Reducing the uncertainty of reservoir characterization requires to better identify the small-scale structures of the subsurface from the available data. Studying the seismic response of meter-scale, stratigraphic heterogeneities typically relies on the generation of reservoir models based on outcrop examples and their forward seismic modelling. To bridge geological information and seismic modelling, these methods allocate values of acoustic properties, such as mass-density and P-wave velocity, according to discretized properties like layer-type lithology or facies units. This strategy matches the current workflow in seismic data inversion in industry, where modelling workflows are based on lithofacies distributions. However, from stratigraphic modelling, we know that meter-scale heterogeneities occur within certain facies and lithologies. Here, we evaluate the difference on the seismic response between allocating acoustic properties in a grain size–based, semi-continuous manner versus discretized manners based on lithology and facies classifications. To do so, we generate a reference geological simulation that we populate with acoustic properties, mass-density and P-wave velocity, using three different strategies: (1) based on grain size distribution; (2) based on facies distribution; and (3) based on lithology. The method we propose includes the generation of realistic geological simulations based on stratigraphic modelling and the transformation of its output into acoustic properties, honouring the intra-lithology and intra-facies, small-scale structures. We, then, generate seismic data by applying a forward seismic modelling workflow. The synthetic data show that the grain size–based simulation allows the identification of small-scale, stratigraphic heterogeneities, such as beds with strong density and velocity contrasts. These stratigraphic structures are smoothened or may completely disappear in the facies and lithology discretized simulations and, therefore, are not (well) represented in the synthetic seismic data. Recognizing meter-scale, stratigraphic heterogeneities is relevant for the characterization of the fluid flow in the reservoir. However, current discrete and lithology-based strategies in seismic inversion are not able to resolve such heterogeneities because real subsurface properties are not discrete properties but continuous, unless there are stratigraphic discontinuities such as erosional surfaces or faults. This research works towards a better understanding of the relationship between changes in these continuous properties and the observed seismic data by introducing greater complexity into the discretized geological simulations. Here, we use synthetic seismic images with the goal of eventually aiding in fine-tuning seismic inversion methodologies applied to real seismic data. One pathway is to foster the development of inversion approaches that can leverage stratigraphic modelling to get stronger geological priors and replace the standard but inadequate multi-Gaussian prior.

The Main Buntsandstein Subgroup in the Roer Valley Graben in the southern Dutch subsurface is a sand-prone sedimentary interval deposited in a fluvial-aeolian environment, and is currently investigated for its suitability as target for low-entalpy geothermal exploration. The current depositional models in the Roer Valley Graben do not fully address the facies heterogeneities within and between Buntsandstein sedimentary units and their impact on the prediction of reservoir architecture. A detailed analysis of the Main Buntsandstein sedimentary facies heterogeneities to de-risk future sustainable energy operations is therefore crucial. In the present study, the sedimentology and lithostratigraphy of the Buntsandstein are assessed in a multidisciplinary analysis by use of a subsurface dataset composed of well cores, gamma-ray logs, and thin section data. The deposition of the Main Buntsandstein sediments in the Roer Valley Graben is dominated by different fluvial processes, with minor aeolian reworking. River planform style evolved through geological time from highly mobile and ephemeral to more perennial in nature. These changes in river style seem to be dictated by a decrease in climatic aridity along with a decrease in tectonic activity. The depositional processes resulted in the development of six lithofacies associations, developing three different types of reservoir architectures with their own set of heterogeneities at different spatial scales. Amalgamated, stacked sandstones have the highest net-to-gross (N/G) with a high degree of lateral and vertical connectivity, and the highest average porosity and permeability. Compensational-stacked sandstone reservoirs have a lower N/G and are the most heterogeneous due to the frequent occurrence of cemented intervals as well as mud drapes in the sandstone bodies. Marginal isolated sandstones show a well preserved relationship between reservoir properties and depositional facies, while more data are needed to resolve the spatial connectivity and lateral continuity of these sandstone bodies. The results of this study enhance the understanding of Lower Triassic reservoir architecture and sedimentary heterogeneities in the Roer Valley Graben that can be applied well beyond the area and provide a solid basis for future investigation of the relationship between sedimentary facies, diagenesis, and reservoir quality.

