Abstract The Wildmoor Sandstone Formation, proved in three boreholes drilled at Birmingham University, is dominated by fine- to medium-grained sandstones deposited in a braided river environment, within which channel lag, channel fill and abandoned channel facies are recognized. Minor proportions of aeolian sandsheet are present, as are dolocretes, not previously reported in the formation. The sandstones are feldspathic and lithic arenites, and typically are clay-poor. Early dolomite dominates the diagenetic overprint, and is preferentially developed in channellag deposits. Burial diagenetic effects are minor. Late calcite occurs as a pore-filling phase and within fractures. Minor fractures and granulation seams are oriented parallel to the NE-SW Birmingham Fault. ‘Conventional’ granulation seams, with comminution of detrital material, and more complex seams containing comminuted dolomite cement with a millimetre-wide halo of dolomite cement are present, the latter implying that the sandstone was dolomitecemented at the time of fracturing. Several scales of heterogeneity will affect groundwater solute transport. The palaeosols and abandoned channel mudstones may act as barriers to vertical flow at the decimetre scale. Dolomite-cemented channel-lag deposits may act similarly at smaller scales. Granulation seams have permeabilities of two-three orders of magnitude lower than their host sandstones, but their limited occurrence may limit their impact on larger scale flow. Matrix permeability is controlled by grain size and dolomite cement. The fines in the fine-grained, ripple cross-laminatied sandstones were extensively washed out during coring, and this lithology may be a source of sand yields in some sandstone boreholes. Although no enhancement of particle yields was seen during packer testing, the possibility remains that more comprehensive failure may occur at higher pumping rates.
Abstract Diagenetic francolite (carbonate fluor-apatite) occurs as overgrowths on detrital apatite grains in sandstones from the Lower Jurassic Statfjord Formation of the North Sea. The francolite overgrowths are considerably more enriched in the rare-earth elements (up to 1 wt % Ce 2 O 3 ), Sr and F than the detrital cores. These elements were carried in aqueous solution, probably in the form of complex ligands involving organically sourced carbon and halogens. It is possible that reported aberrant neodymium isotope model ages within the Statfjord Formation are the result of mobilization and relative fractionation of Sm and Nd during diagenesis, rather than a result of changes in provenance.
This report describes investigations into methodologies that can be applied to overcome imaging problems associated with carbonate minerals on scanning electron microscope-based cathodoluminescence (SEM-CL) systems. The problem arises due to the persistent nature of luminescence from carbonate minerals, which causes ghosting or streaking across SEM-CL images. Two methodologies were tested:
• The first methodology (Lee 2000) applied very long image acquisition times that, in certain situations proved capable of producing excellent images at higher resolution than is possible using optical-based CL systems. However, the image acquisition times are too slow (c. 40 minutes per image) to be useful in most day-to-day situations.
• The second methodology (Reed and Milliken 2003) uses an optical filter to remove the portion of the CL spectrum responsible for the persistent luminescence (in this case in the yellow to red portion of the visible light spectrum). This enabled capture of SEM-CL images at far faster acquisition times (c. 5 minutes per image) than was possible without the filter. However, the resulting ‘filtered’ images suffer from relatively poor contrast and zoning apparent in these images did not always match zoning observed in optical Cl or unfiltered SEM-CL images.
Poor image contrast was observed in the filtered images because the luminescence in the studied carbonates is predominantly due to activation by substitution of Mn, which predominantly occurs in the orange to red portion of the visible spectrum. Therefore, this type of luminescence was effectively excluded by the filter. Consequently the measured signals from the detector reflect the much less intense intrinsic luminescence of the carbonate or luminescence activated by other substituted cations (e.g. rare earth elements) or thermally activated luminescence.
Although filtered SEM-CL carbonate imaging met with variable levels of success, the filtered imaging approach will prove useful in the SEM-CL analysis of quartz in mixed quartz-carbonate-bearing lithologies such as carbonate-cemented sandstones, which have previously been hindered by the persistence of the luminescence from the carbonates.
Abstract Two examples of Dinantian basin-margin carbonates from the United Kingdom provide potential analogues for fracture and porosity distributions in hydrocarbon reservoirs in dolostones. In both examples pore systems have been extensively modified by dolomitization, fracturing and related mineralization. However, the detailed processes and the end results show some significant differences, highlighting the importance of developing an understanding of the specific pore-system modifying processes when characterizing and modelling porosity distributions in these settings. In the first example, predominantly low-porosity and low-permeability limestones in Lower Carboniferous inliers in Leicestershire (Cloud Hill) and south Derbyshire (Ticknall) had their pore systems further degraded by extensive dolomitization. Subsequent fracturing and related mineralization were responsible for significant porosity generation adjacent to fractures. In contrast, in the Sellafield area (west Cumbria), dolomitization was strongly controlled by fractures that were also mineralized by sulphate-rich brines. In the north of the area fractures were filled with an assemblage of barite-fluorite-hematite-calcite that is resistant to corrosion by low-temperature meteoric groundwater. However, in the south of the area the fractures were cemented by anhydrite, which is readily corroded by saline, but sulphatepoor, groundwater formed by percolating meteoric recharge from the east. Progressive dissolution has been ongoing since Tertiary uplift, and has rejuvenated fracture porosity within the dolomitized limestones.
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