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Incised valley fills are complex as they correspond to multiple sea‐level cycles which makes interpretation and correlation of stratigraphic surfaces fraught with uncertainty. Despite numerous studies of the stratigraphy of incised valley fills, few have focused on extensive core coverage linked to high fidelity dating in a macro‐tidal, tide‐dominated setting. For this study nineteen sediment cores were drilled through the Holocene succession of the macro‐tidal Ravenglass Estuary in north‐west England, UK. A facies and stratigraphic model of the Ravenglass incised valley complex was constructed, to understand the lateral and vertical stacking patterns relative to the sea‐level changes. The Ravenglass Estuary formed in five main stages. First, incision by rivers (ca 11 500 to ca 10 500 yrs bp) cutting through the shelf during lowstand, which was a period of fluvial dominance. Secondly, a rapid transgression and landward migration of the shoreline (10 500 to 6000 yrs bp). Wave action was dominant, promoting spit formation. The third stage was a highstand at ca 6000 to ca 5000 yrs bp, creating maximum accommodation and the majority of backfilling. The spits narrowed the inlet and dampened wave action. The fourth stage was caused by a minor fall of sea level (ca 5000 to ca 226 yrs bp), which forced the system to shift basinward. The fifth and final stage (226 yrs bp to present) involved the backfilling of the River Irt, southward migration of the northerly (Drigg) spit and merging of the River Irt with the Rivers Esk and Mite. The final stage was synchronous with the development of the central basin. As an analogue for ancient and deeply buried sandstones, most of the estuarine sedimentation occurred after transgression, of which the coarsest and cleanest sands are found in the tidal inlet, on the foreshore and within in‐channel tidal bars. The best‐connected (up to 1 km) reservoir‐equivalent sands belong to the more stable channels. [ABSTRACT FROM AUTHOR]

The spatial distribution of clay minerals in sandstones, which may both enhance or degrade reservoir quality, is poorly understood. To address this, clay mineral distribution patterns and host‐sediment properties (grain size, sorting, clay fraction abundance and bioturbation intensity) have, for the first time, been determined and mapped at an unprecedentedly high‐resolution in a modern estuarine setting (Ravenglass Estuary, UK). Results show that the estuary sediment is dominated by illite with subordinate chlorite and kaolinite, although the rivers supply sediment with less illite and significantly more chlorite than found in the estuary. Fluvial‐supplied sediment has been locally diluted by sediment derived from glaciogenic drift deposits on the margins of the estuary. Detailed clay mineral maps and statistical analyses reveal that the estuary has a heterogeneous distribution of illite, chlorite and kaolinite. Chlorite is relatively most abundant on the northern foreshore and backshore and is concentrated in coarse‐grained inner estuary dunes and tidal bars. Illite is relatively most abundant (as well as being most crystalline and most Fe–Mg‐rich) in fine‐grained inner estuary and central basin mud and mixed flats. Kaolinite has the highest abundance in fluvial sediment and is relatively homogenous in tidally‐influenced environments. Clay mineral distribution patterns in the Ravenglass Estuary have been strongly influenced by sediment supply (residence time) and subsequently modified by hydrodynamic processes. There is no relationship between macro‐faunal bioturbation intensity and the abundance of chlorite, illite or kaolinite. Based on this modern‐analogue study, outer estuarine sediments are likely to be heavily quartz cemented in deeply‐buried (burial temperatures exceeding 80 to 100°C) sandstone reservoirs due to a paucity of clay grade material (<0·5%) to form complete grain coats. In contrast, chlorite‐enriched tidal bars and dunes in the inner estuary, with their well‐developed detrital clay coats, are likely to have quartz cement inhibiting authigenic clay coats in deeply‐buried sandstones. [ABSTRACT FROM AUTHOR]

