Irish Atlantic Margin Research Articles

Subsurface storage capacity in structural traps in underexplored sedimentary basins: Hydrogen (H 2 ) and carbon dioxide (CO 2 ) storage on the Irish Atlantic margin

Abstract Methodologies for storage assessment developed for basins with dense data coverage are typically not optimally applicable to underexplored sedimentary basins. To address this, a methodology and workflow for storage assessment in underexplored basins is presented which uses existing datasets to identify structural traps and populate a fluid-in-place equation which can be used for a variety of gases including CO 2 and H 2 . This is then applied to the Irish Atlantic margin; Jurassic, Triassic and Carboniferous reservoirs are investigated to understand their reservoir quality and extent, and related seals. Structural trap types are described and the theoretical capacities of three candidate sites with varying data coverage are calculated. The results highlight the potential for underexplored sedimentary basins on the Irish Atlantic margin to support offshore renewable energy projects and reduce Ireland's CO 2 emissions. This workflow is applicable to a variety of underexplored sedimentary basins and emphasises the utility of legacy hydrocarbon datasets for early-stage subsurface storage assessment. Other aspects of energy storage are also discussed, including man-made salt caverns, other candidate reservoir-seal pairs, and the potential for collaborative infrastructure development with CO 2 emitters and renewable energy projects.

Assessing the Rotation and Segmentation of the Porcupine Bank, Irish Atlantic Margin, During Oblique Rifting Using Deformable Plate Reconstruction

AbstractPublished plate reconstructions have provided insights regarding the formation of the North Atlantic, in which the motion of the Porcupine Bank, on the Irish Atlantic margin, underlain by orogenic pre‐rift crustal basement terranes, is investigated and restored. Previous reconstructions of the Porcupine Bank mainly relied on potential field data rather than seismic constraints and failed to reveal the role of inherited crustal terranes during rifting and subsequent crustal deformation. In this study, five deformable plate tectonic models with distinct structural inheritance trends are established in GPlates by adjusting a previously published restoration model for the North Atlantic. For each model, driving factors such as the inclusion of the Orphan Knoll, the Flemish Cap poles of rotation, and the motion of the eastern border of the Porcupine Basin are also considered. To assess the validity of deformable plate models, crustal thickness estimates obtained from gravity inversion and seismic data modeling are compared with those calculated via deformable plate models. The preferred deformable plate model proposes the subdivision of the Porcupine Bank into four blocks with each block experiencing poly‐phased rotation and shearing prior to the final continental breakup, implying strong inheritance and segmentation of the Porcupine Bank and Porcupine Basin. The reconstructed paleo‐positions of the Flemish Cap and Porcupine Bank within deformable regions reveal evolving conjugate relationships during rifting, which are assessed using regional seismic transects from both margins. Finally, extensional obliquity between both margins is quantitatively restored, showing time‐variant orientations due to the rotation and shearing of associated continental blocks.

Kinematic interaction between stratigraphically discrete salt layers; the structural evolution of the Corrib gas field, offshore NW Ireland

The kinematic interaction of thin salt layers during basin evolution has received little attention to date, despite there being several basins which contain multiple thin salt layers across NW Europe. This study utilises high-quality 3D seismic reflection data coupled with borehole data to investigate the evolution of the structure containing the Corrib gas field which is composed of two distinct salt structures. Located in the Slyne Basin offshore NW Ireland, the structure consists of a NE-SW oriented Permian salt anticline which folds the overlying Mesozoic stratigraphy. Upper Triassic salt acts as a second mechanical detachment and forms an elongate salt roller parallel to the crest of the Permian salt anticline. The Upper Triassic salt roller forms the footwall of a listric fault which downthrows the anticlinal crest of the folded Jurassic section to the SE. The Permian salt anticline began rising during the Late Triassic and Early Jurassic driven by low-strain regional extension. During the main phase of rifting, a combination of basement tilting, gravity gliding, and salt welding resulted in a steep increase in the amplitude of the Permian salt anticline. The relief on the salt-cored fold resulted in gravity gliding on the Triassic salt, forming the parallel salt roller and listric fault. The throw distribution on the listric fault is driven by the combined mechanisms of gravity sliding on the Triassic salt and by inflation of Triassic salt in the footwall of the fault. The listric fault is reactivated post-rift, suggesting modification of the Permian and Triassic salt structures. This study improves the understanding of the kinematic interaction of thin salt layers during syn-rift and post-rift deformation, with implications for hydrocarbon exploration on the Irish Atlantic margin and further afield.

