ALBERT

Description: The Lower Cretaceous Goldeneye gas field lies in the Captain turbidite fairway of the Moray Firth and has recently ceased production. Its situation and dimensions have made it an excellent candidate for CO 2 sequestration. The field was extensively modelled for the original development planning when uncertainty was assessed from the perspective of volumetrics and field behaviour. The subsequent need to assess its suitability as a CO 2 store has given the opportunity for a look back at an uncertainty analysis with the benefit of full-field performance, and to perform a new analysis aimed at different issues concerning behaviour during CO 2 injection. Both sets of analyses required coupled static–dynamic modelling runs in which the key static parameter ranges of the field were varied, including depth conversion, internal geometries and aquifer properties. For the field development work the parameter ranges were explored to assess in-place volumes and field behaviour under natural aquifer and depletion drive; for the CO 2 uncertainty work, however, parameter ranges were explored to demonstrate storage capacity and CO 2 containment. The look-back showed that the field volumes indicated by production data landed in the upper part of the original uncertainty range and that there was definitive spare capacity in the field relative to the planned injection volume.

Description: This paper provides details of the trace element and total organic carbon (TOC) content of the Lower Callovian Kellaways Sand. Heavy metals are associated with mineral phases such as pyrite, iron and manganese oxyhydroxides, organic matter and clay minerals, and correlations are varied. Using a multi-disciplinary approach and geochemical comparisons it has been shown that the Kellaways Sand is particularly enriched in Cr. Cr concentration data suggest two separate components, one bound with Ni and MnO, and the other occurring as an accessory resistate phase Fe–Ti–Cr oxide (ulvöspinel) with up to 4.7 wt% Cr2O3. The closest correlations between matrices are Zn and Ni with Fe2O3; V, Ni, Rb and Ga with MnO, MgO and K2O; Ni and MnO with TOC; and Cd with Zn. Average concentrations for trace elements and TOC are generally enriched in both the Oxford and Kellaways clays. Variable concentrations of SO4 in associated groundwaters indicate that oxidation of FeS2 influences the ranges and concentrations of trace elements observed through Eh and pH controlled adsorption or desorption reactions. The natural and variable concentrations of the metals that also occur in associated groundwaters (including Hg, Cd, Ni, As and Pb) have important implications for the associated, risk-based hydrogeological assessments completed for landfill sites contained within the Oxford Clay, and affect the choice of priority determinants significantly for the monitoring control and trigger levels.

Description: Investigation of core and outcrop samples of the Cenomanian Natih-B Member (North Oman) indicates that the different lithofacies present experienced rather different early diagenesis shortly after deposition. Transmitted-light, cathodoluminescence and backscattered scanning-electron microscopy, as well as stable-isotopic, X-ray diffraction and total organic carbon (TOC) analyses were employed to delineate the major controls on the cyclic pattern of early diagenesis and hydrocarbon source potential.The Natih-B intrashelf basinal carbonates are composed of pelagic sediments that exhibit high-frequency cyclicity marked by decimetre-thick lithofacies alternations, mainly between: Lithofacies A compacted, partially bioturbated, skeletal, organic carbon-rich mudstone to wackestone; and Lithofacies B uncompacted, extensively bioturbated, skeletal, sparry-calcite rich wackestone to packstone. Individual units are composed variously of authigenic and biogenic calcite (58.1–97.6%, average 78.5%) and organic carbon (0.3–13.7% TOC, average 3.6%), together with minor quartz, clay, pyrite, dolomite and phosphatic material (fish debris). Lithofacies A contains relatively more organic carbon, clay, pyrite and dolomite than Lithofacies B and constitutes an excellent source rock. Diagenetic textures of Lithofacies A are dominated by compactional deformation of burrow fabrics, faecal pellets and solution seams, in addition to zoned/bright luminescent, non-ferroan sparry and isopachous calcite cement in and around uncompacted foraminifer tests, in an uncemented matrix. In contrast, Lithofacies B does not show any signs of compaction other than microstylolites and is dominated by zoned/dull luminescent, non-ferroan calcite microspar replacement, in addition to pore-filling, predominantly dull-luminescent, non-ferroan, sparry calcite cement. Moreover, Lithofacies B shows evidence of isopachous and meniscus-style cementation, together with geopetal structures and mictritic peloids. Stable-isotopic compositions of both lithofacies were determined from whole-rock samples (δ13C = −0.9 to +0.9‰, average +0.3‰; δ18O = −5.6 to −3.7‰, average −4.8‰) and sparry calcite (both cement and matrix) subsamples (δ13C = −0.6 to +1.2‰, average +0.6‰; δ18O = −5.7 to −3.7‰, average −4.3‰); all results being relative to Vienna Pee Dee Belemnite.These petrographic and isotopic characteristics suggest that the Natih-B abundant calcite cements and replacements were precipitated early, prior to compaction, mainly from ‘normal’ (open, oxic) seawater at slightly elevated depositional temperatures. Some of the slightly negative δ13C values, however, indicate an addition of isotopically light carbon, probably derived from organic-matter oxidation by organisms living in marine pore waters. Based on evidence of extensive seafloor bioturbation and cementation, and their position within the depositional succession, the tops of Lithofacies B (wackestones to packstones) are interpreted as ‘discontinuity surfaces’ that cap shallowing-upward, fifth-order cycles, formed as a function of sediment starvation and increased bottom-current activity during relative sea-level stillstand/turnaround. In contrast, Lithofacies A (mudstones to wackestones) is believed to reflect high organic production coupled with high sedimentation rate and rapid burial. These conditions limited total infaunal colonization and extensive calcite precipitation, and preserved organic matter together with some escape burrows and in-place fauna, suggesting episodic sediment influx when more accommodation was available and seafloor diagenesis was minimized during relative sea-level rises. The relatively higher amounts of pyrite and dolomite in Lithofacies A likely indicate organic-matter degradation by bacterial sulphate reduction in anoxic pore waters during shallow burial.