The specific mineralogy of clay grain coats controls the ability of the coat to inhibit quartz cementation in sandstones during prolonged burial and heating. How and why clay‐coat mineralogy varies across marginal marine systems is poorly understood, even though these eogenetic phenomena strongly influence subsequent mesodiagenesis and reservoir quality. The novel development of the ability to predict the distribution of clay‐coat mineralogy would represent an important development for sandstone reservoir quality prediction. In marginal marine sediments, clay minerals occur as grain‐coats, floccules, mud intraclasts, clay‐rich rock fragments or as dispersed material. However, the relationships between clay mineralogy, the amount of clay, and its distribution is poorly understood. This study focused on the Ravenglass Estuary, UK. The key aim was to develop and apply a novel methodology utilising scanning electron microscope – energy dispersive spectrometry, for the first time, on grain coats in modern sediments, to differentiate the clay‐coat mineral signature from that of the bulk sediment, and reveal the distribution of clay minerals across marginal marine sediments. The study showed that marginal marine sediments principally have their clay mineral assemblage present as clay‐coats on sand grains. These clay‐coats have a mixed clay mineralogy and are spatially heterogeneous across the range of marginal‐marine depositional environments. The study further showed that clay‐coat mineralogy is governed initially by the hydrologically‐controlled segregation of the clay minerals within inner estuarine depositional environments, and subsequently by the selective abrasive removal of specific clay mineral types during reworking and transport into the outer estuary and the marine environment. The highest relative abundance of grain‐coating chlorite was in sand‐flat and tidal‐bar depositional environments. The availability of an analogue data set, and an understanding of the controlling processes of clay‐coat mineralogy, offer crucial steps in building a predictive capability for clay‐coat derived elevated reservoir quality in deeply buried sandstones. [ABSTRACT FROM AUTHOR]

In the quest to use modern analogues to understand clay mineral distribution patterns to better predict clay mineral occurrence in ancient and deeply buried sandstones, it has been necessary to define palaeo sub-environments from cores through modern sediment successions. Holocene cores from Ravenglass in the NW of England, United Kingdom, contained metre-thick successions of massive sand that could not be unequivocally interpreted in terms of palaeo sub-environments using conventional descriptive logging facies analysis. We have therefore explored the use of geochemical data from portable X-ray fluorescence analyses, from whole-sediment samples, to develop a tool to uniquely define the palaeo sub-environment based on geochemical data. This work was carried out through mapping and defining sub-depositional environments in the Ravenglass Estuary and collecting 497 surface samples for analysis. Using R statistical software, we produced a classification tree based on surface geochemical data from Ravenglass that can take compositional data for any sediment sample from the core or the surface and define the sub-depositional environment. The classification tree allowed us to geochemically define ten out of eleven of the sub-depositional environments from the Ravenglass Estuary surface sediments. We applied the classification tree to a core drilled through the Holocene succession at Ravenglass, which allowed us to identify the dominant paleo sub-depositional environments. A texturally featureless (massive) metre-thick succession, that had defied interpretation based on core description, was successfully related to a palaeo sub-depositional environment using the geochemical classification approach. Calibrated geochemical classification models may prove to be widely applicable to the interpretation of sub-depositional environments from other marginal marine environments and even from ancient and deeply buried estuarine sandstones. [ABSTRACT FROM AUTHOR]

A study of the Brent Group sandstones, in well 211/13-A33 in the Thistle Field, northern North Sea, UK, from a sequence of shoreface, foreshore, delta plain and finally barrier shoreline depositional environments, was undertaken with the aim of contrasting the roles of depositional and diagenetic processes on reservoir quality. These subarkosic and sublitharenite sandstones were buried to ~9000 ft TVDSS and have experienced a maximum burial temperature of about 95 °C. This study utilised wireline and core analysis data, with core samples studied using petrography, XRD, SEM-CL, and SEM-EDS. The dominant diagenetic cements are kaolinite and siderite, with minor quantities of pyrite, chlorite, illite, K-feldspar, calcite and quartz cement. A common sequence of diagenetic events occurred in sandstones from all four depositional environments. The dominant reservoir quality controlling factors are depth-controlled mechanical compaction and variable kaolinite growth, with grain size being an important secondary control. The shoreface sandstones have the highest degree of compactional porosity-loss because they have the greatest quantity of detrital ductile grains. Quartz cement is a minor phase in all formations because these sandstones have only just exceeded the temperature threshold for quartz cement growth. Siderite is found in all formations but with the greatest quantity found in the shoreface and delta plain sandstones, since these environments were most enriched in detrital iron minerals such as biotite and chlorite. The shoreface sandstones host stratigraphically-localised, pore-occluding calcite cement that has compartmentalised the reservoir. Overall, the foreshore and delta plain sandstones have the best reservoir quality because they have (i) few ductile grains resulting in low degrees of compaction, (ii) low quantities of pore-filling cement and (iii) they tend to be medium-grained. In contrast, the shoreface sandstones and barrier shoreline sandstones have poorer reservoir quality as they have (i) moderate to large quantities of ductile grains, (ii) moderate quantities of pore-filling cement and (iii) they tend to be fine- or very fine-grained. Detrital sand mineralogy and grain size vary as a function of depositional environment and these have subsequently played a major role in linking diagenesis and reservoir quality to depositional environment. [Display omitted] • The marine Brent Group in the Thistle Field was deposited during a regressive-transgressive cycle. • Reservoir quality is dominated by variable compaction, kaolinite cement growth and grain size. • The regressive shoreface and transgressive barrier shoreline sandstones are fine grained and underwent significant compaction and abundant kaolinite cementation. • The foreshore and delta plain sandstones are medium grained and underwent little compaction. • Quartz cement is a minor component as these sandstones were not deeply buried or hot enough. [ABSTRACT FROM AUTHOR]

