deeper parts of the basin, intrusions that have not propagated up faults typically have saucer-shaped sill morphologies. This transition in the morphology of intrusions related to fault interaction also highlights how dykes observed in outcrop may link with sills in the subsurface. Our interpretation of the seismic data also reveals subsurface examples of bifurcating intrusions with numerous splays, which have previously only been studied in outcrop. Supplementary material: Figures showing uninterpreted seismic lines are available at https://doi.org/10.6084/m9.figshare.c.4395974
Mafic volcanic rocks, typically basalts of mainly late Cretaceous age, have been intersected by Gippsland Basin wells. Intersections of volcanic rocks primarily occur along the basin-bounding Rosedale Fault System in the northern part of the basin, where they exhibit a close spatial correspondence with high-CO2 content gas accumulations. Though petrographic data indicates that the basalts have been variably altered to clays and carbonates, they provide the top seals to numerous hydrocarbon accumulations, most notably at the Kipper Field. Despite the widespread distribution of these volcanics and their relevance to petroleum systems, they have received only sporadic attention over the past few decades. Here we combine petrophysical, geomechanical, geophysical and geochemical datasets to elucidate the origin of the volcanic record of the Gippsland Basin, and to evaluate their potential role in the decarbonisation of the basin, for example through providing opportunities for intra and sub-basaltic storage of CO2.
The Upper Paleocene–Eocene rock record in the Faroe–Shetland Basin is punctuated by a series of unconformities that reflect a persistent tectonic instability throughout the syn- to early post-breakup period, a duration of about 20 myr. A particular focus is on a Late Paleocene subaerial unconformity, herein termed the Flett unconformity, which has been argued to have formed in response to a transient pulse of mantle convective uplift associated with a proto-Iceland plume. However, ambiguity over its presumed correlation with the Faroe Islands Basalt Group combined with stratigraphic and palaeogeographical analysis of the Upper Paleocene–Eocene succession indicates that it is just one of a series of subaerial unconformities that are spatially linked to the same part of the Faroe–Shetland Basin. Most of these Late Paleocene–Eocene unconformities post-date volcanism, and their formation coincides with vertical motions associated with phases of uplift, inversion and compressional deformation linked to the growth of structures, such as the Wyville Thomson and Munkagrunnur ridges, and the Judd Anticline. These deformation phases are broadly coeval with intraplate and plate-boundary events in the wider NE Atlantic region. The possibility that the Flett unconformity had a similar tectonic origin should not be discounted.
Previous basin modelling of the Faroe–Shetland Basin (FSB, offshore UK) has suggested mid-Cretaceous petroleum generation, which predates the deposition of the working Paleogene reservoirs and traps. To justify the time discrepancy between generation, reservoir, and trap formation, factors such as intermediary accumulations and overpressure have been invoked. However, across much of the FSB, the Cretaceous sequences that overly the Kimmeridgian source rock are heavily intruded by Paleogene-aged intrusions. Recent modelling has shown that the emplacement of the intrusions, coupled with lower radiogenic heat production from underlying basement, leads to estimates of petroleum generation occurring up to 40 myr more recently than suggested by previous models. In this work, we seek to better understand the role that igneous intrusions have exerted on petroleum generation and migration in the FSB. Models with varying thicknesses of Paleogene intrusions are compared with those that consider the Cretaceous sequence as purely sedimentary (i.e. similar to assumptions in previous modelling). The estimated times of petroleum generation are compared with geochronological constraints on the ages of oils (i.e. c . 90–68 Ma) along with the deposition and formation of other petroleum system elements. By considering only the effect of igneous intrusions, the expulsion onset from the source rock is retarded by up to 12 myr. In addition, our models show the impact of the intrusions on petroleum saturation and migration, suggesting that intrusions have potentially compartmentalized the basin, trapping petroleum beneath or within the sill complex. Finally, our findings suggest that basin models in regions impacted by significant magmatism need to consider the impact of intrusions to more accurately constrain both petroleum generation and migration. Thematic collection: This article is part of the New learning from exploration and development in the UKCS Atlantic Margin collection available at: https://www.lyellcollection.org/topic/collections/new-learning-from-exploration-and-development-in-the-ukcs-atlantic-margin
The Ceduna Sub-basin is the main depocentre of the frontier Bight Basin, which formed as a result of the late Jurassic-Cenozoic separation of Australia and Antarctica. The sedimentary fill of the Ceduna Sub-basin is dominated by two structurally distinct deltaic lobes of Cenomanian and Santonian-Maastrichtian age with combined thickness >12 km. This region is the focus of growing exploration interest, and thus improved knowledge of its origin and evolution is essential for reducing exploration uncertainty. However, because the Ceduna sub-basin is located completely offshore in water depths up to 5 km, to date there has been little exploratory drilling in this region. With primary data from the sub-basin itself lacking, we have collected a variety of new thermochronological and geochronological datasets from the onshore margins and hinterland to the Ceduna sub-basin, which have a bearing on the evolution of the offshore region. These datasets include:1. Zircon U-Pb ages from several samples of drillcore from the Lower Cretaceous Loongana Formation, which is preserved in the Denman Basin, a shallow depression that underlies the onshore Eucla Basin. Age populations within these data suggest that sediment input at this time was predominantly from the N and W.2. Zircon U-Pb ages from several samples of drillcore from the Winton Formation, an Albian-Cenomanian-age fluvial-lacustrine sequence from the Eromanga Basin. This sequence has been proposed as an analogue for the Cenomanian deltaic lobe in the Ceduna sub-basin, which has yet to be penetrated by drilling.3. AFTA and VR data from outcropping rocks in the Eyre Peninsula, and subsurface rocks retrieved by drilling in the Polda sub-basin, to the NE of the Ceduna sub-basin. These data point to substantial exhumation of this region during the late Cretaceous.4. Zircon U-Pb and fission track ages from the Turonian-Maastrichtian sequence penetrated by the offshore Gnarlyknots-1 well. Interpretation of these ages suggests that this sequence was largely sourced from recycled Permian-Early Cretaceous cover and underlying basement rocks eroded from the proximal, NE basin margin. The integration of these onshore and offshore datasets provides new, valuable insights into the Cretaceous palaeogeography of the Ceduna sub-basin, the tectonic processes controlling the input of clastic sediments, and the prospectivity of this frontier exploration region.
