CLAY DIAGENESIS IN SANDSTONES AND SHALES: AN...

Clay Minerals (1989) 24, 127-136

CLAY D I A G E N E S I S IN S A N D S T O N E S A N D SHALES: AN I N T R O D U C T I O N

C. V. J E A N S

Department of Applied Biology, Pembroke Street, Cambridge CB2 3DX

Sequences of alternating arenaceous and argillaceous lithologies are commonplace in the siliciclastic hydrocarbon reservoirs of the North Sea and elsewhere. Individual lithological units may vary in thickness from millimetres to tens or hundreds of metres. The first part of this introduction will discuss general aspects important for the understanding of the patterns of clay diagenesis associated with these sequences, whereas the second part will deal with the papers presented in this issue of Clay Minerals and how they fit into this general area of interest.

Hypotheses of clay mineral diagenesis

Detailed research into the clay mineral diagenesis of siliciclastic reservoirs has until very recently not produced a satisfactory hypothesis to explain the inter-relationships exhibited by alternating arenaceous and argillaceous lithologies. Petrographic evidence leaves no doubt that in sandstones the clay cements and pore-linings are neoformed and have been precipitated from the pore-fluids. In contrast, the interpretation of X-ray diffraction evidence of the deeply buried mudstones of the US Gulf Coast and elsewhere suggests that in mudstones the predominant reactions involve transformation in which pre-existing and detrital clay mineral structures are inherited. This transformational hypothesis (Hower et al., 1976) has been very widely accepted, but it is generally unacceptable to those familiar with the variations in clay mineral assemblages associated with alternating arenaceous/ argillaceous sequences. Is it likely that there are two completely different processes controlling the clay mineral diagenesis of sandstones and mudstones when their clay mineralogies may show strikingly similar patterns of variations?

Although these difficulties have been realized by working clay mineralogists for some time, it is only in the last few years that they have surfaced in the literature. Primmer & Shaw (1987) have demonstrated for mudstone samples from the Gulf Coast sequences and the North Sea--classic areas of the transformational hypothesis--that there is considerable evidence of neoformed clay porefillings in the larger pores. More rigorous analysis of this fundamental problem (Nadeau & Bain, 1986) has been a by-product of advances made in the understanding of the processes involved in the smectite ~ illite reaction associated with burial diagenesis and the nature of the clay particles involved. Evidence from a variety of different techniques leaves little doubt that the transformational hypothesis is very largely an artifact of over-reliance on a single technique (XRD) and that the smectite ~ illite change observed in the burial diagenesis of many mudrocks results from the dissolution of the

�9 1989 The Mineralogical Society

128 C.V. Jeans

smectite particles and the neoformation of illite crystals (fundamental particles) if the resulting pore-fluid composition is suitable.

This new approach brings to the working clay mineralogist an acceptable explanation of what is observed in field studies and is likely to prove a major impetus to a better understanding of clay diagenesis.

Clay mineralogy of alternating arenaceous/argillaceous sequences

Three factors are particularly important in determining the clay mineral assemblages exhibited by these alternating facies:

(1) mineralogical and chemical nature of the detrital and colloidal sediment load; (2) extent to which individual components within this load are segregated by variations

in the depositional energy of the environment and by the effects of differential flocculation;

(3) variations in the chemistry of the depositional fluids and subsequent porewaters.

If a single detrital source is assumed for the clay fraction of an alternating sequence, the resulting clay mineral assemblages are determined largely by (i) the action of differential segregation and (ii) variations in the depositional and porewater chemistry. This mineral response between the depositional load and the porewaters of the early sediment provides a facies-fingerprint which is likely to persist through burial diagenesis and into the higher grades of metamorphism. It is interesting to realize that very similar conclusions have been reached in the study of exhalent ore deposits: Stanton (1989) has demonstrated how the initial mineralogy of the ore deposits provides a very distinctive fingerprint into the highest grades of metamorphism and how the same 'Precursor Principle' can be applied to the facies interpretation of high-grade metamorphic terrains.