We have developed a generalisable machine learning framework for reservoir quality prediction in deeply buried clastic systems. Applied to the Lower Jurassic deltaic sandstones of the Tilje Formation (Halten Terrace, North Sea), the approach integrates sedimentological facies modelling with mineralogical and petrophysical prediction in a single workflow. Using supervised Extreme Gradient Boosting (XGBoost) models, we classify reservoir facies, predict permeability directly from standard wireline log parameters and estimate the abundance of porosity-preserving grain coating chlorite (gamma ray, neutron porosity, caliper, photoelectric effect, bulk density, compressional and shear sonic, and deep resistivity). Model development and evaluation employed stratified K-fold cross-validation to preserve facies proportions and mineralogical variability across folds, supporting robust performance assessment and testing generalisability across a geologically heterogeneous dataset. Core description, point count petrography, and core plug analyses were used for ground truthing. The models distinguish chlorite-associated facies with up to 80% accuracy and estimate permeability with a mean absolute error of 0.782 log(mD), improving substantially on conventional regression-based approaches. The models also enable prediction, for the first time using wireline logs, grain-coating chlorite abundance with a mean absolute error of 1.79% (range 0–16%). The framework takes advantage of diagnostic petrophysical responses associated with chlorite and high porosity, yielding geologically consistent and interpretable results. It addresses persistent challenges in characterising thinly bedded, heterogeneous intervals beyond the resolution of traditional methods and is transferable to other clastic reservoirs, including those considered for carbon storage and geothermal applications. The workflow supports cost-effective, high-confidence subsurface characterisation and contributes a flexible methodology for future work at the interface of geoscience and machine learning.

Sandstones from the Main Buntsandstein Subgroup represent a promising deep geothermal target in the subsurface of the Netherlands considering their widespread distribution and temperatures locally reaching 140-150 °C at depths of ~ 3 to 5 km. The Main Buntsandstein Subgroup is a sand-prone interval, but the reservoir quality of these sandstones is known to be heterogeneous as result of an interplay between depositional and diagenetic processes. This makes the Buntsandstein sediments an uncertain and risky geothermal play.

In this project, we assess the syn- and post-depositional history of these sediments. The aim is to define structural, sedimentary, and diagenetic heterogeneities within the Main Buntsandstein sediments and assess their impact on reservoir quality. This will help reduce uncertainties for geothermal operations in the Triassic in the southern Netherlands and beyond.

The structural analysis of the study area using seismic and well data reveals that the Main Buntsandstein sediments represent an early syn-rift sequence and that their present-day distribution is strongly controlled by faulting. In parallel, the study of the sedimentology and stratigraphy conducted on core and wireline data indicates that the depositional environment evolves through the Buntsandstein stratigraphy, resulting in the development of different reservoir architectures. Diagenesis has largely altered the primary relationship between sedimentary facies and porosity and permeability. Overall cementation seems to have a larger impact on reducing reservoir quality than compaction, with quartz, dolomite, and illite representing the most abundant types of cement. The analysis of fractures using core and image logs suggests that the fracture density is driven by the lithological variability within the Main Buntsandstein and that fracture joints and stylolites locally may contribute to enhancing the system permeability.

The integrated assessment of the results allows the development of prospect play maps for the Buntsandstein in the southern Netherlands, addressing uncertainties and providing future recommendations for further exploration and optimizing geothermal operations in the Triassic.

Sedimentation on river floodplains is a complex process that involves overbank flooding, crevasse splaying, and river avulsion. The resulting floodplain stratigraphy often exhibits floodplain aggradation cycles with alternating fine-grained overbank flooding deposits that underwent significant petrogenesis, and coarser-grained, avulsion-belt deposits largely devoid of pedogenic impact. These cycles are linked to lateral migration and avulsion of channels driven by internal dynamics, external factors, or a combination of both. To better understand the spatial and vertical variability of such floodplain aggradation cycles, we map these in three dimensions using a photogrammetric model of the lower Eocene Willwood Formation in the northern Bighorn Basin, Wyoming, USA. This allows identifying 44 floodplain aggradation cycles in ∼300 m of strata with an average thickness of 6.8 m and a standard deviation of 2.0 m. All the cycles are traceable over the entire model, pointing to their spatial consistency over the 10 km2 study area. At the same time, rapid lateral thickness changes of the floodplain aggradation cycles occur with changes up to 4 m over a lateral distance of 400 m. Variogram analyses of both field and numerical-model results reveal stronger consistency of floodplain aggradation cycle thicknesses along the paleoflow direction compared to perpendicular to paleoflow. Strong compensational stacking occurs at the vertical scale of 2–3 floodplain aggradation cycles (14–20 m), while full compensational stacking occurs at larger scales of more than six floodplain aggradation cycles (>41 m). The lateral and vertical thickness variability of the floodplain aggradation cycles, as well as their compensational stacking behavior, are interpreted to be dominantly driven by autogenic processes such as crevasse splaying and avulsing that preferentially fill topographic lows. External climate forcing may have interacted with these autogenic processes, producing the laterally persistent and vertically repetitive floodplain aggradation cycles. The spatial variability of floodplain aggradation cycles demonstrated in this study highlights again the need for three-dimensional data collection in alluvial floodplain settings rather than depending on one-dimensional records.

