Muds and Mudstones: Physical and Fluid-Flow Properties

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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. the deformation of thick mudstone packages. (3) Geochemical porosity. This is considered to be the fluid volume in which geochemical reactions occur and is the value required for geochemical modelling. Based on halide This volume data, Pearson argues that the geochemical The papers in this volume are written by people porosity is similar to the solute diffusion working on both petroleum and environmental porosity and is only 30-70% of other issues. Both communities take an active interest porosity values. These differences are in muds and mudstones since abundant petrol- unimportant for transport calculations but eum is generated from and trapped by mud- lead to uncertainties when pore water stones, and because huge amounts of domestic compositions are modelled. and industrial waste are retained by muds. The permeability mine field is reviewed by An example, in stark financial terms, of their Dewhurst et al., who consider pores, fi'actures significance is that mudstones cause 90% of and faults as possible fluid conduits. Mudstones drilling problems for the petroleum industry exhibit a ten orders of magnitude permeability and cost the petroleum drilling industry around range with a three orders of magnitude range at a $600 million per year. single porosity. Currently, predictions of fluid Three general themes, closely linked but flow and pressure in sedimentary basins are studied separately within sub-disciplines of severely impeded by a lack of basic porosity- earth sciences, form the basis of the volume: permeability relationships for mudstones. Based (1) transport properties, including absolute on pore-size distributions and a sadly diminutive and relative permeability, diffusion rates number of permeability data for well-character- and the role of faults, fractures and shears ized mudstones, there is a strong hint that much as fluid conduits; of the variation is related to grain size, with (2) mechanical properties, including deforma- coarser-grained muds having higher permea- tion and the development of faults and bilities at a given porosity. Equally, the rate of fractures; loss of permeability with porosity appears to be (3) geochemical properties, in particular the influenced by grain size, with compaction occur- colloidal properties of mudstones. ring by collapse of the largest pores through which most fluid flow occurs. As faults appear to act both as fluid conduits Physical properties and barriers, under what conditions do they The first five papers of the volume consider the exhibit enhanced permeability? Flow must be basic physical properties of mudstones. Porosity, episodic and is perhaps limited to periods of probably the most fundamental property of a active fault movement, since both field and sediment, is reviewed by Pearson. Even this is laboratory evidence shows that matrix permea- subject to uncertainty! Pearson argues that the bilities in muddy fault zones are very low and Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

4 A. C. APLIN ET AL. similar to those of the wall rocks. Experimental undisturbed nmdstones behave in a brittle evidence, including that presented by Peters & manner, showing distinct peak strength before Ma|tman, in this volume, suggests that faults and undergoing failure and strain weakening to a shear zones can be sites of preferred fluid flow, residual strength. In contrast, at very high mean for water and perhaps petroleum, during active effective stresses, mudstones tend to behave in a deformation. Dilation enhances permeability ductile manner, with the maintenance of peak and is more likely to occur in overconsolidated strengths to large strains. In this paper, Pet|ey (uplifted) and brittle (lithified) muds (Ingrain & suggests that a 'transitional regime' can be Urai). defined for most mudstones, in which undrained Hydraulically formed microfractures remain a shear deformation leads to the maintenance of mystery. They are rarely seen in mudstones but peak strength to a given axial strain in a manner are often inferred from the fact that observed which is similar to ductile deformation, before fluid pressures do not exceed minimum stress the initiation of strain weakening to a residual (e.g. DOppenbecker et al. 1991). For oil petrol- strength. Pervasive microfracturing characterizes eum companies, this is important because the the transition zone so that the mudstone behaves probability of petroleum leakage through in a ductile manner on the macro-scale and in a caprocks appears to be enhanced when fluid brittle manner on the micro-scale. A critical pressures