Muds and Mudstones: Physical and Fluid-Flow Properties
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Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 Muds and mudstones: physical and fluid-flow properties ANDREW C. APLIN 1, ANDREW J. FLEET 2 & JOE H. S. MACQUAKER 3 1Fossil Fuels and Environmental Geochemistry Postgraduate Institute." NRG, Drummond Building, University of Newcastle, Newcastle upon Tyne NE1 7R U, UK 2Department of Mineralogy, The Natural History Museum, Cromwell Road, London S W7 5BD, UK 3Department of Geology, University of Manchester, Oxford Road, Manchester M13 9PL, UK Muds and mudstones are the prime control on lags behind that of other sediments. Their fluid flow in sedimentary basins and near-surface physical and bulk properties are poorly defined, environments. As the world's commonest sedi- particularly as they relate to behaviour at depth; ment type, they act as aquitards in sedimentary for instance, what mudstone permeability should basins, restricting water flow, and they influence be applied when carrying out a particular fluid- the development of overpressure. In petroleum flow modelling exercise, or under what con- systems they act as source rocks for nearly all oil ditions does flow through fractures dominate and much gas, determine migration directions flow through the capillary matrix? between source and trap in most settings, and act A search of the Science Citation Index from as seals to many reservoirs. In near surface 1981 to 1998 revealed 13 380 articles containing environments, they not only control natural flow, the word 'mud' or 'shale' in the title, keyword or but have also been used over the centuries to abstract; about 750 per year. 5986 articles con- restrict leakage, most pertinently in recent times tain 'mud' or 'shale' in the title alone. However, from waste disposal sites. surprisingly few articles and books consider the This book focuses on fluid flow through muds key physical properties of muds, despite the and mudstones. Such flow controls processes volumetric importance of muds and the fact such as water escape from a mud during burial, that an understanding of their properties is upward or downward petroleum expulsion from fundamental to a whole range of processes of a source-rock sequence, leakage from a petrol- importance to the petroleum, environmental eum reservoir, or containment of leachate in a and engineering industries (Table 1). In the clay-lined landfill site. geological literature, sedimentology, palaeontol- Despite the significance of muds and mud- ogy, geochemistry, mineralogy, diagenesis and stones, their fine-grained nature means that our palaeogeography are dominant themes (e.g. knowledge of their composition and properties Weaver & Beck 1971; Blatt et al. 1980; Potter Table 1. Muds and mudstones in the industrial sector Discipline Process Petroleum exploration and production Drilling problems/performance Pore pressure prediction Vertical migration of petroleum Seal capacity and caprock integrity Waste containment Landfill liners Storage of nuclear and hazardous waste Contaminant transport Engineering Landslide prediction Foundation design Subsidence Swelling and shrinkage Heavy clay industry Brick and ceramic raw material From: AvLIN, A. C., FLEET, A. J. & MACQUAKER,J. H. S. (eds) Muds and Mudstones: Physical and Fluid Flow Properties. Geological Society, London, Special Publications, 158, 1-8. 1-86239-030-4/99/$15.00 9The Geological Society of London 1999. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 2 A. C. APLIN ETAL. et al. 1980; Chamley 1989; Weaver 1989; O'Brien which, as for most sedimentary rocks, is based on et al. 1990; Velde 1992), with distinctly less grain size and texture: emphasis on physical and mechanical properties. Notable exceptions to this generalization are the Non-fissile important compilations of Rieke & Chilingarian Fissile mudstone mudstone (1974; physical properties), Bennett et al. (1991; > 2/3 silt Silt-shale Siltstone microfabric) and Maltman (1994; deformation). 1/3-2/3 silt Mud-shale Mudstone A recurring theme of the papers in this volume > 2/3 clay Clay-shale Claystone is thus the surprise expressed at the remark- ably limited database describing the funda- A classification centred on these parameters is mental properties (e.g. thermal conductivity, useful as grain size is closely linked to mineralogy permeability, strength, compressibility, mechan- and the physical properties of muds (for ical behaviour, pore size distributions) of well example: Burland 1990; Midttome & Roaldset; characterized muds. Links between properties Dewhurst et al., this volume). But although and sedimentary makeup remain critical because simple and robust, there are several difficulties the primary sedimentological and mineralogical with Blatt et al.'s classification. Firstly, it is diversity of mudstones ensures an equally diverse relatively difficult to determine the grain size range of physical properties. Without the basic distribution of lithified muds. Chewing the data, accurate predictions and models of the sample to estimate its 'grittiness' is a common processes