Alluvial stratigraphy builds up over geologic time under the complex interplay of external climatic and tectonic forces and internal stochastic processes. This complexity makes it challenging to attribute alluvial stratigraphic changes to specific factors. Geological records indicate pronounced and persistent climatic changes during the Phanerozoic, while the effects of these changes on alluvial stratigraphy remain insufficiently documented. We provide evidence for 405 k.y. long-eccentricity climate forcing of alluvial stratigraphy in the lower Eocene Willwood Formation of the Bighorn Basin, Wyoming (USA). Two ∼90-m-thick intervals, characterized by a relative paucity of sand, dominance of sinuous-river channels, and floodplain sediments with better-developed paleosols, coincide with eccentricity maxima as determined through integrated stratigraphic methods. These intervals are interspersed with three contrasting intervals, marked by relatively high sand content, prevalent braided-river channels, and less-developed paleosols, corresponding to eccentricity minima. A comprehensive genetic model that integrates climate, source-to-sink system, and alluvial dynamics to explain these findings remains to be elucidated. Given the consistent presence of the 405 k.y. eccentricity cycle throughout Earth’s history, it is plausible to infer that its influence may be discernible across a wide array of alluvial stratigraphic records.

The lower Triassic Main Buntsandstein Subgroup represents a promising, but high-risk geothermal play in the Netherlands. Although the gross thickness in boreholes locally exceeds 200 m, the spatial distribution, geometries and preservation of these sedimentary units remained uncertain due to the lack of seismic data with sufficient resolution and the sparse borehole network. This creates uncertainty in the quantification of the aquifer dimensions that is essential for the planning of geothermal operations.

In this study, seismic interpretation and 2D palinspastic restoration of new and reprocessed seismic data were conducted and combined with borehole data to assess the tectonic evolution of the Roer Valley Graben in the southeastern Netherlands and its control on the spatial distribution of the Main Buntsandstein Subgroup sediments. Our results show that the central and southern parts of the Roer Valley Graben were active depocenters in the Early to Middle Triassic times dominated by fluvial sandstone deposition, providing important play elements for prospective leads on geothermal exploration. The northern part of the basin was a more marginal area where mostly fine-grained sediments were deposited. To the northwest, differential subsidence resulted in the development of areas where the Buntsandstein thickness is reduced to ∼150 m.

After deposition, the Main Buntsandstein sediments were compartmentalised by faulting related to post-depositional tectonic activity, locally reducing the lateral extent of the geothermal target areas down to 1–2 km in a ∼NE–SW direction. On the platform areas adjacent to the Roer Valley Graben and to the southeast, Jurassic sediments are largely absent and the Main Buntsandstein sediments are present at depths shallower than 2 km. These platforms are promising targets for further investigation, as the relatively shallow burial depths, compared to the central part of the Graben, may have contributed to the preservation of more favourable reservoir properties.

In the past decades it has become evident that the often cyclic cross-shore migration of longshore bars is significantly influenced by wave climate. This study demonstrates that this cyclic migration, whether landward or seaward, leads to the formation of low-angle seaward-inclined stratification (SIS) spanning the beach and upper shoreface in ground-penetrating radar (GPR) data from prograding sandy coastal barriers including strandplains. Previously documented radargrams of these systems are reinterpreted using this knowledge of cross-shore dynamics of longshore bars. Five distinct wave-climate-related radar architectures are identified. A notable observation is the prevalence of SIS as the dominant upper-shoreface to beach structure in most radargrams, despite its infrequently described occurrence from outcrop observations. To address this disparity, this paper also focuses on recognizing SIS in outcrop and core data. Compelling evidence of SIS was discovered in a late Messinian coastal barrier parasequence in SE Spain, and the associated sedimentary architecture is described in detail. It is hypothesized that this example holds generic significance, illustrated using core data from Middle Jurassic strata of the Norwegian continental shelf, and suggesting that the sedimentary architectures of numerous other ancient prograding barrier deposits may exhibit variations within a similar SIS framework.

