As the world progressively shifts to a low/neutral carbon economy over the course of the twenty-first century, geoscience will continue to play a vital role in the energy sector and beyond. Static and dynamic characterization of the subsurface, a key component of the petroleum industry for decades, will be required to evaluate and unlock a variety of different initiatives including carbon, capture and storage (CCS), hydrogen (or any other gas) storage, thermal energy storage and geothermal energy development. Onshore and offshore Ireland and Great Britain, Lower Triassic sandstones are likely to represent one of the primary geological targets for future geoenergy applications. This paper provides a review of the Triassic Sherwood Sandstone Group depositional system across the British and Irish region of NW Europe including its overall stratigraphic context, palaeogeography, sediment provenance and transport directions, and present-day distribution. In addition to geological outcrop data, there is a wealth of subsurface data available from historical petroleum wells. Looking forward, one important focus of geoscience research will be to extrapolate away from these well-defined control points to predict sandstone facies distribution, diagenetic history and hence overall reservoir quality in new undrilled areas that will be of interest to the geothermal, CCS and gas storage sectors.

Matrix-rich sandstones (matrix content >15%) have long been recognized in the ancient sedimentary record, most commonly in deep-marine depositional systems. The origin of the matrix component, whether it was detrital or diagenetic, has been widely debated in the literature and the issue became known as the ‘greywacke problem’. However, with increased research focus in clay-rich sediment gravity flows that emplace matrix-rich sandstones (hybrid event beds and related mud-rich strata) of fundamentally primary origin, it is timely to revisit the greywacke problem in the context of current state of knowledge of HEBs and associated transitional flow deposits and matrix-rich sandstone models. In the present study, literature-based datasets for greywackes and HEBs were compiled and assessed for patterns in bed structure, matrix origin, matrix cut-off percentage, depositional environment, and geological age in order to address some key questions pertinent to the greywacke problem. The dataset indicates that the bed structure of numerous ancient greywackes is similar to HEBs and its variants. The majority (61%) of studies in the greywacke database suggested a ‘secondary’ origin of the matrix whereas 39% of the studies advocated a ‘primary’ genesis. In comparison, a ‘primary’ origin of matrix was inferred for all HEBs and related deposits. Nevertheless, a mixed nature of matrix comprising both detrital and diagenetic components cannot be ruled out in both greywackes and HEBs. The majority of greywackes (82%) and HEBs (87%) display similar range of matrix contents (>15%), therefore the matrix cut-off threshold for matrix-poor and matrix-rich sandstones can be confidently placed at 15–20% matrix. This dataset illustrates that both greywackes (86%) and HEBs (82%) occur most commonly in deep-marine environments. Greywackes (48%) and HEBs (37%) of Paleozoic and Cenozoic age respectively dominate in the complied database. Analysis of these factors, integrated with learnings from HEB and related deposits suggest that many ancient deep-water greywackes may be re-interpreted as HEBs with significant original detrital matrix.

Highly efficient sediment gravity flows can bypass mid fan channels and lobes and deposit significant volumes of sand, mud and particulate organic matter in outer fan and basin plain settings. The Serpukhovian to Bashkirian fill to the Shannon Basin, western Ireland, includes deep-water fan deposits (Ross Sandstone Fm) that gradationally overlie basin floor shales (Clare Shale Fm). As part of a broader progradational succession, the upward transition from muddy basin floor to sandy fan preserves the stacked deposits of settings present prior to and outboard of mid-fan channels and lobes. Three fully cored boreholes and associated wireline data constrain the facies tracts in an 18 km long panel orientated oblique to original depositional dip. Two distal successions dominated by hybrid event beds (HEBs) are recognised, separated by a prominent condensed section. The lower Cosheen system includes m-thick, tabular HEBs with prominent linked debrites that pass down dip into much thinner sandstones overlain by sand-speckled mudstone caps that thicken distally before thinning. The latter are interpreted as secondary mudflows released following reconstitution of more thoroughly mixed sections of the up-dip linked debrites. Significant bypass and accumulation of mud by this mechanism helped heal local topography and maintain a relatively flat sea floor promoting an overall tabular geometry for the deposits of larger volume hybrid flows reaching the distal sector of the basin. The overlying distal Ross system fringe is characterised by very fine to fine-grained sandstones and is lateral to compensationally-stacked lobes further to the west. It has a progradational (at least initially) stacking pattern, facies transitions developed over shorter length scales, and includes outsized event beds but these are thinner than those in the Cosheen system. Common banding and evidence for turbulence suppression by dispersed clay rather than entrained mud clasts indicate these were transitional flows. In this case, event beds are inferred to taper distally, with significant mud emplaced by plug flow retained as caps to sandy event beds rather than bypassing down-dip. Different flow transformation mechanisms thus impacted how mud was partitioned across the fringe of the two systems and this influenced bed geometries, larger scale bed stacking patterns and stratigraphy. Whereas the flow efficiency concept stresses the ability of flows to carry sand in a basinward direction, it is also imperative to consider the variable efficiency of mud transport given the operation of clay-induced flow transformations. These can either promote bypass or trigger premature fallout of mud with implications for how systems fill accommodation, bed -scale facies transitions and the burial and preservation of particulate organic carbon fractionated along with the clay in deep-water system fringes.

