Clay Minerals (1986) 21,603-616

A N SEM S T U D Y OF P H Y L L O S I L I C A T E D I A G E N E S I S IN S A N D S T O N E S A N D M U D S T O N E S

IN THE W E S T P H A L I A N C O A L M E A S U R E S U S I N G B A C K - S C A T T E R E D E L E C T R O N

M I C R O S C O P Y

J . M. H U G G E T T

BP Petroleum Development Limited, Britannic House, Moor Lane, London EC2 Y 9BU

(Received 18 July 1985; revised 31 July 1985)

A B S T R A C T : Back-scattered electron imaging was used in conjunction with energy-dispersive spectral analysis to study phyllosilicate minerals in polished thin-sections of Carboniferous (Westphalian) sandstones and associated mudstones from the East Midlands and from the diageneticaUy higher grade area of Abernant in the South Wales coalfield. Chemical diagenesis proceeded more rapidly in the sandstones than in the mudstones. In the East Midlands, greater flow of freshwater through the sandstones than the mudstones produced a higher degree of alteration of muscovite to kaolinite in the sandstones. Chlorite is present in the Well 1 and Well 2 East Midlands sandstones but not the mudstones nor any Well 3 samples. The chlorite may be an early diagenetic replacement of biotite. Much chlorite has been replaced by kaolinite. In the Abernant samples chlorite is present in both the sandstones and mudstones. In the former it appears to have replaced detrital biotite. In the latter the chlorite is too fine-grained to discern whether it is detrital or authigenic. The chlorite may have formed during the Hercynian orogeny. Much of the mica and chlorite in the Abernant sandstones has been replaced by illite, possibly in the post-orogenic period. The scarcity of kaolinite in the Aberr.ant samples reflects the lack of freshwater leaching.

This paper compares phyllosilicate diagenesis in interbedded Westphal ian mudstones and sandstones, from a range of diagenetic grades, using back-scat tered electron imaging (BEI) of carbon-coated, polished thin-sections in an SEM with a semi-quantitative EDS analysis facility. X R D was used for phyllosilicate identifications. Use of polished specimens overcomes the problems of artefacts in the characterist ic spectra encountered with X- ray analyses obtained by fracture-surface SEM studies. Analyses can be obtained from grains of 2/~m diameter or greater. In BEI mode, areas of different mean atomic number produce the image contrast. Thus minerals can be identified by calibrating the grey tones in the image against known samples (e.g. quartz) and by the use of characterist ic morphologies.

The opt imum magnification of the Camscan Series 4 microscope in BEI mode is from x40 to x5000, which facilitates petrographic studies of mudstones as well as sandstones and therefore their direct comparison. The combinat ion of the imaging resolution o f an SEM, the image contras t of BEI and the semi-quantitative analytical facilities is consequently a very useful tool for detailed studies of diagenetic t ransformations of phyllosilicate minerals.

�9 1986 The Mineralogical Society

604 J. M. Huggett

S A M P L I N G

Samples were obtained from three BP wells in the East Midlands and from an NCB borehole in the anthracite zone of the South Wales Coalfield at Abernant, Mid-Glamorgan. The East Midlands samples were from depths of ~700 m in Well 1,900 mSn Well 2 and 1500 m in Well 3. Whilst not maximum burial depths, the amount of uplift is unlikely to have varied significantly between wells. Well 3 samples may therefore be assumed to have been the most deeply buried, and those from Well 1 the least buried. The Abernant samples were obtained from a present-day depth of ~750 m, though maximum burial was probably much greater. The burial history is complicated in South Wales by folding and faulting during the Hercynian orogeny. Clearly burial depth is an inappropriate measure of maximum diagenetic grade. Vitrinite reflectance was therefore used as this is a guide to thermal maturity. Vitrinite reflectance values for the East Midlands are low--R 0 max% = 0.41-0.66 (personal communication, P. M. Curran, BP Research Centre, Sunbury- on-Thames) in the vicinity of the three wells. This range of values corresponds to a low diagenetic grade. Vitrinite reflectance values from the Abernant Colliery indicate R 0 max% = 2.32 (Gill et aL, 1979), which corresponds to a high diagenetic grade.

The approximate thicknesses of the sampled sandstones and mudstones, and the sample depths, are shown in Table 1.