Nature of the detrital clay load of rivers

The clay-grade detritus carried by rivers to the sea consists of both material developed within soil profiles by transformational and neoformational processes, and unaltered material inherited from the bedrock. The inherited part of the clay load is likely to be particularly stable, and we can assume that it does not play a major role in the evolution of the sediment during diagenesis. The soil components can be subdivided into the relatively well- defined mineral phases and the poorly-defined and highly unstable hydroxides, hydrated oxides and organic complexes involving A1, Fe and Si. The occurrence and reaction of these during river transport, deposition and subsequent entombment in a sediment is poorly understood. Wilson (1987), reviewing soil smectites, has pointed out that the great majority consist of Fe-rich beidellite. These differ from bentonitic smectites (the most frequently recorded type in fossil sediments) in (i) their composition (bentonitic smectites are typically montmorillonites rich in octahedral AI), (ii) their formation of various non-exchangeable complexes with A1, organic and other material which show anomalously high basal spacings (18-20 A), and (iii) their occurrence as interstratified phases in well-ordered crystals resulting from the transformation of original mica and chlorite crystals. The absence of these soil smectites, their interstratifications and complexes is a result of either being overlooked (or not looked for) or being altered beyond recognition during diagenesis. Are these smectite complexes with anomalously high basal spacings hidden in the XRD heap which frequently represents the 001 spacing of smectite?

Clay diagenesis in sandstones and shales: an introduction 129

My own experience in examining (by transmission electron microscopy) the clay crystal morphology of fossil marine sediments suggests that there is, in many instances, a surprisingly high proportion of well-defined crystal faces even in the most detrital of clay formations. Where do the components for this new crystal growth come from ?--perhaps the widespread breakdown of the less stable soil clays and the poorly crystalline and non- crystalline substances.

We know little about the detailed mineralogy and chemistry of the fine-grained detrital load of rivers and its ultimate fate in the brackish and marine realms. It is an area ripe for the well-trained clay mineralogist.

Differential flocculation and settling

The interaction between the surface charge of the particles and the chemistry of the water in which they are suspended is particularly important in controlling the depositional behaviour of fine-grained particulate mineral and amorphous matter. Water of low cation concentration will allow fine-grained material to remain dispersed and to settle out in relation to their effective settling diameters. Some minerals tend to be particularly fine-grained (smectites, for example) and may settle much more slowly than mica, chlorite or quartz. The particles remain dispersed in water of low cation concentration because the charge distribution around each particle is such that when any two particles come into close proximity they will repel each other. With increasing cation concentration (divalent cations are more effective than monovalent) this charge distribution pattern is modified and individual particles can approach each other more closely and allow the forces of mass attraction to come into play, with the formation of multi-particle associations (floccules). Each mineral and colloid has its characteristic range of surface charge density, and therefore there will be a tendency for like-particles, or different particle types but with similar surface- charge densities, to form floccules. If a river carries in suspension a fine-grained detrital load through increasingly cation-rich zones (freshwater to marine transition), differential flocculation will occur causing the separation of particles flocculating at low cation concentrations from those flocculating at high concentrations. Floccules have much greater effective settling diameters than the particles from which they are made. Certain minerals form relatively dense, tight floccules of relatively high bulk specific gravity (e.g. mica, chlorite, kaolinite) whereas others (e.g. smectite) form loose floccules of low bulk specific gravity which are easily torn apart in turbulent water. The combination of differential flocculation and settling of an assemblage of mixed mineral and amorphous particles may cause a segregation into well-defined chemical and mineralogical zones.

The flocculation behaviour of any particular mineral particle depends on its surface charge density--i.e, the ratio between the total surface charge and the surface area of the particle. The source of the surface charge is twofold: either the adsorption of ions from the surrounding electrolyte solutions or the imbalance of charge within the particle. The preferential adsorption of cations or anions from electrolyte solutions will vary with the electrolyte concentration, and the resulting surface charge will vary accordingly. In contrast, the surface charge resulting from imbalances within the particle will be constant and independent of the solution in which the particle is suspended. The resulting surface-charge density of any particular particle depends on the interplay of these independent factors.