The influence of multiple salt layers on rift‐basin development; The Slyne and Erris basins, offshore NW Ireland

AbstractIn contrast to much of the European Atlantic margin, the influence of salt on basin evolution has received little attention to date in the basins west of Ireland, despite salt being proven in several boreholes. Using an extensive seismic reflection database coupled with data from exploration wells and shallow boreholes, this study maps the distribution and composition of salt layers and investigates their role in the structural evolution of the Slyne and Erris basins offshore west of Ireland. Two salt‐prone intervals have been proven. The Upper Permian Zechstein Group is present throughout the Slyne and Erris basins, whereas the Upper Triassic Uilleann Halite Member is only developed in the Northern Slyne and Southern Erris sub‐basins. Both sedimentary units mechanically detach pre‐, post‐ and intra‐salt stratigraphy. Both salt layers underwent halokinesis during basin development, creating a variety of salt‐related structures. Salt pillows and salt rollers formed in the Zechstein Group, causing folding and rafting in the overlying Mesozoic section, driven by active faulting within the pre‐salt basement. Halokinesis in the Uilleann Halite Member caused thin‐skinned crestal collapse of the overlying Jurassic section above anticlines cored by Zechstein salt. Where both Triassic and Permian salt are present, unique structural geometries are formed when two stratigraphically discrete but kinematically linked halokinetic structures develop. The most common structural configuration consists of a Zechstein salt pillow and an Uilleann Halite salt wall separated by Lower Triassic sandstones. The fold axis of the salt pillow trends parallel to the strike of the salt wall. The results of this study provide a framework for the evolution of halokinetic structures in other basins on the Irish Atlantic margin, give a greater insight into the Permian and Triassic palaeogeography of the region and have more general implications for the evolution of salt‐related structures in rift basins with multiple stratigraphically discrete salt layers.

Investigating the Porcupine Atlantic margin, offshore Ireland, through integration of new seismic reflection and gravity data

While the offshore Irish Atlantic margin and related rift basins have been intensively studied for several decades, the Porcupine Bank, straddling between the well-studied Porcupine and Rockall basins, is a poorly understood region due to lacking sufficient geophysical data coverage. In this study, ten newly acquired long-offset multichannel seismic profiles extending across the western Porcupine Bank margin, combined with potential field data, are used to investigate the crustal architecture, tectonic history, and rift-related magmatism along the margin. Significant margin-parallel and margin-perpendicular structural variations are observed and these are used to map the crustal architecture in terms of rifted margin domains. In the transitional zone between continental and oceanic crust, both peridotite ridges with shallow reflective basement and exhumed serpentinized mantle with deeper and smoother basement are interpreted, similar to the conjugate Iberian and Newfoundland margins, as well as further south at the Goban Spur margin. In addition to inferred variations in extension rate during poly-phased rifting episodes, the reactivation of pre-existing inherited Caledonian and Variscan structural fabrics are proposed to have influenced the variable geometries and distributions of the crustal domains along the Porcupine Atlantic margin. Northwestward increasing volcanism and related reflectivity patterns support the transition from magma-poor rifting in the southeast to magma-rich rifting in the northwest. Rigid plate reconstructions of the Irish Atlantic and the Newfoundland margins, particularly involving the Flemish Cap, back to the Early Campanian period, show asymmetric rifting and final continental breakup migrating toward the Porcupine Bank region. This asymmetry is possibly due to oblique extension between the two margins, and/or oblique deformation on the Porcupine Bank side due to its rotation during the opening of the Porcupine Basin.

From rifting to hyperextension: Upper Jurassic–Lower Cretaceous tectono‐stratigraphy of the Porcupine Basin, Irish Atlantic Margin