A decline in conventional hydrocarbon reserves coupled with technological advances and growing energy demand has driven a shift in exploration of energy rich Australian Basins, with a progressive focus on unconventional energy sources (e.g. Coal Seam Gas, Shale Gas and Enhanced Geothermal Systems). Understanding natural fractures is critical to assessing the prospectivity of unconventional plays, as structural permeability in the form of interconnected natural fracture networks commonly exert a prime control over fluid flow in reservoir units due to low primary permeabilities. Structural permeability in the Northern Perth, South Australian Otway, and Northern Carnarvon basins is characterised using an integrated geophysical and geological approach combining wellbore image logs, core, 3D seismic attribute analysis and detailed structural geology. Integration of these methods allows for the identification of faults and fractures over a range of scales (mm-km), providing crucial permeability information. New stress orientation data is also interpreted, allowing for stress-based predictions of fracture reactivation. The resulting fracture orientations from each basin are compiled into a map of structural permeability of the Australian continent, demonstrating orientation variations which cannot be explained through fracture formation and reactivation prediction based on known stress orientations. The importance of validating remotely sensed fractures is demonstrated in the Otway Basin; analysis of core shows open fractures are rarer than image logs indicate, due to the presence of fracture-filling siderite, an electrically conductive cement which may cause fractures to appear hydraulically conductive in image logs. Although the majority of fractures detected are favourably oriented for reactivation under in-situ stresses; fracture fills primarily control which fractures are open, demonstrating that lithological data is often essential for understanding potential structural permeability networks and the orientations at which open fractures may form. The Carnarvon Basin is shown to host distinct variations in fracture orientation; a result of the in-situ stress regime, regional tectonic development, and local structure. A detailed understanding of the structural development, from regional-scale (100s km) down to local-scale (km), is demonstrated to be important when attempting to understand natural fracture orientations, and hence, structural permeability.
Natural fractures can be identified in wellbores using electric resistivity image logs; however, the challenge of predicting fracture orientations, densities, and probable contribution to subsurface fluid flow away from the wellbore remains. Regional interpretations of fracture sets are generally confined to areas featuring an extensive reservoir analog outcrop. We have made use of extensive data sets available in Western Australia’s Northern Carnarvon Basin to map subsurface natural fractures, contributing to a regional understanding of fracture sets that can be applied to broader parts of the basin. The Northern Carnarvon Basin is composed of distinct structural domains that have experienced differing tectonic histories. Interpretation of regional fractures was achieved through an integrated approach, incorporating electric resistivity image logs from 52 Carnarvon Basin wells and seismic attribute analysis of two 3D seismic data sets: Bonaventure_3D ([Formula: see text]) and HC_93_3D ([Formula: see text]). Integration of these two data sets allows for a regionally extensive identification of natural fractures away from well control. Fractures of differing age and character are identified within the basin: Outboard areas are dominated by fractures likely to be open to fluid flow that are parallel to subparallel to the approximately east–west present-day maximum horizontal stress, providing possible flow conduits between potential damage zones identified alongside the north–northeast/south–southwest-striking faults that constitute the major structural trend of the basin, and inboard areas dominated by northeast–southwest to north–northeast/south–southeast fractures formed in fault damage-zones alongside normal, and inverted-normal, faults at those orientations. Finally, fractures observed in wells from the Rankin Platform and Dampier Subbasin occur at neither of these orientations; rather, they closely parallel the strikes of local faults. Additionally, variation is seen in fracture strikes due to isotropic present-day stress magnitudes. This methodology extends fracture interpretations from the wellbore and throughout the region of interest, constituting a regional understanding of fracture sets that can be applied to broader parts of the basin.
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