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Complex Carbonate Pore Systems of the Carboniferous Hodder Mudstone Formation, Bowland Basin, UK


Diagenesis in muddy, carbonate-rich sediments can reduce or enhance porosity at varying scales. The Lower Carboniferous Hodder Mudstone Formation in the Bowland Basin is a potential UK shale-gas play and provides an opportunity to understand the evolution and controls on porosity in an organic- and carbonate-rich mudstone. This is achieved through the characterization of pore types and mineral components in samples from a suite of wells along the northern margin of the Bowland Basin. The work utilises petrographic, XRD, X-ray CT, C/O isotopes, electron microprobe and N2 gas-adsorption techniques. Samples were divided into nine lithofacies which provided a framework to establish compositions, textures, pore types and depositional environments. Lithofacies were then grouped into associations: (i) mudstones – >50 % clay-sized particles; (ii) calcisiltites – calcareous, silt-rich mudstones, and (iii) limestones – bioclast-rich, resedimented wackestones and packstones. Mudstones (~40% of samples) exhibited rare planar to convolute laminae, but were mostly unlaminated. Calcisiltites (~30 % of samples) were largely unlaminated, but where calcisiltites grade into mudstones they formed discontinuous ripple laminations. Limestones (~30 % of samples) exhibited ripple laminations except for the rare occurrences of storm-brecciated, crinoidal beds within mudstones. Within the cores lithofacies fine upwards from coarse-grained, muddy limestones to medium-grained calcisiltite and then organic-rich mudstones. Pore types included interparticle, intercrystalline and intraparticle forms. Macropores (>4mm) exhibited vuggy intercrystalline pore morphologies within veins localised in the calcisiltites; while micro- to nano-pores (<62.5µm) occurred within pyrite framboids (intraparticle), between clay minerals and grains (interparticle) and in organic matter particles. In the mudstones, pores within pyrite framboids and clay minerals were <300nm in diameter and comprised a large percentage of the pore volume. The limestones exhibited <1nm sized-pores due to carbonate cementation and pore-filling kaolinite. Although diagenesis plays a vital role in modifying primary depositional fabric, the work demonstrates that pore morphology and volume are mainly controlled by lithofacies. The importance of this work is in the characterization of pore structures within a potential future UK shale-gas play and in the identification of process controls in carbonate-dominated mudrocks.