The fine-grained components of soils (Table 1) are divided into (i) variable-charge colloids and clays of low activity (CEC <24 mEq/100 g clay) and (ii) high-activity clays (>24 mEq/100 g clay). The former group is characterized by variable surface charge--i.e, the

130 C. V. Jeans

TABLE 1. Distribution of variable and constant surface charge in common soil components. Most of the least stable components are characterized by

variable surface charges.

Variable surface charge Constant surface charge

Al-intedayered chlorite Kaolinite Halloysite Imogolite Allophane Hydrous Al-oxides Hydrous Fe-oxides Hydrous Mn-oxides Humified organic matter

(complex formation with AI, Fe)

Mica (illite) Vermiculite Smectite Chlorite Sepiolite/palygorskite(?)

effects of ion adsorption from the surrounding electrolyte solutions dominate any surface charge derived from imbalances within. In contrast, the latter group does not show this charge variability as the internal charge imbalances dominate any effects of surface adsorption. Whitehouse et al. (1960) provide a general understanding of the effects of differential flocculation and settling of kaolin and the constant surface charge clays (montmorillonite, illite, mixed-layer montmorillonite/illite, vermiculite and chlorite) under laboratory conditions. There is no equivalent information available for the variable-charge species although their physics and chemistry and that of the soils in which they occur have received considerable attention from soil scientists (e.g. Theng, 1980; Uehara & Gillman, 1981; Barrow, 1987).

Fossil sediments

What role has been played by differential flocculation and settling in determining the clay mineral patterns observed in sediments? Some clay geologists (Millot, 1967, pp. 217-218, for discussion) accept that it is important, although others (e.g. Gibbs, 1977) suggest that its effectiveness in nature is reduced or lost by organic or metallic coatings. My own experience from the Cretaceous and Jurassic sediments of on-shore UK (Jeans, 1986, p. 431) indicates that on the large scale it is the most important factor in developing the clay mineral patterns observed in these sediments. However, when detailed comparison is made between (i) the inferred physico-chemistry and depositional energy of the environment and (ii) the actual clay mineral assemblages, the effects of differential settling are much less obvious. The alternating arenaceous/argillaceous facies is an ideal testing ground. If a consistent detrital source is assumed there should be marked differences in the clay mineralogy of adjacent sandstone and mudstone units as long as there has been relatively little interchange between the pore-fluids of the two lithologies. There are considerable differences in mineralogy but they are restricted in general to the inferred detrital clay component, whereas the neoformed component of the assemblages tend to be similar. Does this mean that in routine clay analysis less obvious differences are being overlooked--for example, variations in mineral chemistry and the abundance of poorly crystalline substances---or does differential flocculation and settling play only an insignificant role in segregating the variable-charge soil components from the constant-charge clay minerals?

Clay diagenesis in sandstones and shales: an introduction 131

The best situation to study this problem is in sediments where individual lithologies have stewed in their own pore-fluids. Differences in the neoformed clay assemblages associated with the individual units in the alternating arenaceous/argillaceous facies are not uncommon, but have usually resulted from the invasion by allochthonous porewaters into the coarser, more porous lithologies. Fig. 1 shows patterns of neoformed clay mineral assemblages which might be associated with a single sandstone-mudstone couplet.

C O N T R I B U T I O N S TO T H I S S P E C I A L I S S U E

The papers in this issue of Clay Minerals divide into three groups. The main group (11 papers) deals with the chemistry, stability and timing of clay mineral neoformation in various hydrocarbon reservoirs and mudstones. The second group (three papers) explores various facets of the composition of mudstones and how this may influence the chemistry of porewaters of the adjacent sediments. The third group (two papers) deals with the range of mineral and chemical reactions associated with the experimental chemical enhancement of the poroperm characters of illite-cemented sandstones,

Illite (or mica as I prefer to call it) is the most widespread neoformed clay mineral in North Sea reservoirs. According to its crystal shape, thickness and rigidity, it may or may not be particularly troublesome during field development and production.