Summary Conventional modelling methods such as object-based, variogram-based, or multiple point statistics modelling often struggle to reproduce realistic levels of connectivity particularly in scenarios where the net:gross ratio is high, but connectivity is low, such as deep-marine depositional systems like the Ross Formation (west of Ireland). Compression-based modelling allows for the generation of hierarchical models conditioned to wells with both facies proportion and geobody connectivity defined separately as modelling inputs. This method can be combined with conventional methods providing greater model flexibility for reproducing different degrees of connectivity for individual depositional elements. The workflow differs from a conventional modelling workflow by transforming the grid after the facies modelling step: this transformation allows for both facies proportions and connectivity to be honoured. The grid transformation must therefore be considered when defining the modeling inputs and designing the training images. The inverse transformation is applied to the conditioning data to ensure they are appropriately honoured at the end of the workflow. The method is illustrated using the well-characterised Ross Formation outcrops in the west of Ireland.

Hybrid event beds (HEBs) are tiered deep-water deposits that can include significant intervals of organic-rich muddy sandstone and sandy mudstone. They are emplaced by decelerating sediment gravity flows in which turbulence becomes extinguished due to increasing cohesion. Whilst several studies have addressed the distribution of organic matter (OM) in turbidites, the extent to which OM is segregated between the component HEB divisions has yet to be quantitatively addressed for clastic systems. Here we document bed scale fractionation of terrestrial OM in HEBs drawn from a range of deep-water sub-environments (basin floor sheets, outer fan fringes and mid-fan lobes) preserved in the Ross Sandstone Formation, western Ireland, a tropical Pennsylvanian deepwater fan complex. A suite of bulk geochemical techniques (TOC, Rock-Eval pyrolysis, delta 13C isotopes, and XRF scanning) and petrographic analyses were applied to HEB-dominated core intervals retrieved from four behindoutcrop boreholes. Results shows that muddy sandstones (H3 divisions) of the HEBs have significantly higher OM (average TOC = 1.2 wt% and up to 2.7 wt%) and mud contents than co-genetic cleaner sandstones (H1 divisions; average TOC = 0.2 wt% and up to 0.7 wt%). Muddy caps (H5 divisions) to the event beds have higher mud but relatively low OM contents (average TOC = 0.7 wt% and up to 1 wt%) compared to H3 divisions, implying textural fractionation of OM components, greater burn-down linked to slower suspension settling and/or downward propagation of oxidation fronts. HEBs can dominate distal lobe stratigraphy and are thus an important but under-represented sink for terrestrial carbon with enhanced preservation of OM in H3 divisions on account of rapid en-masse deposition, high mud abundances offering enhanced protection, and where thick, emplacement beyond the reach of oxidation fronts descending from the sea floor. The present study has important implications for understanding how carbon is buried and distributed in deep-water successions.

The Neogene Tabernas Basin, SE Spain, provides important evidence at outcrop for the interplay between tectonic deformation of the sea floor, slope instability and turbidity current behaviour. Dextral-oblique strike-slip faults and associated folds propagated along the basin axis to deform the palaeo-sea floor, creating structurally-controlled depressions in which turbidity currents were trapped and ponded. EW-trending syn-depositional faults define a narrow sub-basin that subsided asymmetrically as a negative flower structure. The sub-basin contains an expanded succession (>300 m of ponded turbidite sheets, debrites and slumps) along its northern margin flanked by the principal fault strand defined by a wide zone of sheared and calcite-veined marl. A narrower fault zone with a smaller displacement marks the southern margin of the sub-basin and the fill close to it is thin with internal discordances, evidence of local failure and southward thinning of sandstone sheets. Both northern and southern faults ‘died’ at the same stratigraphic level and were overstepped by basin floor turbidites showing evidence of weaker and longer-range topographic confinement. As turbidites healed and aggraded out of the sub-basin to progressively onlap the southern margin of the basin, major gravity failures occurred emplacing thick (>100 m) mass-transport complexes. The first initially reworked the southern part of the sub-basin fill together with the early onlap wedge, the second remobilised the onlap wedge, and the third records failure of the upper part of the slope well above the wedge. The first two were toe-confined failures, the third and furthest travelled was confined by basin axis topography. All three failures are lateral to or directly overlain by ‘megabed’ sheets on the basin floor, implying either a common trigger (earthquakes) or slope instability following reflection of large volume flows. Tabernas turbidites highlight the role of basin tectonics (as opposed to up-dip supply and sea level fluctuations) in directly impacting on deep-water processes and stratigraphy. Small deep-water transtensional sub-basins opened up along long transfer faults accommodating regional extension.