approach the minimum stress (Gaaren- question is then: what happens to the large-scale stroom et al. 1993; Ingrain & Urai). Important permeability of the mudstone along the defor- questions relating to issues surrounding micro- mation path (see Ingram & Urai; Dewhurst et ai. fractures are whether they actually occur and if and Peters & Maltman)? so, how much fluid is lost, and over what time- The effects of chemical diagenesis on mud- scale is permeability enhanced? In their paper, stone properties are highlighted by Bjorlykke. He Ingrain & Urai suggest that most mudstone reminds us that, although mudstone compaction caprocks will fail by shear, leading to enhanced is usually modelled as a purely mechanical permeability if the mudstones are dilated. effect, above 60~ time-temperature dependent Dilation is more likely to occur in stronger chemical processes may also be important, as (overconsolidated and/or lithified) mudstones. they are in sandstones. There are few quantitative Midttome & Roaldset's review of thermal con- data to constrain the rate of chemical compaction ductivity carries similar messages to Dewhurst or its impact on permeability and overpressure et al.'s review of permeability. The lack of basic development, but, as Bjorlykke points out, thermal conductivity data, especially for well- chemical compaction may proceed with a much characterized mudstones, means that there is lower dependence on effective stress. In addition, no clear understanding of what controls the chemical processes will lithify the mud, increas- range of measured values. Use of inappropriate ing its brittleness and thus its propensity to values in computer-based thermal models of develop dilatational fractures (see also Ingram & sedimentary basins can lead to dramatic differ- Urai and Petley). ences in temperature histories and thus the time at which oil and gas were generated. One example shows a 50~ variation in the modelled, present- Experimental studies day temperature of some upper Jurassic sedi- The next three papers in the volume are related in mcnts from the northern North Sea, depending that they are all experimental studies of mud which of two different methods was used to permeability. Although the rationale for the estimate matrix conductivity. The need to work was different in each case (geological, standardise the technique used to measure storage of radioactive waste, landfill liner), the thermal conductivity is emphasized, as is the ideas and data are readily transferable between need to understand the influence of sediment sub-disciplines. texture (mineralogy, grain size, sedimentology) There are only a handful of published studies on thermal conductivity. of permeability anisotropy and the way that this The deformation and mechanical failure of evolves with increasing stress. Clennell et al. mudstones is considered by Petley. Combining a examine the development of permeability aniso- new set of high mean effective stress (2-50 MPa), tropy in well characterized kaolinite, smectite, undrained shear experiments on London Clay silty clay and a remoulded, natural mud, with a summary of similar experiments on other experimentally compacted to 4 MPa. Aniso- mudstone datasets, he constructs a new frame- tropies are less than 3 for natural clays and work for the behaviour of mudstones during kaolinite, but is 8 at a porosity of 60% for pure undrained shear deformation. Previous work has Ca montmorillonite. These values are similar to suggested that at low mean effective stress, most previously published values (Tavenas et al. 1983; Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

INTRODUCTION 5

A1-Tabbaa & Wood 1987; Leroueil et al. 1990), flowed through pressure-induced cracks or but much less than those predicted from microfractures, not through the intergranular models in which clay plates become increasingly pores. Gas permeabilities were similar to the aligned with increasing stress (up to 100; Arch & intrinsic permeabilities and depended on the Maltman 1990). Photomicrographs suggest that number of pressure-induced flow paths. From a compaction-driven reorientation of clay plates is petroleum perspective, it is intriguing to consider not easy and that directional permeability is not these results in the context of the abundant simply controlled by levels of average particle geochemical and geophysical evidence for orientation. A two-tier microstructural hierarchy vertical migration of gas through mudstones in is proposed, comprising randomly aligned sedimentary basins; debate will continue as to the particle domains and shear faces between relative importance of capillary and fracture flow domains. Permeability anisotropy models based as gas flow mechanisms in