The troublesome variety is very thin, exceptionally elongated and flexible, and is known familiarly as 'fibrous illite'; in fact the crystals are ribbon-shaped, not fibrous. Electron microscopy suggests frequently that more than one crystal morphology is present in any reservoir. Often there is an earlier phase of more robust equidimensional crystals, which is overgrown or changes its growth form to a ribbon-shaped morphology. It is frequently unclear whether these robust earlier crystals are no more than matted accumulations of ribbon-shaped crystals. What factors control this change in morphology? Is it a change in the chemistry of the porewaters which should be reflected in the crystal-chemistry of the illite crystals? Or is some change in the physical conditions of the pore solutions--pressure, temperature, ion diffusion pattern, flow rate--responsible? Warren & Curtis have tackled the chemical aspects of this problem in the Upper Carboniferous reservoir of the Bothamsall Oilfield (East Midlands, UK) and in examples (unlocated) of Rotliegendes reservoirs from the North Sea. Using the analytical transmission electron microscope they have demonstrated systematic differences in the chemistry of the illites from the two reservoir types, but there is no evidence in the Bothamsall reservoir of there being chemical differences related to crystal morphology or whether the illite was replacive, pore-lining or pore-filling. This suggests that the illite of each reservoir precipitated in equilibrium with the porewaters. A similar state of equilibrium is described by Jahren & Aagaard in iUites and chlorites from a number of Norwegian Jurassic reservoirs with present-day temperatures and depths ranging from 70~176 and 1600--4400 m: the chlorite shows composition trends related to temperature in which the AI Iv increases and the octahedral vacancy decreases with increasing temperature.

Illite development in mudstones is another aspect of the illite cement problem. Within this low-permeability lithology with limited porewater circulation we must expect much greater heterogeneities in porewater composition and the availability of suitable parental ions for crystal growth than in highly porous sandstone reservoirs. These will be responsible for considerable variations in chemistry and morphology of crystal growth, as well as in the timing and temperature of reaction. It is no surprise that the investigation by Freed & Peacor

132

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C. V. Jeans

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FIG. 1. Hypothetical patterns of neoformed clay assemblages associated with a sandstone- mudstone couplet from the alternating arenaceous/argillaceous facies. The sandstone and mudstone units represent the maximum effects of differential flocculation and settling in a particular depositional environment. These patterns of clay assemblages result from: (A) Increasing relative compaction of the mudstone with pore-fluid movement into the sandstone unit. The sandstone's porewaters are displaced to regions of lower pore-fluid pressure. The pore solutions of the mudstone and sandstone units are assumed to have been exactly similar at deposition but, subsequently, they have been differentiated by reaction with their host sediments.

(B) Effects of pore-fluids, completely allochthonous to the couplet, entering the sandstone unit at the various compaction stages shown in (A).

Clay diagenesis in sandstones and shales: an introduction 133

of the predominantly argillaceous Tertiary sediments from a number of Gulf Coast wells shows considerable differences in temperature and reaction rate for the smectite -* illite 'transformation'. Similarly, an account of the Devonian continental red beds (Old Red Sandstone) of northern Scotland by Hillier & Clayton demonstrates that there is little consistent relationship between the extent of the smectite --, illite reaction and the temperature determined by spore colouration or vitrinite reflectance. If we want reliable temperature measurements for diagenesis, organic methods are proving much more reliable than those involving mineralogical reactions. The findings of Hansen & Lindgreen in their XRD and TEM study (supported by computer simulations) of the mixed-layer clay diagenesis in Upper Jurassic mudstones from on-shore and off-shore Denmark provide additional evidence of these limitations.