AbstractHyperextended basins are increasingly recognized along the outboard parts of continental margins as aborted basins created during continental break‐up. Many of the concepts for understanding and modelling basin evolution and fill were developed for regions that have undergone modest crustal stretching (β < 2) and may not be valid in basins where the crust and upper mantle are heavily modified by extreme stretching. The present study uses extensive 2D and 3D seismic and well data to analyse the Late Jurassic–Cretaceous tectono‐stratigraphic evolution of the Porcupine Basin, bracketing the timing of hyperextension. It is an instructive basin, offshore west of Ireland, preserving low‐magnitude strain in the north, with increasing degrees of hyperextension in the south. Detailed mapping of strain domains (proximal, necking and hyperextended) across the Porcupine Basin reveals five main rift segments, each with a distinctive geometry and strain history. During early low‐strain rifting, inherited crustal structures strongly influenced the rift architecture by controlling the location and geometry of fault‐controlled marine depocentres. The transition from hyperextension to post‐rift subsidence was marked by locally developed, unconformity‐bounded, marine sequences that draped the underlying rift topography. Whilst these ‘transition sequences’ are dated as Tithonian above the necking domain, similar but younger Early Cretaceous transition packages developed in the hyperextended domain, suggesting extension migrated towards the rift axis during hyperextension. Early post‐rift sequences were broadly distributed across the rift centre and basin flanks before strong, thermally‐controlled subsidence of the hyperextended crust, along with hinging of the necking domain, locally to the point of slope failure, gave rise to axially‐focused marine deposition. Hyperextension may have left the basin susceptible to intra‐plate stress changes accounting for several unconformities within the post‐rift fill. This study provides an improved basin‐wide understanding of the tectono‐stratigraphic evolution of hyperextended basins.

The Rockall and the Orphan Basins of the Southern North Atlantic Ocean: Determining Continuous Basins across Conjugate Margins

Reconstructions of the opening of the North Atlantic Ocean generally result in the Orphan Basin, offshore Newfoundland, Canada, lying approximately conjugate to the rift basins on the Irish Atlantic margin at the onset of seafloor spreading toward the end of the Early Cretaceous. Most of these plate reconstructions have involved rigid plates with plate motions based solely on the interpretation of oceanic magnetic anomalies. In particular, these reconstructions often show the Rockall Basin, west of Ireland, forming a continuous Mesozoic basin with the West Orphan Basin, offshore Newfoundland. However, more recent plate reconstructions involving deformable plates have called this conjugate relationship into question. The goal of this study is to investigate the validity of this potentially continuous basin system by reconstructing and restoring present-day seismically-constrained geological models both spatially and temporally back to their original configurations pre-rift. By comparing the reconstructions in terms of sedimentary package thicknesses and crustal thicknesses in 3D, using both rigid and deformable plate reconstructions to orient the reconstructed models, we are able to test different basin connectivity scenarios using a multidisciplinary approach. Our analysis provides subsurface geophysical support for the hypothesis that the Rockall Basin was originally conjugate to and continuous with the East Orphan Basin during Jurassic rifting, later linking to the West Orphan Basin as rifting evolved during the Early Cretaceous. This complex basin evolution example highlights the need for using 3D rifting mechanism models to properly understand the fundamental driving forces during rifting and has significant implications for assessing basin prospectivity across conjugate margin pairs.

From orogenic collapse to rifting: A case study of the northern Porcupine Basin, offshore Ireland

Offshore Ireland, the North Atlantic opening is generally interpreted as successive, 10-to-15-Myr-long, rifting events during the Mesozoic. However, their interaction is poorly documented in terms of structural inheritance and fault reactivation. From extensive seismic and well data in the northern Porcupine Basin, we show that extension actually evolved over an abnormal period of ca. 220 Myrs with overlapping tectonic stages that, all together, reflect the Irish Atlantic Margin construction from the Palaeozoic.During the Carboniferous, extension initiated throughout the Variscan and Caledonian thickened crusts and controlled the deposition of generally slightly-tilted, continental clastics. Two subsequent rifting events, during the Triassic-Lower Jurassic and Upper-Jurassic-Lower Cretaceous, controlled shallow-to-deep marine sedimentation in separated sub-basins that progressively widen and connect once extension localised along bounding faults.We propose that extensional tectonics, first controlled by a general orogenic collapse reflecting an early stage of stretching, is then followed by several rifting periods that ultimately evolved toward the hyper-thinning of the margin and shaped an aborted rift propagator. This evolution is due to pre-existing orogenic-related structures and boundary conditions variations. Such a continuum of deformation, although pulsed, implies varying structural interactions of crustal scale that are often recorded in syn-tectonic sediments.