R E S U L T S

Petrography

The detrital mineralogies of the sandstones do not vary significantly between the four locations. The sandstones are all sub-lithic quartz-arenites. The detrital mineralogy consists predominantly of quartz with subordinate feldspar, minor muscovite, chlorite, iUite and rare biotite. Authigenic minerals are quartz overgrowths, kaolinite, chlorite, siderite, calcite, ankerite, ferroan dolomite and minor pyrite. Authigenic kaolinite is only abundant in the East Midlhnds samples and authigenic chlorite only in the Abernant samples. Two carbonates only (usually siderite and ferroan dolomite) are generally present. Some pressure solution has occurred, particularly in the Abernant sandstones. Detailed descriptions of the petrography and diagenesis of other similar Westphalian sandstones from the East Midlands are given by Huggett (1982, 1984).

The mudstones from the East Midlands differ slightly from those from Abernant (Table 2), the former containing abundant silt-grade quartz, feldspar and muscovite whilst the latter consist almost entirely of clay with minor silt-grade muscovite and chlorite. Minor kaolinite was identified by XRD in all the mudstones, though it was not observed with SEM in the Abernant mudstones.

In Well 3 both the sandstones and mudstones contain detrital muscovite, illite and authigenic kaolinite. In Wells 1 and 2 the sandstones contain chlorite, muscovite, illite and authigenic kaolinite, but the mudstones contain no chlorite. In the Abernant borehole muscovite, illite and chlorite are present in both the sandstones and mudstones.

Phyllosilicate diagenesis

Phyllosilicates identified by EDS with the SEM were chlorite, muscovite, biotite, illite and kaolinite (Table 2). Over 150 analyses were obtained. The analyses quoted in the text

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TABLE 2. Phyllosilicates present in the sandstones and mudstones of Wells I-3 and the Abernant borehole. (d) = detrital; (a) = authigenic.

Sandstones Mudstones

Well 1 muscovite (d) muscovite (d) chlorite (d) rare kaolinite (a?) kaolinite (a) illite (d) minor iUite (d)

Well 2 muscovite (d) muscovite (d) chlorite (d) rare kaolinite (a ?) rare biotite (d) illite (d) kaolinite minor iUite (d)

Well 3 muscovite (d) muscovite (d) kaolinite (a) rare kaolinite (a?) minor illite (d) illite (d)

Abernant borehole muscovite (d) muscovite (d) chlorite (d) chlorite (d) illite (a + d) illite (d)

have been calculated as oxides normalized to 100%. The analyses are listed in Tables 3 -6 and the points at which they were taken are shown in Figs 1-10. Because these are only semi-quantitative analyses, the data have been used to demonstrate the general nature of the diagenetic reactions rather than to calculate specific reactions for individual phyllosilicate grains as in a previous study (White et al., 1985). Slightly high SiO 2 values are probably artefacts caused by adjacent quartz grains.

The analyses are presented as A F K plots (Fig. 12) and have also been plotted by the method of Velde (1977) with the co-ordinates MR3-2R3-3R 2 (Fig. 11). The calculations involved in producing these diagrams are:

M R 3 = N a + + K + + (2Ca 2+)

2R 3 = (A13+ + Fe 5+ _ MR3)/2

3R 2 = (Mg 2+ + FEZ+)/3

The M R a pole represents the bulk composit ion of feldspars, these being the most alkali-rich minerals associated with clay minerals; kaolinite and other pure alkali-free dioctahedral phases plot at the 2R 3 pole. Talc and other pure alkali-free trioctahedral phases plot at the 3R 2 pole.

Illite as a detrital clay is present in all samples. As it does not appear to have participated in the diagenetic reactions it is not discussed.

East Midlands samples

Well 1. The mudstone samples contain mostly illite with lesser amounts of silt and clay-grade muscovite, quartz and authigenic pyrite (Fig. 1). The muscovite appears fresh,

S E M phyllosilicate diagenesis in sandstones and mudstones

TABLE 3. Semi-quantitative EDS analyses of phyllosilicates from Well 1.

607

693.29/1 693.29/2 700.10/1 700.10/2 700.10/3 700.10/4 700.10/5 Analysis (Fig. 1) (Fig. 2)

sio 2 54.79 55.51 56.16 50.62 56.23 50.42 52.30 TiO 2 0.73 0.57 0.35 0.06 0.10 0.00 0.02 A1203 30.00 30.56 31.00 45.71 30.89 45.87 37.24 Fe203/FeO 5.10 2.68 2.32 1.15 2.20 0.78 2.08 MnO 0.00 0.02 0.03 0.00 0.07 0.01 0-00 MgO 0.36 1.14 1.24 0.00 0.53 0.23 3-01 CaO 0.06 0.00 0.02 0.00 0.01 0.02 0.27 Na20 0.00 0.05 0-00 0-04 0-01 0-09 0-13 KzO 8.96 8.47 8.88 2-42 9-96 0.78 3.95

i.e. the layers are not splayed apart, but analyses indicate that it is very slightly K-depleted (see Table 3, analyses 693.29/1 and 693.29/2) compared with values of 10-11% for fresh muscovite (Deer et al., 1966). No chlorite is present in the mudstone. Minor kaolinite was detected by XRD but it was not identified with the SEM.