Petroleum-rich basins at a mature stage of exploration and production offer many opportunities for large-scale Carbon Capture and Storage (CCS) since oil and gas were demonstrably contained by low-permeability top-sealing rocks, such as shales. For CCS to work, there must be effectively no leakage from the injection site, so the nature of the top-seal is an important aspect for consideration when appraising prospective CCS opportunities. The Lower Cretaceous Rodby Shale and the Palaeocene Lista Shale have acted as seals to oil and gas accumulations (e.g., the Atlantic and Balmoral fields) and may now play a critical role in sealing the Acorn and East Mey subsurface carbon storage sites. The characteristics of these important shales have been little addressed in the hydrocarbon extraction phase, with an understandable focus on reservoir properties and their influence on resource recovery rates. Here, we assess the characteristics of the Rodby and Lista Shales using wireline logs, geomechanical tests, special core analysis (mercury intrusion) and mineralogical and petrographic techniques, with the aim of highlighting key properties that identify them as suitable top-seals. The two shales, defined using the relative gamma log values (or Vshale), have similar mean pore throat radius (approximately 18 nm), splitting tensile strength (approximately 2.5 MPa) and anisotropic values of splitting tensile strength, but they display significant differences in terms of wireline log character, porosity and mineralogy. The Lower Cretaceous Rodby Shale has a mean porosity of approximately 14 %, a mean permeability of 263 nD (2.58 × 10−19 m2), and is calcite rich and has clay minerals that are relatively rich in non-radioactive phases such as kaolinite. The Palaeocene Lista Shale has a mean porosity of approximately 16% a mean permeability of 225 nD (2.21 × 10−19 m2), and is calcite free, but contains abundant quartz silt and is dominated by smectite. The 2% difference in porosity does not seem to equate to a significant difference in permeability. Elastic properties derived from wireline log data show that Young's modulus, material stiffness, is very low (5 GPa) for the most shale (clay mineral)-rich Rodby intervals, with Young's modulus increasing as shale content decreases and as cementation (e.g., calcite) increases. Our work has shown that Young's modulus, which can be used to inform the likeliness of tensile failure, may be predictable based on routine gamma, density and compressive sonic logs in the majority of wells where the less common shear logs were not collected. The predictability of Young's modulus from routine well log data could form a valuable element of CCS-site top-seal appraisals. This study has shown that the Rodby and Lista Shales represent good top-seals to the Acorn and East Mey CCS sites and they can hold CO2 column heights of approximately 380 m. The calcite-rich Rodby Shale may be susceptible to localised carbonate dissolution and increasing porosity and permeability but decreasing tendency to develop fracture permeability in the presence of injected CO2, as brittle calcite dissolves. In contrast, the calcite-free, locally quartz-rich, Lista Shale will be geochemically inert to injected CO2 but retain its innate tendency to develop fracture permeability (where quartz rich) in the presence of injected CO2. [ABSTRACT FROM AUTHOR]