these cases. on a mixing of plates, ellipses and spheres are more applicable to muds and predict anisotropy levels of 2-3. Greater anisotropies may only be Case studies possible in laminated or vertically stacked, Caprock leakage from petroleum reservoirs is heterogeneous mudstone sequences. further considered by Ingram & Urai. Where the Permeability changes as a result of failure are capillary threshold pressure of a top seal is important to all situations where fluid retention greater than the buoyant pressure of a petroleum is important: oil or gas field caprocks, hazardous column in a fill-to-spill reservoir, leakage is only waste storage sites and landfill liners. Peters & possible through faults or fractures. Ingram & Maltman present an experimental investigation Urai propose some simple but pragmatic pre- of strain and shear-related changes in the dictive tools to risk the likelihood of top seal hydraulic conductivity of natural boulder clays, leakage through faults and fractures. For which operate as landfill liners in many parts of faults, fault density and the internal geometry/ the UK. Many of these clays are overconsolid- stratigraphy of the mudstone are critical because ated due to the removal of Quaternary ice. of the increased risk of leakage when relatively Shearing of overconsolidated clays is accommo- permeable silts/thin sands with low capillary dated by dilation, with potentially increased threshold pressures and relatively high permea- permeability. With increasing strain, the bilities are juxtaposed by faults. hydraulic conductivity of an overconsolidated Leakage may also occur through extensional sample increased by less than 20% and by less or dilatant shear fractures. By pointing out that than 10% at failure. stronger (overconsolidated and/or cemented) Two-phase flow in fine-grained materials is mudstones are more likely to dilate than weaker another poorly constrained process of import- mudstones, Ingrain & Urai define strategies, ance to both the petroleum and radioactive based on parameters which can either be easily waste communities. Gas generated within storage measured or estimated from wireline data, to sites may increase the pressure within clay-liners evaluate the risk of dilatational fractures. One and lead to its fracture, potentially compromis- important question is exactly how muds become ing the long-term performance of the repository. lithified into mudstones, since these are the In similar vein, migration or remigration of processes which increase strength and the tend- petroleum through thick sequences of mudstones ency to deform by brittle fracture. These is a common, critical, but poorly understood processes remain poorly constrained in mud- phenomenon, in many parts of the world (e.g. stones, in part because it is difficult to observe Gulf Coast, Niger Delta, North Sea Tertiary). mineral cements except when they are obviously Debate continues over the relative importance of pore-filling. migration through capillary networks (Leith et al. Overpressure (that is, fluid pressures in excess 1993), faults (Hooper 1991) and microfractures of hydrostatic) develops in sedimentary basins (Caillet 1993) resulting from high fluid pressures where the rate of pressure generation by sediment or changes in the local stress regime. loading or gas generation exceeds the rate at In the radioactive waste context, Harrington & which it can be dissipated by fluid flow. In this Horseman have performed gas injection experi- context mudstones are critical in that they are the ments on compacted bentonite (landfill liner or most common, low permeability unit in basins liner for nuclear waste repository) and a natural and allow the retention of high fluid pressures mudstone, the Boom Clay. Intrinsic perme- over geological timescales. On the other hand, abilities were around 100-500 nD for the Boom the low permeability of mudstones makes it Clay and 5-10 nD for the bentonite at porosities impossible to directly measure the pore pressure, of close to 40%. In these experiments, gas only which must therefore be inferred from other Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

