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 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.