PhyUosilicate minerals present in the sandstone samples are detrital muscovite and ferroan chlorite, authigenic kaolinite and minor detrital illite. The muscovite and chlorite grains are up to 500 gm long. No analyses of fresh muscovite or chlorite were obtained, and none of the grains were of fresh appearance.

All muscovite analyses indicate K-depletion (e.g. Table 3, 700.10/1 and 700.10/3). Muscovite is commonly intergrown with kaolinite and associated with pore-filling books of kaolinite. Analyses of the kaolinite intergrowths in Fig. 2 (Table 3,700.10/2 and 700.10/5) indicate some contamination, probably due to fine intergrowths of muscovite which are too narrow to be resolved with the SEM, occurring within bands of kaolinite. Analysis of the kaolinite books in Fig. 2 (Table 3, 700.10/4) indicates that they are almost 'pure' kaolinite. The two morphological forms of kaolinite may be due to two distinct mechanisms of formation. The kaolinite intergrowths may be in situ replacements and the kaolinite books may have precipitated from solution.

Chlorite grains also have kaolinite intergrowths. Some grains show patchy grey and white contrast probably due to variation in iron content.

Well 2. The muscovite in the mudstone samples appears fresh but is very depleted in K (K~O mostly 3-5%). Variation in iron content is reflected in the range of grey tones shown by the muscovite (Fig. 3). Some muscovite grains show faint tonal banding, which with their very low K20 values (Table 4, analysis 920.60/2) suggests fine-scale kaolinite intergrowths; these may correspond to traces o f kaolinite detected by XRD. No chlorite was detected in the mudstone samples.

In the sandstones, grains of detrital muscovite and ferroan chlorite occur commonly with kaolinite intergrowths (Figs 4 and 5). None of the analyses of the phyllosilicates in Fig. 4 is of a pure phase. The light grey areas have a composition which is probably leached biotite (Table 4, analyses 924.40/3 and 924.40/4), and the intermediate grey areas have a composition which is probably leached muscovite, perhaps contaminated with kaolinite. The biotite has less K than the muscovite suggesting preferential leaching of the biotite interlayer cations. The biotite also has very high SiO 2 values, probably due to 'contamination' by adjacent quartz grains. The dark grey areas infilling the areas between

608 J. M. Huggett

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SEM phyllosilicate diagenesis in sandstones and mudstones 609

and beyond the muscovite fragments are probably kaolinite with muscovite contamination, either from the visible fragments or from fine-scale intergrowths. (This example indicates some of the uncertainties which arise when attempting to identify fine-grained mixtures of phases and to recognize incomplete reactions.)

Fig. 5 shows an iron-rich chlorite (Table 4, analyses 924.36/2 and 924.36/3) intergrown with kaolinite (Table 4, analyses 924.36/1 and 924.36/4). The mottled appearance of the chlorite may reflect variable iron content. The minor K content of both chlorite and kaolinite may be due to a few mica layers or to contamination from adjacent illite. In this example very thin remnants of chlorite are visible intercalated with the replacive kaolinite.

Well 3. In the mudstone samples there is abundant silt-grade muscovite, but no chlorite was identified. In a few grains, intergrowths of more than one phase, e.g. analyses 1475.07/1 and 1475.07/2 (Table 5 and Fig. 6), indicate muscovite and kaolinite. Analyses of grains without visible intergrowths from the same sample (1475.07/3-1475.07/6) indicate very similar compositions. These grains may also be intergrowths of kaolinite and muscovite but too fine to be resolved with the SEM used.

In the sandstone samples fresh muscovite is very rare and no chlorite was detected. The muscovite is generally depleted in K and has common broad intergrowths of kaolinite up to 20 #m wide. However, some grains have been protected from leaching pore-fluids by detrital illite (Fig. 7), have very few kaolinite intergrowths, very little loss of K (Table 5, analysis 1479.3 / 1), or physical separation of the phyllosilicate 1 ayers.

Abernant borehole

The mudstones contain less silt-grade material than the East Midlands mudstones and also differ in having some silt-grade ferroan chlorite in addition to muscovite and illite. No intergrowths were visible in the phyllosilicates (Fig. 8), but may be beyond the limit of detection. The analyses indicate a moderate Fe and Ti content, and low K (Table 6, 746/1-746/5).