6 A.C. APLIN ET AL. physical properties. In their paper, Skar et al. The relevance of these results to the petroleum estimate pore pressures of mudstones on the sector is striking since, as we have already Halten Terrace using soil mechanics con- pointed out, that industry is still struggling to solidation theory to relate measured porosities define the pathways by which petroleum migrates to effective stress (=overburden stress-pore through kilometre-thick mudstone sequences pressure; Hottmann & Johnson 1965). Their (e.g. Goff 1984). Preferential pathways are often analysis suggests distinct variations in pore inferred (e.g. Hooper 1991; Roberts & Nunn pressure which cannot be easily explained by 1995; Larter et al. 1996) but there is little disequilibrium compaction resulting from rapid, consensus as to the nature of the pathways late Tertiary sedimentation. Skar et al.'s expla- (faults, fractures, silts) and the circumstances nation is that pressure has been transferred under which they are able to conduct fluid. laterally from the deep Rgts Basin through the Klakk Fault Complex. At the present time, the Future directions strong pressure difference across the fault complex indicates that it is sealing. The key question, The papers in this volume provide a brief which is equally relevant to fault seal in snapshot of some of the current research on reservoirs, is thus understanding and predicting fine-grained sediments. By highlighting defects in the circumstances under which faults transmit our knowledge, it provides pointers to the future. fluid (see Dewhurst et aL). In this case, Skar et al. Even if the coming millennium is not ultimately argue that the fault transmitted fluid and thus remembered as the millennium of mud, in its first pressure is a result of a basinal reorientation of decade the following areas seem likely to be stress in the early Pliocene. amongst those in which progress will be made. The last two papers concern the pore structure (1) Gathering of basic data for the physical and flow properties of mudstones which are properties of well-characterized mudstones, includ- currently close to the earth's surface. Dorseh & ing compressibility, strength, absolute and relative Katsube's interest lies in the porosity and pore- permeability, thermal conductivity, mechanical size distribution of mudstones of the Cambrian behaviour and petrophysical properties. If appar- Conasuga Group, which has been investigated as ently mundane, this task is critical as it will a possible site for the storage of radioactive underpin all efforts to provide a predictive contaminants. As also shown by Pearson's work, understanding of the environmental behaviour different techniques resulted in different porosity of mudstones in a range of settings and on both values, ranging between around 4 and 10% for human (e.g. drilling) and geological timescales. mudstones that have been buried to over 4 km. Proper account should of course be taken of Values determined by mercury injection are the sedimentological diversity exhibited by lowest because this technique only detects pores mudstones. with diameters greater than 3 nm. Deeply buried (2) Mechanisms of single and multiphase fluid mudstones contain significant porosity with .[tow. In addition to the continuing need for single throats smaller than 3 nm. This nanoporosity phase permeability data, it will also be important may be inaccessible to contaminants which occur to constrain the relative importance of capil- as organic chelates or as colloids, which will laries, faults and fractures on flow both through therefore follow preferential flow pathways and across mudstone sequences. Experimental through larger pores. studies of the hydromechanical behaviour of Preferential groundwater flow pathways mudstones, especially at high fluid pressures through the shallow buried, Tertiary Bartonian where hydrofracturing may occur and at which Clay in Belgium are also inferred by Walraevens permeability may be strongly related to stress will & Cardenal, based on the inconsistency between be important. Establishing ways in which data laboratory permeability (10-a~ s -1) tests and gathered from short-term laboratory experiments the distribution of hydraulic heads in the region. can be confidently extrapolated to explain Hydraulic conductivities of 10-9m s -I gave an processes on geological timescales will, as always, excellent agreement between measured and be a point of discussion. All these ideas are also modelled hydraulic heads, thus implying the relevant to the placing of constraints on the presence of preferential flowpaths. Recharge to migration mechanisms of gaseous and liquid the underlying Ledo-Paniselian aquifer occurs petroleum, a problem of real importance for through the Bartonian Clay. Hydrogeochemical both petroleum and environmental scientists. data are used to support the notion of localized, Again, the relative importance of fi'acture v. preferential flow pathways, which may in prin- capillary flow is critical, as is the definition of ciple be faults, fractures or thin, interconnected wettability. Although some ideas can be silts or sands. imported from the extensive studies of Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