The illite crystals making up these cements incorporate K in their structure and can be dated radiometrically by the K-Ar method. Their minute dimensions mean that problems of contamination and of recrystallization (re-equilibriation) within porewaters changing in chemistry and physical conditions with time are important and that great care has to be taken with sample preparation and interpretation. These and other limitations of the K-Ar method are discussed quantitatively by Hamilton, Kelly & Fallick: anybody who makes use of radiometric dates in diagenetic studies should become familiar with their strengths and shortcomings. It is a salutary experience to appreciate the difficulties that Ehrenberg & Nadeau were confronted with when they attempted to use K-Ar dates to provide a time framework for the illite cements of the Middle Jurassic Garn Formation (Haltenbanken Area, mid-Norwegian Shelf) and to determine their relationship with the grade of metamorphism and hydrocarbon emplacement. Glasmann and his colleagues have used a combination of isotope and fluid inclusion studies and K-Ar dating to establish a complex diagenetic history for the notoriously complex Heather Field (UK, North Sea) in the Middle Jurassic shallow-water Brent Group which involves several major changes in porewater chemistry. Burley & Flisch have made use of radioisotope K-Ar dating in comparing the timing of hydrocarbon emplacement in relation to illite cement precipitation and the development of illite in the adjacent mudstones of the Upper Jurassic reservoirs of the Piper and Tartan Fields (UK, North Sea).

A question raised in Ehrenberg & Nadeau's paper on the Garn Formation concerns the widely used rule that the introduction of hydrocarbons into sediment brings mineral diagenesis to an end--unless the hydrocarbons are subsequently replaced by connate waters. These authors suggest that mineral reactions are not necessarily brought to a halt particularly if the degree of water saturation is high. In the literature there are many reports of marked differences between diagenesis from the oil and water zones. It is my own suspicion that some of these will turn out to be facies-controlled diagenesis and have little to do with oil-water contacts. Other examples, including the Tartan, Piper and Heather Fields discussed in this issue, appear to involve real differences. In these three oilfields the evidence is based solely on K-Ar dating, and it is rather surprising that there is no evidence of the modification of cement textures at the oil-water contact or transition. Ehrenberg & Nadeau's doubts are probably valid: it is likely that some degree of mineral diagenesis can proceed when there is partial water saturation; however, they provide no petrographic evidence of this form of mineral growth. My own experience in the North Sea and in the literature suggests there are very few instances where such evidence has been observed or published.

The field studies so far discussed are characterized by diagenetic features of the non- intrinsic type, and it appears that intrinsic diagenesis has played only a minor role in

134 C. V. Jeans

developing the reservoir qualities of importance for production. Two papers deal with reservoirs where facies-controlled diagenesis is very important. The Upper Jurassic non- marine/marine reservoir in Quadrant 13 described by Jeans & Atherton shows evidence of extensive over-pressuring during its burial, and that the different sediment types and facies stewed in their own juices. Diagenesis is strongly facies-controlled and any effects of varying maximum temperature and depth of burial are minimal when compared to the facies imprint. The Brent sandstones of the NW Hutton Field are demonstrated by Scotchman, Johnes & Miller to show a strong facies control in their diagenetic patterns which has subsequently been modified by the effects of enhanced temperatures, increased grain pressure and the invasion of allochthonous pore-fluids, all associated with the more deeply buried parts of the reservoir.

Well-bore collapse, formation damage, restricted log interpretation and deleterious effects on adjacent or interbedded sandstones are some of the difficulties facing the oil explorationist when they have to deal with mudstones in the sub-surface. If these difficulties could be overcome, considerable advances would be forthcoming. However, these problems have proved to be rather intangible. For example, at the 1984 Cambridge Meeting, Hall, Astill & McConnell (1986) presented experimental evidence that cleared the main suspect for causing overpressures and subsequent well-bore collapse--the dehydration reactions of smectite minerals during burial. In this present issue three papers consider various aspects of mudstones. Fletcher & Sposito examine how the complex ion-exchange reaction of Wyoming bentonite can be modelled in order to obtain thermodynamic data for its reactivity with a variety of cations. Jones, Hughes & Tomkins describe how a North Sea smectite-rich shale formation of Cretaceous age has acted as an ion-exchange membrane controlling both the expulsion of water during compaction and the chemistry of the porewaters of the formation into which the compactional waters are being expelled. Skipper, Refson & McConnell explore the interaction between water molecules and talc using atomic pair potentials, the start of a series of investigations using computer modelling into the understanding of clay- water-cation systems, such as may occur in the exchange of water and cations between drilling fluid and the shale rocks of a well bore. Although to the practising geologist these investigations may seem esoteric, they are an essential stepping stone in furthering the ability to avoid problems when drilling mudstones and for realizing the full potential of log interpretation. For example, it is a long established fact that the porewaters of freshwater, brackish water and marine shales are chemically distinct (Kiihnel, 1963; Spears, 1973, 1974), and there is no reason why suitable logging tools cannot distinguish between them if the ion- exchange between the drilling fluids and the formational waters can be avoided or fully understood so that the primary chemical pattern can be recognized.