The hierarchies of the stratigraphic discontinuity surfaces observed in ancient tidalites are qualitatively assessed, aiming to evaluate their role as possible preferential conduits for fluid migration. Three outcrop examples are presented from microtidal settings of southern Italy: (i) siliciclastic tidalites consisting of quartz-rich cross-stratified sandstones generated by strong two-directional tidal currents flowing along a tidal strait; (ii) carbonate tidalites, which accumulated in a Cretaceous lagoon and tidal flat where peritidal cycles formed vertically-stacked sequences of biopeloidal and fenestral packstones, wackestones and bindstones during repeated phases of Milankovitch-scale sea-level changes; (iii) mixed, siliciclastic-bioclastic tidalites, deposited in a bay and recording offshore-transition, to shoreface wave-dominated and tide-influenced environments. Observations made during this study suggest that fluid movement can be controlled by the presence of main bounding surfaces that occur at different dimensions, from large (hectometre)-scale, to medium (decametre)-scale, to smaller (metre)-scales. These surfaces produced either by depositional or erosional processes, are characterised by different features and geometries in siliciclastic, carbonate and mixed siliciclastic-bioclastic tidalites arguably revealing complex internal pathways for fluid flows. These results suggest that fluids propagating along the main discontinuities follow a dominant sub-horizontal direction of propagation, associated with minor sub-vertical movements, due to local internal surface geometries and interconnections and a general lack of fractures. This surface-based approach to the study of fluid-flow transmission within stratified rocks represents a conceptual attempt to predict fluid mobility and reservoir potential in tidalite-bearing siliciclastic, carbonate and mixed reservoir rocks.

Orbital driven climate control on sedimentation produces regional, stratigraphically repetitive characters and so cyclostratigraphic correlation can improve correlation and identify stratigraphic trends in borehole sections. This concept is commonly used to correlate marine and lacustrine strata. However, in the alluvial domain, its use is more challenging because internal, local dynamics controlling sedimentation may interfere with the expression of cyclic climate forcing. Intervals of low net-to-gross may be important for successful application in this domain as they tend to better document regional changes. This study applies climate-based stratigraphic correlation concepts to improve well correlations, characterise vertical sand distribution, and identify potential reservoir targets in a generally low net-to-gross interval. Coarsening upward sedimentary repetitions (cyclothems) are identified and correlated with high certainty in nineteen well sections in the upper Carboniferous Westoe and Cleaver formations of the Silverpit Basin. Local sedimentary dynamics provide variability in the character of the cyclothems and several types of cyclothem are classified. Correlation of sections using cyclothems recognised on wireline logs is done twice: once manually and once semi-automatically. The semi-automated correlation is based on calculation of deviation curves which depict stratigraphic changes that are less dependent on absolute wireline values and follow vertical trends more clearly. The correlations provide composite stratigraphies that are analysed using vertical proportions curves. Both approaches yield similar results in terms of stratigraphic trends. However, for detailed correlation of wells, the manual correlation is better at accounting for any local variability within the system. The same two zones of higher net-to-gross ratios are found using both correlation methods. These are linked to palaeoclimatic changes driven by long eccentricity and the proposed climate stratigraphic model has predictive value for identifying sandstone occurrence. The climate-based stratigraphic correlation improves the assessment reservoir distribution and properties on small (10–20 m thickness) and large (100–200 m thickness) stratigraphical scales.