Hyperextended 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.

With this contribution we use a pair of overlapping 3D seismic surveys and two exploration wells to document the response of long-lived slope channels to the onset of bottom currents sweeping the lower slope in the NE Rockall Basin, offshore Ireland. Downslope gravity current activity, linked to a phase of uplift, prevailed throughout the Eocene and led to the formation of multiple channels, most notably a large-scale sinuous channel complex (Channel 4 Complex) tied to a persistent sediment entry point on the margin. Channels fed lobes on the floor of the basin, with increased axial tilting forcing gravity currents to flow parallel to the base of slope. A phase of margin-wide differential subsidence and basin deepening in the Late Eocene then activated bottom current circulation across the basin. Northward-flowing bottom currents first erosionally refashioned the lower slope creating a prominent unconformity with contourites that then initially forming infill drifts. Bottom currents swept along and obliquely upslope, building plastered drifts that straddled the lower sections of the still active channels. The drifts modified and amplified the spurs separating the active channels, with interaction between along and downslope processes accreting sediment that built and maintained channel relief and allowed the channel mouths to extend further basinward over and across the earlier lobes. Contourite-forced channel extension on account of lower slope depositional re-profiling represents another manifestation of the interplay between slope channels and bottom currents. In the case of NE Rockall, the accreted base of slope wedge and mounded geometry resembles similar features elsewhere along the eastern Rockall margin. Those have previously been ascribed to mass-transport and base-up channel initiation, however, the similarities are so striking that we propose that these also are the result of lower slope re-profiling by bottom currents.

Isolated, detached sands provide opportunities for large-volume stratigraphic traps in many deepwater petroleum systems. Here we provide a review of the different types of sandbody detachments based on published data from the modern-day seafloor and recent (generally Quaternary-present), shallow-buried strata. Detachment mechanisms can be classified based on their timing of formation relative to deposition of the detached sandbody as well as their process of formation. Syndepositional detachment mechanisms include flow transformation associated with slope failure (Class 1), turbidity current erosion (Class 2), and contourite deposition (Class 3). Post-depositional detachment is related to subsequent erosive processes and truncation of the pre-existing sandbody, either by submarine channels (Class 4), mass-transport events (Class 5), post-depositional sliding or faulting (Class 6) or bottom currents (Class 7). Examples of each of these mechanisms are identified on the modern seafloor, and show that detached sandbodies can form at different locations along the continental slope and rise (from upper slope to basin floor), and between or within different architectural elements (i.e., canyon, channels and lobes). This variation in formation style results in detached sands of highly variable sizes (tens to hundreds of kilometres) and geometries across and along the depositional profile, which are dependent upon the erosive and/or depositional processes involved, as well as the seafloor topography of the area in question. Whilst modern seafloor systems may not always represent the final stratigraphic architecture in the subsurface, they provide important insights into the development of detached sandbodies and therefore serve as potential analogues for subsurface stratigraphic traps.

Oil reservoirs hosted in deep-water slope channel deposits are a challenge to manage and model. A six-level hierarchical arrangement of depositional elements within slope channel deposits has been widely recognized, and dimensional (width and thickness) and stacking (amalgamation ratio and volume fraction) data have been acquired from published studies to establish parameters for a representative slope channel system. A new static modelling workflow has been developed for building models of channel complexes based on a simplified hierarchical scheme using industry-standard object-based modelling methods and a new plugin applying the compression algorithm. Object-based modelling using the compression algorithm allows for independent input of volume fractions and amalgamation ratios for channel and sheet objects within a hierarchical modelling workflow. A base-case channel complex model is built at the resolution of individual sandstone beds, conditioned to representative dimensional and stacking characteristics of natural systems. Inclusion of explicit channel axis and margin regions within the channels governs bed placement and controls inter-channel connectivity where channels are amalgamated. The distribution of porosity and permeability within these beds mimics grain-size trends of fining in the vertical and lateral directions. The influence of various geological parameters and modelling choices on reservoir performance have been assessed through water-flood flow simulation modelling. Omission of the compression method in the modelling workflow results in a three-fold increase in oil recovery at water-breakthrough, because the resultant unnaturally high amalgamation ratios result in overly-connected flow units at all hierarchical levels. Omission in the modelling of either the bed-scale hierarchical level, or of the axial and marginal constraints on the bed placement in models that do include this level, results in a two-fold increase in oil recovery at water-breakthrough relative to the base-case, because in these cases the channel-channel connections are too permissive.