• The Acorn CO 2 Storage Site is a geological CCS project, offshore, UK. • A coupled geomechanical and petrographic study has been performed on the site. • The site is here shown to be suitable for long-term CO 2 injection and storage. Extraction or injection of fluids within the subsurface causes fluctuations of fluid pressures and thus stress conditions. It is paramount to have knowledge of the geomechanical strength of a system's lithologies, and the factors that control it, in order to maintain optimal conditions during extraction/injection. If the yield strengths of the reservoir or caprock are overcome, particularly in the near-wellbore region where stress is amplified, these fluctuations could potentially compromise the system, through compactional or dilatational failure. Here we have used a novel combination of methods to determine the geomechanical and petrographic properties of the reservoir and caprock lithologies to assess suitability of the proposed Acorn CO 2 Storage Site, offshore north-east Scotland, for long-term injection and storage of CO 2. The Acorn CO 2 Storage Site has a highly porous and transmissible sandstone reservoir, with bulk mineralogy that will be stable under CO 2 -rich conditions, making it ideal for receiving at least 152 MT CO 2 injected over ∼20 years and storage of >1000 years post-injection, as part of the ACT-Acorn Development Plan. However, due to the high porosity and low cementation of the sandstone reservoir, it has low yield strength and is vulnerable to disaggregation and porosity-reduction if injection rates are too high and stress/pressure conditions exceed their yield strength. The results presented here provide quantitative constraints on the porosity reduction expected should yield occur and place limits on CO 2 injection rates. The shale caprock, with a high swelling clay content and very low permeability, present ideal Carbon Capture and Storage seal properties. [ABSTRACT FROM AUTHOR]

Research to date has identified cost and lack of support from stakeholders as two key barriers to the development of a carbon dioxide capture and storage (CCS) industry that is capable of effectively mitigating climate change. This paper responds to these challenges through systematic evaluation of the research and development process for the Acorn CCS project, a project designed to develop a scalable, full-chain CCS project on the north-east coast of the UK. Through assessment of Acorn's publicly-available outputs, we identify strategies which may help to enhance the viability of early-stage CCS projects. Initial capital costs can be minimised by infrastructure re-use, particularly pipelines, and by re-use of data describing the subsurface acquired during oil and gas exploration activity. Also, development of the project in separate stages of activity (e.g. different phases of infrastructure re-use and investment into new infrastructure) enables cost reduction for future build-out phases. Additionally, engagement of regional-level policy makers may help to build stakeholder support by situating CCS within regional decarbonisation narratives. We argue that these insights may be translated to general objectives for any CCS project sharing similar characteristics such as legacy infrastructure, industrial clusters and an involved stakeholder-base that is engaged with the fossil fuel industry. • Cost reduction and stakeholder support are essential for CCS industry development. • Acorn is aimed to develop a full-chain CCS system at minimum cost by the 2020s. • Infrastructure re-use and staged investment and development enable cost reduction. • Stakeholder and local community engagement is key to promote a just transition. • The Acorn experience can be implemented in regions with similar CCS needs. [ABSTRACT FROM AUTHOR]

Carbon Capture and Storage (CCS) is an essential tool in the fight against climate change. Any prospective storage site must meet various criteria that ensure the effectiveness, safety and economic viability of the storage operations. Finding the most suitable site for the storage of the captured CO 2 is an essential part of the CCS chain of activity. This work addresses the site selection of a second site for the Acorn CCS project, a project designed to develop a scalable, full-chain CCS project in the North Sea (offshore northeast Scotland). This secondary site has been designed to serve as a backup and upscaling option for the Acorn Site, and has to satisfy pivotal project requirements such as low cost and high storage potential. The methodology followed included the filtering of 113 input sites from the UK CO 2 Stored database, according to general and project-specific criteria in a multi-staged approach. This criteria-driven workflow allowed for an early filtering out of the less suitable sites, followed by a more comprehensive comparison and ranking of the 15 most suitable sites. A due diligence assessment was conducted of the top six shortlisted sites to produce detailed assessment of their storage properties and suitability, including new geological interpretation and capacity calculations for each site. With the new knowledge generated during this process, a critical comparison of the sites led to selection of East Mey as the most suitable site, due to its outstanding storage characteristics and long-lasting hydrocarbon-production history, that ensure excellent data availability to risk-assess storage structures. A workshop session was held to present methods and results to independent stakeholders; feedback informed the final selection criteria. This paper provides an example of a criteria-driven approach to site selection that can be applied elsewhere. • CO 2 storage site selection for the Acorn CCS project in the North Sea. • Criteria-driven approach based on geological, economic, and safety aspects. • Multi-staged methodology including filtering, comparison and ranking of sites. • East Mey selected thanks to its storage characteristics and hydrocarbon history. [ABSTRACT FROM AUTHOR]