Geology and seepage in the NE Atlantic region

Abstract Exploration for hydrocarbons in the NE Atlantic mainly focuses on the central eastern margin. The western margin has remained virtually unexplored, with no exploration wells drilled so far. A cost-efficient way to infer the presence of natural hydrocarbons in the poorly explored regions of the NE Atlantic is the application of synthetic aperture radar (SAR). This study presents four areas, the Western Barents Sea Margin, the Irish Atlantic Margin, East Greenland and Jan Mayen, where clustered oil-slick data indicate possible active oil seepage. The eastern margin of the NE Atlantic contains numerous oil-slick observations, but along the western margin the number of observations is limited, partly due to a persistent sea-ice coverage. Based on the tectonostratigraphic setting, it is suggested that Triassic and Jurassic source rocks are the most likely candidates for the generation of seeps in the areas studied. Near Jan Mayen and East Greenland, Cenozoic source rocks could also be present. SAR data are a useful tool in an early stage of exploration, but further work is needed to improve the understanding of the subsurface below the observed oil slicks in the NE Atlantic to determine the origin of the seepage.

The thermal history of the western Irish onshore

We present here a low-temperature thermochronological study that combines the apatite fission-track and (U + Th)/He dating methods with a pseudo-vertical sampling approach to generate continuous and well-constrained temperature–time histories from the onshore Irish Atlantic margin. The apatite fission-track and (U + Th)/He ages range from the Late Jurassic to Early Cretaceous and the mean track lengths are relatively short. Thermal histories derived from inverse modelling show that following post-orogenic exhumation the sample profiles cooled to c . 75 °C. A rapid cooling event to surface temperatures occurred during the Late Jurassic to Early Cretaceous and was diachronous from north to south. It was most probably caused by c . 2.5 km of rift-shoulder related exhumation and can be temporally linked to the main stage of Mesozoic rifting in the offshore basins. A slow phase of reheating during the Late Cretaceous and Early Cenozoic is attributed to the deposition of a thick sedimentary sequence that resulted in c . 1.5 km of burial. Our data imply a final pulse of exhumation in Neogene times, probably related to compression of the margin. However, it is possible that an Early Cenozoic cooling event, compatible with our data but not seen in our inverse models, accounts for part of the Cenozoic exhumation. Supplementary material: Details on the apatite fission-track and (U + Th)/He methods are available at www.geolsoc.org.uk/SUP18765 .

Tectonostratigraphic evolution of the northern Porcupine Basin, Irish Atlantic margin, during the Late Jurassic–Early Cretaceous, implication for a regional compressional event

The conventional interpretation of the Jurassic–Lower Cretaceous succession in the Porcupine Basin suggests an extensional setting with progressive deepening of the basin. However, well data show a prominent gap of several million years between the Upper Jurassic and Lower Cretaceous. A data base of 15 key wells and approximately 5,000 km of seismic reflection data were examined in the northern Porcupine Basin, in order to understand the nature, controls and mechanisms of this unconformity. Seven seismic markers, constrained by well data, are mapped. It is shown that during the Late Jurassic (possibly the Oxfordian–Kimmeridgian), the basin experienced extension and synrift deposition. During the latest Jurassic–earliest Cretaceous (possibly the Tithonian–early Berriasian), a series of north-trending structural highs and lows developed and extensive areas in the northern Porcupine Basin experienced folding, uplift and erosion. Evidence from the study suggests that compression, uplift and erosion played an important role in the shaping of the depositional and structural architecture of the basin and caused formation of the regional Base Cretaceous Unconformity in the northern basin. It is suggested that the deformation in the northern Porcupine Basin during the latest Jurassic–earliest Cretaceous may be related to the initial closure of the Alpine Tethys during the late Tithonian. This tectonic event may also have resulted in compressional deformation and formation of the Base Cretaceous Unconformity elsewhere in Western Europe.

Structural and stratigraphic evolution of the Connemara discovery, Northern Porcupine Basin: significance for basin development and petroleum prospectivity along the Irish Atlantic Margin

ABSTRACT A detailed seismic stratigraphic interpretation of a previously unpublished, well-calibrated 3D seismic volume and regional 2D seismic lines in the Northern Porcupine Basin, west of Ireland has provided significant insights into the basin development, sedimentary fill and petroleum prospectivity within this area of the Irish Atlantic continental margin. The results of the seismic interpretation presented here provide a detailed description of the evolution of the Connemara discovery and have resulted in a revised model for the tectonic development and stratigraphic evolution of the fault block structure containing the oil accumulation. The new seismic stratigraphic analysis also provides the basis for an alternative interpretation of Early Cretaceous ‘clinoform’ features based on the results of the 3D seismic interpretation which supports a genesis through structural rotation of onlapping horizons rather than depositional downlap favoured by previous interpretations. This new-found structural and stratigraphic understanding not only has significant implications for determining the main controls on the Connemara discovery, but also provides a basis upon which to place other prospective structures in their regional context, including the identification and risking of exploration plays and prospects in the area.