listed in Table 1 will continue to prove way of distinguishing silt and clay, but is scarcely elusive. quantitative! Secondly, by dividing these fine- This volume seeks to take stock of our grained sediments up into two subsets of a larger knowledge of muds and mudstones, as it relates group, when the subsets themselves are difficult to physical properties and fluid flow, through a to distinguish, confers no obvious advantage to series of papers which review particular topics most geologists and thus on its own would not be (e.g. porosity, permeability), or discuss experi- adopted. Thirdly, using fissility as a descriptive mental results, or present specific case studies. parameter is fraught with uncertainty as fissility Together the papers consider the physical commonly develops with increased weathering. properties of muds and mudstones from the Fourthly, not taking into account the mineralogy near-surface to the deep basinal. They do not and origin of the grains within the fine-grained provide encyclopaedic answers but try and focus sediment has the potential to confuse, part- on some of the key issues which need to be icularly where fine-grained carbonate rocks are resolved if fluid flow through muds and mud- encountered. stones is to be understood. Caught between common usage and the strict use of a term, we have chosen to use 'mud and mudstone' in their more general sense: to indicate clastic sediments and sedimentary rocks which Defining muds and mudstones are formed primarily from particles smaller than Both stratigraphic (Blatt 1970) and geochemical 1/16 mm (62.5 ~m). These terms have been (Garrels & Mackenzie 1970) data tell us that fine- adopted because the name mudstone is consist- grained clastic sediments are the world's com- ent with sandstone and limestone. Furthermore, monest sediment type, comprising more than it is less ambiguous than the term 'shale', which 65% of the sediment pile. They are often to be effective relies partly on the degree of described, in hand specimen, by a confusing weathering. plethora of terms (for example, clay, mud, mud- Often grey and apparently homogeneous in stone, shale, claystone, siltstone, argillite) and hand specimen, the rich diversity of mudstones is qualifiers (for example, silty, clay-rich, silt-rich, only revealed by X-radiography or under the shaly). Defining clay as particles finer than 1/256 optical or electron microscope (O'Brien & Slatt mm (c. 4 pm) diameter and silt as particles with 1990). These techniques reveal significant text- diameters between 1/256 and 1/16 mm (62.5 pro), ural, grain size and compositional variability and Blatt et al. (1980) provided a useful, simplified suggest that this variability can be systematic in terminology based purely on grain size. Using both temporal and spatial senses (Macquaker this classification scheme, mud is a sediment et al. 1998). predominantly composed of clay and silt, a Most muds are deposited as an assemblage of mudstone is a sedimentary rock composed of particles ranging in diameter from less than lithified mud and shale is a fissile mudstone. 0.1 ~m to greater than 100 pm. This three orders Blatt et al. (1980) also provide a more detailed of magnitude range, whilst typical of muds, is classification of fine-grained clastic sediments, unusual in other clastic sediments; for example, Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 INTRODUCTION 3 very few sandstones contain particles ranging in relevant measure of porosity depends on what size from the very finest sand (62 gm) to cobbles process one is trying to model. He distinguishes (0.062 m). The mean grain size of muds is also three types of porosity. very variable, with clay fractions ranging from 10 (1) Physical porosity, which is the volume not to 100%. For example, Picard's (1971) average occupied by grains. composition for 751 recent muds (15% sand, (2) Transport porosity, which is the inter- 45% silt and 40% clay) derives from samples connected physical porosity and can be with clay fractions ranging from 0 to 95%. estimated by mercury injection porosimetry Deposition of distinctive muds in both space and or measurements of water content. Trans- time results in mudstone packages which are port porosity can be sub-divided into lithologically heterogeneous on scales which can advective and diffusive transport porosity, be observed on photomicrographs (Potter et al. which are used when describing the move- 1980; O'Brien & Slatt 1990; Bennett et al. 1991; ment of fluids and solutes. Water diffusion Macquaker & Gawthorpe 1993), X-radiographs porosities equal water content porosities (O'Brien & Slatt 1990), wireline-log data (Bohacs but solute diffusion porosities are smaller & Schwalbach 1992) and even seismic data because solutes cannot access the entire (Cartwright & Dewhurst 1998). Although gener- water content of mudstones as a result of ally ignored to date, heterogeneity poses import- minerals' electrical double layers and the ant future challenges to those seeking to model common occurrence of nanometre-scale the flow of fluid and heat through mudstones and pores in deeply-buried mudstones.