Description: This study provides an evaluation of the sorption capacity of two contrasting mineral liners or barriers widely used in the UK for a range of organic contaminants of varying hydrophobicity commonly found in landfill leachate. Batch tests (involving toluene, trichlorobenzene, trichloroethene and naphthalene) showed that the sorption capacity of Oxford Clay was not only significantly greater than that of Mercia Mudstone, but was also greater than the sorption capacity of many soils or clays reported in the literature. The organic carbon normalized sorption coefficients (Koc) for Mercia Mudstone were comparable with both published and empirically derived Koc values, but the Koc for Oxford Clay was underestimated by literature values by several orders of magnitude. Retardation of these contaminants by Oxford Clay was also under-predicted by estimates based solely on organic carbon content. Amorphous organic matter (the main component of the organic matter in the Oxford Clay as characterized using coal petrography' methods) was believed to be responsible for the elevated sorption capacity of the Oxford Clay liner. Sorption coefficients were reduced in the presence of dissolved organic carbon in leachate, suggesting that published Koc values derived in synthetic groundwater may overestimate the sorption capacity in landfill scenarios. Sorption coefficients and Koc-Kow correlations determined in this study can be used for modelling organic contaminant sorption in Oxford Clay and Mercia Mudstone liners as part of landfill risk assessments in the absence of site-specific data, in particular for Oxford Clay, for which published correlations were shown to be too conservative. For other types of clay liner material, the cautious approach would be to determine site-specific sorption coefficients following characterization of the organic carbon. Further research is needed into the effects of leachate dissolved organic carbon and the composition of clay liner organic carbon on sorption of hydrophobic organic compounds.

Description: The Mississippian North Wales Platform is located on the margins of the East Irish Sea Basin and has been little studied over the last 30 years. The exposed Visean limestones provide new insights into the deposition, porosity evolution, distribution of dolomitization, and Pb–Zn and Cu mineralization on the North Wales carbonate platform. This is of relevance to the characterization of fault-related dolomite hydrocarbon reservoirs and age-equivalent Mississippi Valley-type mineral deposits. In particular, the study demonstrates the intimate relationship between sedimentation, basin-scale tectonism and post-depositional fluid flux. Depositional cyclicity is marked, with metre-scale upward-shallowing cycles in which pervasive marine and meteoric calcite cements occlude matrix porosity and syndepositional fractures. Consequently, subsequent burial diagenetic replacive dolomitization is matrix selective and cements are primarily restricted to fractures. Seven phases of dolomite are defined based on texture and cathodoluminescence petrography, with phases D1–D3 as the most volumetrically significant. Dolomite phases D0–D2 are matrix replacive, cross-cutting stratigraphy and locally fingering along beds for several metres. Dolomite phases D3–D7 are hosted by faults and fractures and also line vugs. Evidence of telogenesis is recorded where burial diagenetic products are post-dated by calcite cements precipitated from meteoric fluids. Dolomitization probably occurred during the Mississippian and continued into the Pennsylvanian. Pb–Zn mineralization is also interpreted to have occurred during the Pennsylvanian, associated with Variscan tectonism. Overall, the North Wales Platform displays a more complex paragenesis than age-equivalent platforms in the Pennine Basin, owing to multiple phases of burial and exhumation. The study demonstrates the importance of linking burial history to detailed field and petrographical data to understand and predict the spatial and temporal controls on diagenetic processes and products within syn- and post-rift sequences.

Description: The Lower Cretaceous Goldeneye gas field lies in the Captain turbidite fairway of the Moray Firth and has recently ceased production. Its situation and dimensions have made it an excellent candidate for CO 2 sequestration. The field was extensively modelled for the original development planning when uncertainty was assessed from the perspective of volumetrics and field behaviour. The subsequent need to assess its suitability as a CO 2 store has given the opportunity for a look back at an uncertainty analysis with the benefit of full-field performance, and to perform a new analysis aimed at different issues concerning behaviour during CO 2 injection. Both sets of analyses required coupled static–dynamic modelling runs in which the key static parameter ranges of the field were varied, including depth conversion, internal geometries and aquifer properties. For the field development work the parameter ranges were explored to assess in-place volumes and field behaviour under natural aquifer and depletion drive; for the CO 2 uncertainty work, however, parameter ranges were explored to demonstrate storage capacity and CO 2 containment. The look-back showed that the field volumes indicated by production data landed in the upper part of the original uncertainty range and that there was definitive spare capacity in the field relative to the planned injection volume.