Many stratigraphic features occur at a scale that is at the edge or below vertical seismic resolution. Thus, they cannot be directly observed in the seismic data, while still having an important effect on the fluid flow within the system. The better understanding of these sub-seismic scale features or heterogeneities can help decrease subsurface uncertainty. Here we present a novel method that integrates forward stratigraphic modelling, petrophysics, and geophysics to decipher the seismic imprint of heterogeneities in wave-dominated, shallow marine environments. The proposed three-stepped method starts with defining geology-related input parameters for BarSim, a stratigraphic forward modelling software that produces models that include stratigraphic architecture, grain size distribution, and facies distribution. Then, the geological data is translated, cell by cell, into petrophysical data (density, Vp, and Vs) using emphirical relationships. Finally, the forward seismic modelling is performed by combining a finite difference approach strategy and angle-dependent full wavefield migration to retrieve the angle gathers This method also allows the generation of large amounts of field-independent data suitable for machine learning applications.

The Buntsandstein subgroup in the southeastern part of the Netherlands represents one of the most promising, but risky, geothermal plays. To understand the main controls on Buntsandstein reservoir quality, we combine petrophysical (porosity and permeability) and petrographic (point counting) data derived from different wells and different depth levels. Results show that porosity ranges from 2 to 18.5 and permeability from 0.001 to 285 mD. Dolomite represents the most abundant cement and show an inverse correlation with porosity. Illite occurs in higher concentrations in samples with values of permeability below 20 mD, while kaolinite becomes the most dominant phyllosilicate cement in samples with higher permeability. By looking at the main cement distribution over the sedimentary facies, it appears that dolomite is strongly related to depositional facies and has a positive correlation with grain size, while illite and kaolinite yield a negative correlation with grain size. Pedogenic dolomite nodules are often reworked as detrital grain into the channel scour deposits and are the main source for dolomite cementation. The current study has shown how diagenesis makes Buntsandstein reservoir complex and heterogeneous, and how reservoir quality is strongly related to the depositional environment.

The Lower Triassic Main Buntsandstein Subgroup represents one of the most promising deep geothermal plays in the Netherlands. Reservoir zones consist of sandstone units deposited in an arid to semi-arid alluvial plain reaching total thicknesses of over 200 m in the Roer Valley Graben. However, the lack of a comprehensive reservoir model integrating stratigraphy, sedimentology, and petrography makes the Buntsandstein a high-risk target. Here, we present a reservoir architecture model of the spatial and temporal variation of sedimentary facies and inherent permeability barriers and baffles, based on the integration of data from 33 wells from different stratigraphic levels and different areas of the Roer Valley Graben.

Core samples and thin sections analysis revealed two major reservoir facies that were interpreted as the products of transport and depositional processes in braided and sinuous river settings. The identified facies were then coupled to wireline logs to assess the spatial and temporal variation in sedimentary architecture. The lower part of the stratigraphy is dominated by braided river reservoir facies with a high degree of connectivity, where regional lacustrine-playa lake sediments represent the main potential permeability barriers. By contrast, the upper part of the stratigraphy is characterized by an increase in the proportion of sinuous river complexes. These latter yield a lower degree of connectivity with different types of baffles such as intercalated fine-grained overbank sediments, abandonment plugs, bar-draping fines, and cemented dolocrete scour fills. These are much more localized compared to the braided complexes-related barriers, making the prediction of the upper stratigraphy architecture uncertain.

This poster outlines a hierarchical, multiscale modelling approach that is adapted from proven hydrocarbon reservoir characterization workflows to determine which 3D sedimentological and stratigraphic heterogeneity types at which temporal and spatial scales and in which configurations are most important for successful long-term CO2 storage. The approach is particularly in saline aquifers that are data-poor but have the potential to store large CO2 volumes. It uses the Representative Element Volume (REV) concept and associated upscaling methodology to characterise sedimentological heterogeneity types, and it leverages novel modelling tools that facilitate rapid model construction and prototyping, geometrically accurate representation of key geological heterogeneities in models, and computationally efficient simulation of all CO2 trapping mechanisms over relevant spatial and temporal scales.