Deep-water lobe deposits are arranged hierarchically and can be characterized by high net:gross ratios but poor sand connectivity due to thin, but laterally extensive, shale layers. This heterogeneity makes them difficult to represent in standard full-field object-based models, since the sands in an object-based model are not stacked compensationally and become connected at a low net:gross ratio. The compression algorithm allows the generation of low-connectivity object-based models at high net:gross ratios, by including the net:gross and amalgamation ratios as independent input parameters. Object-based modelling constrained by the compression algorithm has been included in a recursive workflow, permitting the generation of realistic models of hierarchical lobe deposits. Representative dimensional and stacking parameters collected at four different hierarchical levels have been used to constrain a 250 m-thick, 14 km(2)model that includes hierarchical elements ranging from 20 cm-thick sand beds to more than 30 m-thick lobe complexes. Sand beds and the fine-grained units are represented explicitly in the model, and the characteristic facies associations often used to parameterize lobe deposits are emergent from the modelling process. The model is subsequently resampled without loss of accuracy for flow simulation, and results show clearly the influence of the hierarchical heterogeneity on drainage and sweep efficiency during a water-flood simulation.

Hybrid event beds form when turbidity currents that transport or locally acquire significant quantities of mud decelerate. The mud dampens turbulence driving flow transformations, allowing both mud and sand to settle into dense, near-bed fluid layers and debris flows. Quantifying details of the mud distribution vertically in what are often complex tiered deposits is critical to reconstructing flow processes and explaining the diverse bed types left by mud-bearing gravity flows. High-resolution X-ray fluorescence core scanning provides continuous vertical compositional profiles that can help to constrain mud distribution at sub-millimetre scale, offering a significant improvement over discrete sampling. The approach is applied here to cores acquired from the Pennsylvanian Ross Sandstone Formation, western Ireland, where a range of hybrid event beds have been identified. Raw X-ray fluorescence counts are calibrated against element concentrations and mineral abundances determined on coincident core plugs, with element and element log-ratios used as proxies to track vertical changes in abundances of quartz, illite (including mica), chlorite and calcite cement. New insights include 'stepped' (to higher values) as opposed to 'saw-tooth' vertical changes in mud content and the presence of compositional banding that would otherwise be overlooked. Hybrid event beds in basin floor sheets that arrived ahead of the prograding fan system have significantly cleaner sandy components than those in mid-fan lobes. The latter may imply that the heads of the currents emerging from mid-fan channels entrained significant mud immediately before they collapsed. Many of the H3 debrites are bipartite with a sandier H3a division attributed to re-entrainment and mixing of a trailing debris or fluid mud flow (H3b) with sand left by the forward part of the flow. Hybrid event bed structure may thus partly reflect substrate interaction and mixing during deposition, and the texture of the bed divisions may not simply mirror those in the suspensions from which they formed.

This volume focuses on underwater or subaqueous landslides with the overarching goal of understanding how they affect society and the environment. The new research presented here is the result of significant advances made over recent years in directly monitoring submarine landslides, in standardizing global datasets for quantitative analysis, constructing a global database and from leading international research projects. Subaqueous Mass Movements demonstrates the breadth of investigation taking place into subaqueous landslides and shows that, while events like the recent ones in the Indonesian archipelago can be devastating, they are at the smaller end of what the Earth has experienced in the past. Understanding the spectrum of subaqueous landslide processes, and therefore the potential societal impact, requires research across all spatial and temporal scales. This volume delivers a compilation of state-of-the-art papers covering topics from regional landslide databases to advanced techniques for in situ measurements, to numerical modelling of processes and hazards.

The integration of passive electromagnetic geophysical data and well‐log data for basin characterization and interpretation has been investigated in the Clare Basin, western Ireland. The Clare Basin is overmature and has a clear contrast in electrical resistivity between the Clare Shale Formation, a widespread organic rich shale unit, and the surrounding stratigraphy. The electrical resistivity distribution beneath the Clare Basin was determined by means of three‐dimensional (3‐D) joint inversion of three distinct and differently sensitive electromagnetic parameters: (1) the MT impedance tensor (Z), (2) the geomagnetic transfer function (T), and (3) the interstation horizontal magnetic transfer function (H). Well‐log data from a local exploration well, Doonbeg‐1, were analyzed by means of multivariate statistical methods identifying three groups with distinct resistivity values. The groups were propagated along the basin using the 3‐D electrical resistivity model, showing those regions in the basin with significant organic content at high maturity stage. The lack of continuity of these regions supports the hypothesis of advective fluid heating as the cause of the high maturity levels. The results also help to define the geometry of the basin at depth and have identified an area within the basin, near the Loop Head, where organic‐rich clay/shale is either poorly developed, and/or the organic matter is less mature and less conductive. Finally, the potential of the basin for both CO2 storage and geothermal energy was considered, supporting the use of the Clare Basin as a potential site for geothermal energy but not for the storage of CO2.