Structure across the northeastern margin of Flemish Cap, offshore Newfoundland from Erable multichannel seismic reflection profiles: evidence for a transtensional rifting environment

We present the results from processing and interpreting nine multichannel seismic reflection lines collected during the 1992 Erable experiment over the northeastern margin of Flemish Cap offshore Newfoundland. These lines, combined into five cross-sections, provide increased seismic coverage over this lightly probed section of the margin and reveal tectonically significant along-strike variations in the degree and compartmentalization of crustal thinning. Similar to the southeastern margins of Flemish Cap and the Grand Banks, a transitional zone of exhumed serpentinized mantle is interpreted between thinned continental and oceanic crust. The 25 km wide transitional zone bears similarities to the 120 km wide transitional zone interpreted as exhumed serpentinized mantle on the conjugate Irish Atlantic margin but the significant width difference is suggestive of an asymmetric conjugate pair. A 40–50 km wide zone of inferred strike-slip shearing is interpreted and observed to extend along most of the northeastern margin of Flemish Cap. Individual shear zones (SZs) may represent extensions of SZs and normal faults within the Orphan Basin providing further evidence for the rotation and displacement of Flemish Cap out of Orphan Basin. The asymmetry between the Flemish Cap and Irish conjugate pairs is likely due in large part to the rotation and displacement of Flemish Cap which resulted in the Flemish Cap margin displaying features of both a strike-slip margin and an extensional margin.

Sedimentology, sandstone provenance and palaeodrainage on the eastern Rockall Basin margin: evidence from the Pb isotopic composition of detrital K-feldspar

Abstract The Rockall Basin, west of Ireland, is a frontier area for hydrocarbon exploration, but currently the age and location of sand fairways through the basin are poorly known. A recently developed provenance approach based on in-situ Pb isotopic analysis of single K-feldspar grains by laser ablation multi-collector inductively coupled mass spectrometry (LA-MC-ICPMS) offers advantages over other provenance techniques, particularly when applied to regional palaeodrainage issues. K-feldspar is a relatively common, usually first-cycle framework mineral in sandstones and its origin is typically linked to that of the quartz grains in arkosic and sub-arkosic rocks. Consequently, in contrast to other techniques, the Pb-in-K-feldspar tool characterizes a significant proportion of the framework grains. New Pb isotopic data from K-feldspars in putative Permo-Triassic and Middle Jurassic sandstones in Well 12/2-1z (the Dooish gas condensate discovery) on the eastern margin of the Irish Rockall Basin are reported. These data suggest that three isotopically distinct basement sources supplied the bulk of the K-feldspar in the reservoir sandstones and that the relative contribution of these sources varied through time. Archaean and early Proterozoic rocks (including elements of the Lewisian Complex and its offshore equivalents), to the immediate east, NE and north of the eastern Rockall Margin, are the likely sources. More distal sourcelands to the NW cannot be ruled out but there was no significant input from southern sources, such as the Irish Massif. These data, together with previously published regional Pb isotopic data, highlight the important role played by old, near and far-field Archaean–Proterozoic basement highs in contributing sediment to NE Atlantic margin basins. The Irish Massif appears to have acted as a significant, but inert, drainage divide from the Permo-Triassic to the Late Jurassic and hence younger, Avalonian and Variscan sand sources appear to have been less important on the Irish Atlantic Margin.