ABSTRACT The lower Eocene Willwood Formation of the Bighorn Basin, Wyoming, USA, is an alluvial succession with a sand content varying around 25 palaeoenvironments and palaeoclimates, as well as sedimentological and stratigraphic analysis. Channel dynamics were studied at a relatively low resolution throughout the basin over the geological time from late Palaeocene to early Eocene. Here, a high-resolution study is reported to complement previous research at the basin scale. Efforts are made to document the characteristics and river planform styles of most sandstone bodies encountered through ca 300 m of alluvial stratigraphy in a 10 km2 area of the Deer Creek part of the McCullough Peaks area situated in the basin axis of northern Bighorn Basin. Four channel facies associations are recognized and ascribed to four river planform styles: crevasse channel, trunk channel, braided-like channel and sinuous-like channel, with the latter two types dominant. Braided-like and sinuous-like channel sandstone bodies differ significantly in thicknesses, being on average 6.1 m versus 9.0 m, but they have similar palaeoflow–perpendicular widths of on average 231 m and palaeoflow directions of on average N 003°. Braided-like and sinuous-like river planform styles show no spatial dependency in the 10 km2 study area. Results of this study are in line with existing basin-scale depositional models that are composed of a single axial system fed by several transverse systems dominantly from the west. The feeding of these systems could be influenced by palaeoclimate changes possibly controlling their contribution over time, thereby impacting river planform styles. At the same time, changing water discharge hydrograph, sediment load, and overbank cohesiveness may have equally driven the observed river planform style changes within the basin without a major role of catchments.

A combined study of depositional facies and diagenesis variation was carried out to understand the main controls on aquifer quality of the Middle Buntsandstein in the southeastern part of the Netherlands. Heterogeneities in continental sandstone bodies occur at different spatial scales, ranging from micrometers to hundreds of meters. Commonly, such heterogeneities result from the interaction of depositional processes at various spatial and time scales. These processes partially also influence subsequent diagenetic evolution, hence present-day aquifer properties. Understanding the role of the resulting architectural heterogeneities in controlling the dynamic reservoir behavior is key in determining aquifer properties and improving pre-drilling prediction. The sandstones of the Main Buntsandstein subgroup in the southeastern part of the Netherlands provide an excellent example where different detrital compositions, internal sedimentary architectures, and diverse burial histories have resulted in a wide range of present-day aquifer properties. In the study area, the aquifers are composed of stacked heterogenous alluvial sandstones bodies intercalated with mud-prone intervals deposited in arid to semi-arid conditions. Differences in sediment sources, transport mechanisms, and intrabasinal conditions resulted in a wide distribution of composition and texture. Additionally, the effect of post-depositional burial diagenesis in a basin with complex tectonic history created diverse burial histories across the basin. The study aims to investigate the variation of present-day aquifer hydraulic parameters about changes in aquifer facies and architecture, detrital composition, as well as compaction and cementation during burial. Core sample analysis unfolded a diverse spectrum of sedimentary facies and lithic fragments, which differ between formations. Thin section analysis provides insights about mechanical compaction, cementations, and authigenic phases. By combining these results with petrophysical data on permeability and porosity of core samples, the major controls on present-day aquifer quality can be assessed.

Northern Lights is the CO₂ transport and storage component of Longship, the Norwegian full-scale CCS project. Injection is planned into an under-explored sloping saline aquifer in the northern North Sea, the Johansen and Cook formations of the Lower Jurassic Dunlin Group. To bridge the information gap, well 31/5-7 (Eos) was drilled. The comprehensive dataset acquired was fundamental to interpret the depositional environment and determine the scale and spatial distribution of heterogeneities, as input to 3-D models aimed at improving storage resource assessment and understanding the injected CO₂ plume behaviour over time. The interpreted gross depositional environments of the storage units are marginal- to shallow-marine, arranged in three successive fining-upwards intervals. The lower interval includes coastal deposits with mixed wave- and river influence, correlatable over a large distance, dominated by meso-scale heterogeneities. The middle interval records paralic deposits with a wave- and tidal- interplay generating higher vertical and lateral variability. The upper interval is interpreted as tidal-dominated, predominantly with cm-scale heterogeneities. The repeated fining-upwards trends are ideal for plume redistribution and efficient CO₂ storage, and the reconstructed lateral depositional trends associated with generally good properties indicate a high storage potential. The Eos well data enabled building the properties distribution model, highlighting the importance of well control for storage evaluation.