INTRODUCTION 7 multiphase flow in petroleum reservoirs, the low strengthen the mechanical properties of mud- permeability and sub micron pore size of mudstones is poorly understood but their importance stones ensure that this is an area where progress lie in the fact that they will strongly influence the will require the collaboration of geoscientists fracture, fluid flow and sealing characteristics of with physicists and chemists. mudstones. Equally important will be to define (3) Definition and visualization of three dimen- the links between failure mechanisms and the sional mudstone sequences. Into the future, the basic mechanical properties of the full range of fundamental aim must be to advance the study of mudstone types. mudstones to a similar point to that enjoyed by sandstones and carbonates today. Techniques initially developed for the exploration and pro- References duction of petroleum can now be exploited as a AL-TABBAA, A. & WOOD, D. M. 1987. Some measure- tool for defining and interpreting mudstone ments of the permeability of kaolin. Gdotechnique, sequences. Wireline well log and both 2D and 37, 499-503. 3D seismic data can be used to help define the ARCH, J. & MALTMAN, A. J. (1990). Anisotropic anatomy, petrophysical and geochemical proper- permeability and tortuosity in deformed wet ties of mudstone sequences. In turn, these data sediments. Journal of Geophysical Research, 95, 9035-9047. form the basis for placing the mudstones, and BENNETY, R. H., BRYANT, W. R. & HULBERT, M.H. their specific characteristics, into a chronostrati- (eds) 1991. The Microstructure oJ" Fine-grained graphic, sequence stratigraphic and depositional Sediments, from Mud to Shale. Springer Verlag, framework. Ultimately, stratigraphic data may New York. be tied to the tectonic, weathering, transport and BLATT, H. 1970. Determination of mean sediment depositional processes which impose the grain thickness in the crust: a sedimentological method. size and mineralogical properties which exert a Geological Society of America Bulletin, 81, fundamental control on the muds' physical 255-262. behaviour upon burial. ~, MIDDLETON, G. V. & MURRAY, R. C. 1980. Origin of Sedimental~, Rocks, 2rid Edition, (4) Computer-based modelling. Increased com- Prentice-Hall, Englewood Cliffs, NJ. puting power means that three dimensional BOHACS, K. M. & SCHWALBACH, J. R. 1992. Sequence geological data is becoming easier and easier to Stratigraphy in Fine-Grained Rocks: With Special assimilate and display. For mudstones, this Reference to the Monterey Formation. In: implies that compaction will be treated in SCHWALBACH, J. R. & BOHACS, K. M. (eds) terms of a tensorial rheology; models already Sequence Stratigraphy in Fine-Grained Rocks: exist in civil engineering but have yet to be used Examples from the Monterey Formation. SEPM successfully to explain geological compaction in Field Guide, 7-20. sedimentary basins (Pouya et al. 1998). With BURLAND, J. B. 1990. On the compressibility and shear strength of natural clays. GOotechnique, 40, appropriate calibration to wireline log and 329-378. physical parameters, 3D seismic data can further CAILLET, G. 1993. The caprock of the Snorre Field, define the internal anatomy of mudstone Norway: a possible leakage by hydraulic fractur- sequences and the occurrence of internal faults ing. Marine and Petroleum Geology, 10, 42 50. and fractures (Cartwright & Dewhurst 1998; CARTWRIGHT, J. A. & DEWHURST, D. N. 1998. Layer- Ingram & Urai). Flow simulators will be increas- bound compaction faults in fine-grained sedi- ingly combined with models describing the ments. Geological Society of America Bulletin, chemical interactions between water, organic 110, 1242-1257. fluids and rock minerals. The high specific CHAMLEY, H. 1989. Clay Sedimentology. Springer- Verlag, Berlin. surface area of mudstones implies that particular DOPPENBECKER, S. J., DOHMEN, L. & WELTE, D. H. attention should be given to adsorption and ion 1991. Numerical modelling of petroleum expul- exchange reactions. sion in two areas of the Lower Saxony Basin, (5) Mechanical properties. A more unified northern Germany. In: ENGLAND, W. A. & FLEET, understanding of the mechanical (compaction, A. J. (eds) Petroleum Migration. Geological fracture) properties of mudstones is likely Society, London, Special Publications, 59, 47-64. (see Maltman 1994 and Petley). Historically, GAARENSTROOM, L., TROMP, R. A. J., DE JONG, M. C. & compaction has been treated primarily as an BRANDENBURG, A. M. 1993. Overpressures in the elastoplastic, mechanical process. However, it is Central North Sea: implications for trap integrity and drilling safety. In: PARKER, J. R. 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8 A. C. APLIN ET AL.

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