Drainage reorganization during breakup of Pangea revealed by in-situ Pb isotopic analysis of detrital K-feldspar

Pb isotopes in detrital K-feldspar grains provide a powerful provenance tracer for feldspathic sandstones. Common Pb isotopic compositions show broad (hundred-kilometer scale) regional variation, and this signature can survive weathering, transport, and diagenesis. The feldspar Pb signature can be measured rapidly using laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS), and careful targeting can avoid inclusions and altered regions within grains. Here, we combine a new Pb domain map for the circum–North Atlantic with detrital K-feldspar Pb isotopic data from Triassic and Jurassic sandstones from basins on the Irish Atlantic margin. The Pb isotopic compositions reveal otherwise cryptic feldspar populations that constrain the evolving drainage pattern. Triassic sandstones originated from distant Archean and Paleoproterozoic rocks, probably in Green-land, Labrador, and the Rockall Bank to the NW, implying long (>500 km) transport across a nascent rift system. Later, Jurassic sandstones had a composite Paleo- and Mesoproterozoic source in more proximal sources to the north (

The crustal structure and regional development of the Irish Atlantic margin region

During the past decade, a suite of wide-angle seismic reflection/refraction profiles has been shot in the Porcupine, Rockall and Hatton basins, as well as across the Hatton continental margin. Integration of the wide-angle seismic data with normal-incidence reflection profiles and with gravity and magnetic data reveals a clear picture of the regional crustal and upper mantle structure, and of the large-scale sedimentary geometry of the Late Palaeozoic–Cenozoic basins in the area. The region contains a set of large sedimentary basins resting upon variably thinned continental crust. The crust beneath the basin-bounding structural highs is typically around 25km thick, with the crust beneath the basins being as thin as 6km. The Porcupine Basin contains up to 10 km of sediments. Up to 7km of sedimentary strata are indicated in the Rockall Basin, with 4km of sediment in the Hatton Basin. Several seismically distinct layers, with P-wave velocities of 1.8–5.3 kms −1 , were identified above the basement in the sedimentary basins. A significant variability in thickness of the sedimentary layers is seen on the wide-angle data. These reflect the presence of a number of older Mesozoic rift sedimentary basins. The wide-angle data reveal marked topographic irregularity at the base of the sedimentary succession in the Rockall Basin, interpreted as the result of extensive multi-phase Mesozoic rifting. The Moho beneath the Rockall Basin shows a clear asymmetry, with a steeper gradient along the eastern margin of the basin. The continent–ocean boundary west of the Hatton High has a significant underplated and volcanic component, while that at the southern margin of the Rockall Basin shows evidence of a greater transpressive component. The nature of the lower crust appears to exert a controlling influence on the location of crustal separation. In areas where thinning is accommodated by upper and middle crustal attenuation, the strength of the upper crust (and underlying brittle mantle) has served to preserve the integrity of the continental crust and inhibit seafloor spreading. However, where the lower crust is thinned significantly, as beneath the Hatton continental margin, crustal separation has taken place even though the upper and middle crust remains relatively thick.

The structural framework of the Irish Atlantic Margin

Analysis of the results of regional mapping, integrated with new regional subcrop maps, has yielded significant regional concepts regarding the development of the frontier sedimentary basins west of Ireland. Five provinces of basement and Devono-Carboniferous rocks are mapped across the region. The nature of the basement successions, together with their inherent lineaments and structural fabrics, exerted a major influence on the location and structural segmentation of the basins and in acting as a conduit for Early Cretaceous and Early Cenozoic igneous activity. The major structures in the Porcupine region were N–S throughout its Late Palaeozoic to Cenozoic history, while those in the Slyne, Erris, Rockall and probably Hatton basins were predominantly NNE–SSW to NE–SW. The main structural controls in the Goban region were orientated ENE–WSW and NNW–SSE. The Porcupine Basin is shown to have a more pronounced N–S orientation than has hitherto been proposed. In particular, the basement core of the Porcupine High is shown to extend southwards to the Goban province, thereby isolating the basin for most of its history from the Atlantic region to the west. Separate Permo-Triassic to Jurassic basins occur on the flanks of the main, younger Rockall Basin and their location and orientation were influenced by NE–SW to ENE–WSW structural fabrics. Permo-Triassic sedimentation took place in a series of rift basins in the Porcupine, Rockall, Slyne–Erris and Goban regions. Jurassic rifting was widespread in most of the basins, commencing in Middle Jurassic time in the Slyne Basin but later (Late Jurassic) in the Porcupine and Rockall basins. Early Cretaceous sedimentation was more pronounced in the Porcupine and Rockall basins and shows less control by deep-seated structures. Late Jurassic to Early Cretaceous basin margin uplift on the flanks of the Rockall Basin in particular, is likely to explain the thin to absent nature of such strata in some of the adjacent smaller basins.