Grain-coating chlorite preserves porosity and permeability through the inhibition of quartz cement whereas pore-filling chlorite blocks pore-throats and diminishes reservoir quality. The aim of this study is to determine the origin and principal mechanisms which govern the distribution of grain-coating and pore-filling chlorite in Jurassic deltaic sandstones (Tilje Formation, Smørbukk field, Mid Norwegian Shelf). The study focussed on very high-density sampling for petrographic analysis, from three sections of sandstone core from the same well with contrasting reservoir quality, rather than the low-density sampling approach typically employed. The aim was to gain new understanding of specific controls on porosity and permeability based on core description, core analysis measurements and a suite of petrographic techniques. Results of this study show grain-coating chlorite originated from the thermally-driven recrystallisation of detrital clay coats and/or clay mineral precursors. Pore-filling chlorite has principally derived from the ductile deformation of chlorite-rich Fe-ooids, that were possibly reworked from a proximal evaporitic setting. The distribution of chlorite precursor material, detrital clay coats, and subsequently the distribution of grain-coating and pore-filling chlorite, were controlled by the relative dominance of tidal and fluvial processes active during sediment deposition. Optimum grain-coating chlorite is found in tidal-fluvial sandstones with moderate fluvial influence. Pore-filling chlorite is pervasive in tidal-fluvial channel sandstones deposited during periods of high fluvial discharge, or proximal to the central turbidity maximum zone; marked by an abundance of fluid mud. Tidal channel sandstones with no fluvial influence are pervasively quartz cemented due to an absence of grain-coating chlorite. Grain-coating chlorite and good reservoir quality occurs in heterolithic distributary mouth bar sandstones, however mixing of mud- and sand-prone facies due to intense bioturbation has reduced permeability. Results from this study can be used to predict reservoir quality in the Smørbukk field and in analogous shallow-marine sandstones worldwide.

Formation of alluvial stratigraphy is controlled by autogenic processes that mix their imprints with allogenic forcing. In some alluvial successions, sedimentary cycles have been linked to astronomically-driven, cyclic climate changes. However, it remains challenging to define how such cyclic allogenic forcing leads to sedimentary cycles when it continuously occurs in concert with autogenic forcing. Accordingly, we evaluate the impact of cyclic and non-cyclic upstream forcing on alluvial stratigraphy through a process-based alluvial architecture model, the Karssenberg and Bridge (2008) model (KB08). The KB08 model depicts diffusion-based sediment transport, erosion and deposition within a network of channel belts and associated floodplains, with river avulsion dependent on lateral floodplain gradient, flood magnitude and frequency, and stochastic components. We find cyclic alluvial stratigraphic patterns to occur when there is cyclicity in the ratio of sediment supply over water discharge (Q_s/Q_w ratio), in the precondition that the allogenic forcing has sufficiently large amplitudes and long, but not very long, wavelengths, depending on inherent properties of the modelled basin (e.g. basin subsidence, size, and slope). Each alluvial stratigraphic cycle consists of two phases: an aggradation phase characterized by rapid sedimentation due to frequent channel shifting and a non-deposition phase characterized by channel belt stability and, depending on Q_s/Q_w amplitudes, incision. Larger Q_s/Q_w ratio amplitudes contribute to weaker downstream signal shredding by stochastic components in the model. Floodplain topographic differences are found to be compensated by autogenic dynamics at certain compensational timescales in fully autogenic runs, while the presence of allogenic forcing clearly impacts the compensational stacking patterns.

Despite the importance of channel avulsion in constructing fluvial stratigraphy, it is unclear how contrasting avulsion processes are reflected in stratigraphic-stacking patterns of channelized fluvial sand bodies, as a proxy for how river depocenters shifted in time and space. Using an integrated, geospatially referenced, three-dimensional data set that includes outcrop, core, and lidar data, we identify, for the first time in an outcrop study, a predictive relationship between channelized sand body architecture, paleochannel mobility, and stratigraphic-stacking pattern. Single-story sand bodies tend to occur in vertically stacked clusters that are capped by a multilateral sand body, indicating an upward change from a fixed-channel system to a mobile-channel system in each cluster. Vertical sand body stacking in the clusters implies reoccupation of abandoned channels after "local" avulsion. Reoccupational avulsion may reflect channel confinement, location downstream of a nodal avulsion point that maintained its position during development of the sand body cluster, and/or aggradation and progradation of a backwater-mediated channel downstream of a nodal avulsion point. Sand body clusters and additional multilateral sand bodies are laterally offset or isolated from each other, implying compensational stacking due to "regional" switching of a nodal avulsion point to a new, topographically lower site on the floodplain. The predictive links between avulsion mechanisms, channel mobility, and resultant sand body distributions and stacking patterns shown in our findings have important implications for exploring and interpreting spatiotemporal patterns of stratigraphic organization in alluvial basins.