The need for detailed crystallo-chemical characterization of clay cements in production geology will always be present. How will the clay react to the chemical used for porosity enhancement or to changes in porewater chemistry? Will the reservoir quality be improved or reduced? There are two stages in this clay characterization. The first is to recognize any qualitative and quantitative differences in clay mineralogy within the reservoir and how these vary in relation to facies, depth etc. The second stage is to test how the various clay assemblages react both in the laboratory and in the reservoir to the various chemical treatments planned for porosity enhancement. Two papers deal with aspects of this process. Humphreys, Smith & Strong identify two phases of chlorite cementation in the Upper Triassic sediments of Quadrant 22; one type is limited to the marginal marine facies, whereas the other occurs throughout both the marginal marine and continental fluvial facies. Hughes,

Clay diagenesis in sandstones and shales: an introduction 135

Davey & Curtis describe experiments in which a variety of illite cements are reacted in the laboratory in closed and open systems at 80~ with different chemical agents (distilled water, hydrochloric acid, sodium hydroxide, citric acid and hydroxylamine hydrochloride).

C O N C L U S I O N S

Relatively little research is aimed at the mineralogical inter-relationships exhibited by alternations of sandstone and mudstone. Major obstacles will have to be overcome before progress (of use in petroleum geology) can be achieved. We must know more about the flocculation behaviour of clay minerals and other fine-grained and colloidal components of soils in the changing physico-chemical conditions of the freshwater, brackish water and marine environments. Attention should be focused on (i) how individual and mixed assemblages of soil components react and interact under different conditions, (ii) what patterns of mineral and chemical zonations will result from this differential flocculation and subsequent settling, and (iii) what happens to this detrital pattern during diagenesis--which components will continue unchanged into the fossil sediment, which ones are only temporary residents to be consumed or modified in the complex of chemical and mineralogical reactions between the porewaters and the freshly deposited detritus. Recent advances in the understanding of the formation of green Fe-rich clay minerals in modern marine and brackish-marine environments (Odin, 1988; Bailey, 1988) provide a flavour of the findings to come. It would be no surprise if these early reactions were proved to be at least as varied and complex as those which are known to take place in soils.

Once a fuller knowledge of the effects of differential flocculation and settling (on the fine- grained components of soils) is known the clay-mineral-geochemical budget of a few well- graded river systems should be comprehensively investigated. Such river systems and their depositional areas should be small and, as far as possible, restricted to the continental shelf; they should exhibit a full range of environments from the freshwater to the fully marine, and the soil patterns, chemistry and mineralogy of their detritus source areas should be well known. The rivers should be chosen to allow the effects of similar source materials and different rates and styles of weathering and soil development to be compared.

In the 1960s and 1970s many of the major oil companies with their powerful technical and financial resources turned their attention, with considerable success, to a fuller understand- ing of limestone facies. The present time is appropriate for a similar concentration of effort, skill and resources onto the alternating arenaceous/argillaceous facies. The long-term reward will be a greatly enhanced understanding both from an exploration and production viewpoint of clay diagenesis and of this particular facies in which a considerable proportion of the world's oil reserves are located.

ACKNOWLEDGMENTS

I wish to thank Dr B. W. Bathe (Cambridge University), Dr P. F. Rawson (University College, London) and Dr M. J. Wilson (Macaulay Land Use Research Institute) for discussion and criticism.

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136 C . V . Jeans

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