Wavelet and multitaper coherence methods for assessing the elastic thickness of the Irish Atlantic margin

SUMMARY There have been some inconsistencies in estimates of the effective elastic thickness of the continental lithosphere Te based upon admittance or coherence relationships between gravity and topography. This paper compares multitaper and wavelet methods to analyse the coherence between Bouguer gravity and bathymetric data over the Irish Atlantic margin. The analyses show that similar lateral Te variations can be recovered from the data, but demonstrate that the size of the data window can give rise to a significant downward bias in Te estimates. A seismically constrained 3-D gravity inversion over the Rockall basin shows the presence of surface and subsurface loads whose ratio is loosely correlated with load ratio variations generated from the wavelet coherence method. The Te and load ratio, f variations can be plausibly related to major geological structures on the margin. If the load ratio variations can be interpreted geologically, it implies that spectral based methods to estimate effective elastic thickness must incorporate subsurface loads within the underlying theoretical model. On the Irish Atlantic margin, Te is generally low (6‐18 km) and is associated with a NE‐SW Caledonian trend. The weakest lithosphere is in the southern Rockall basin, Porcupine bank and Porcupine basin and the strongest lithosphere is along the Rockall‐Hatton region. The low Te values are consistent with results from other passive margins. The reasons for such low Te values on the Irish Atlantic margin remain unclear, but may be the consequence of Te being frozen into the lithosphere when loads were emplaced during continental breakup and temperature gradients were high. The process of sedimentation and the presence of fluids may be contributory factors. There is an indication of a geological and rheological divide between the Rockall‐Hatton region and the Rockall basin, possibly associated with the Caledonian orogenic front.

Structural Evolution of a Complex 3D Fault Array in the Cretaceous and Tertiary of the Porcupine Basin, Offshore Ireland

Abstract A high-quality 3D seismic survey, located in the northwest Porcupine Basin (Irish Atlantic Margin), has been used to investigate the geometry and origin of pervasively developed and complexly distributed linked extensional fault arrays, present within Late Cretaceous and Early Tertiary sequences. The faults show a downwards transition from relatively simple, planar fault segment geometries (~N-S-trending) within younger Early Eocene sand-dominated clastic sequences, into complex conjugate arrays in the underlying older Early Eocene to Late Cretaceous shale-dominated sequences. Rectilinear to polygonal structural configurations are developed at the deeper levels. Most of the fault array ultimately terminates downwards into the Late Cretaceous, where structural accommodation may have taken place by localized or more regional bedding plane slip and/or by volume changes resulting from compaction of fine-grained sequences. Locally, reactivated Jurassic syn-rift extensional faults are locally seen to link upwards into the shallow fault array and appear to have controlled both the intensity and facing direction of the shallower faults on a km scale. The seismic data also clearly show that early upslope-throwing faults are cross-cut by later, downslope-throwing faults. Such geometries are comparable to those formed in sandbox models where gravitational collapse of a tilted sequence is the dominant process controlling fault development. Overall, the fault array geometries seen in the Cretaceous and lower Tertiary successions in this area are interpreted to have resulted from gravitational collapse processes during basin subsidence and sediment compaction, and where the main deformation mechanism was non-rigid block rotation. Differential compaction of Cretaceous and lower Tertiary sediments over pre-Cretaceous rift topography and selective reactivation of the Jurassic fault array are also considered important influences on the resultant fault distribution in 3D.

About this title

Ireland is virtually encircled by sedimentary basins which developed in response to periods of rifting and thermal subsidence. These offshore basins have been the focus of intermittent phases of exploration since drilling of the first well in 1970 and, to date, 136 wells have been drilled. Most of the drilling so far has concentrated on structural traps, but recent exploration has begun to focus on a variety of stratigraphic traps, with greater emphasis on results obtained from studies of the Atlantic margin basins. The Petroleum Exploration of Ireland's Offshore Basins contains a set of 27 papers on a wide range of topics relating to recent exploration of the Irish offshore sedimentary basins. These papers address aspects of the structural and stratigraphic evolution, thermal history, petroleum systems, reservoir geology and sea-bed processes in the Irish offshore area. Although the main focus is on petroleum systems and those issues bearing on exploration risk, the exploration effort has yielded fundamental new insight into the wider development of starved passive continental margins. The volume will be of interest to oil industry explorationists and researchers focusing on NW European sedimentary basins and the evolution of the Irish Atlantic margin.