In the sandstone samples, both muscovite and chlorite are present. The chlorite sporadically appears fresh (Fig. 9; Table 6, 740/5). However, both chlorite and muscovite grains are extensively replaced by illite (Fig. 10). From the analyses it is generally uncertain whether biotite or chlorite has been replaced by illite. Relatively large Ti contents suggest

FIG. 1. Well 1 mudstone. Detrital muscovite (analyses 1 and 2) and quartz in a matrix of illite. Bright white 'grains' are authigenic pyrite framboids.

FIG. 2. Well 1 sandstone. Muscovite (analyses 1 and 3) with intergrowths of kaolinite (analyses 2 and 5). Kaolinite books (analysis 4) have partially filled the pore space. The bright white

rhombs with the muscovite/kaolinite intergrowth are siderite. FIG. 3. Well 2 mudstone. The range of grey tones shown by the muscovite grains (analyses 1-6) reflects the variation in iron content. The other grains are quartz and the fine-grained white

mineral is ankerite. FIG. 4. Well 2 sandstone. Muscovite intergrown with kaolinite (analyses 1, 2 and 5). Possible

leached biotite (analyses 3 and 4). The bright white fine-grained mineral is siderite. FIG. 5. Well 2 sandstone. Iron-rich chlorite (analyses 2 and 3) intergrown with kaolinite

(analyses 1 and 4). FIG. 6. Well 3 mudstone. Silt grade 'muscovite' and quartz in an illite matrix. The muscovite appears to have fine-scale kaolinite intergrowths which are not always resolved (analyses 1-6).

The bright white mineral is pyrite.

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SEM phyllosilicate diagenesis in sandstones and mudstones 611

FIG. 7. Well 3 sandstone. Detrital muscovite (analysis 1) protected from leaching pore-fluids by detrital illite (analysis 3). Only minor alteration to kaolinite has occurred at the grain

terminations and in the smaller muscovite fragment (analysis 2). FIG. 8. Abernant borehole mudstone. Less silt-grade material is present than in the mudstones

from Wells 1-3. The pale elongate grains (e.g. analysis 3) are mostly chlorite. FIG. 9. Abernant borehole sandstone. Apparently fresh chlorite. Analysis 5 implies that it is either partially altered to illite, or the analysis is 'contaminated' by the mass of illite to the left of

the chlorite grain. FIG. 10. Abernant borehole sandstone. Chlorite or biotite (analyses 2 and 4) being replaced by illite (analyses 1 and 3). The relatively high Ti values suggest that biotite is more likely to be the

parent mineral.

tha t the or iginal phyl los i l icate in Fig. 10 was biot i te (Table 6, ana lyses 740/1-740/4) . X R D results indicate tha t minor kaol ini te is present , a l though it was not observed by SEM.

Discussion of the MR 3-2R 3-3R 2 plots (Fig. 11)

The muds tone ana lyses in Well 1 (Fig. 1 l a ) c luster a round illite, in Well 2 (Fig. 1 l b ) the range t rends slightly t owards kaolini te , and in Well 3 (Fig. 1 lc ) there is a b road spread of ana lyses between illite and kaolinite. This reflects the increase in authigenic kaol ini te with depth. In the A b e r n a n t borehole (Fig. 1 ld ) the range lies between the areas where 'pure ' illite and ' pure ' chlori te would plot, suggest ing tha t the grains ana lysed are in te rgrowths o f illite and chlori te , beyond the resolut ion of the S E M used.

612 J. M. Huggett

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SEM phyllosilicate diagenesis in sandstones and mudstones 613

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The sandstones show a greater spread of analyses than the mudstones because the grains are larger and analyses could more often be obtained from a single phase in the former, rather than from a fine-scale multiphase intergrowth. In the East Midlands wells the spread is also greater because chlorite occurs in the sandstones but not the mudstones. The 'pure' chlorite analyses are generally from relatively large unaltered grains. 'Pure' kaolinite analyses in Fig. 1 l a are from kaolinite books (Fig. 2). There are no pure kaolinites in Fig. 1 lc because all the analyses were of intergrowths. The chlorite analyses plotting midway between 2R 3 and 3R 2 in Fig. 1 ld are from the fresh chlorite shown in Fig. 9. The gradual loss of chlorite with increasing well-depth is shown by the narrowing spread of sandstone phyUosilicate analyses from Wells 1 to 3. This loss of chlorite with depth is due to replacement by kaolinite (Fig. 5).

Discussion of the AFK plots (Fig. 12)

Of the three elements, A1 is most abundant in all analyses except those of a few iron-rich chlorites (Figs 12a,b). In the East Midlands wells, sandstone and mudstone analyses show a strong move away from Fe in the phyUosilicates towards A1, with increase in depth (Figs 12a-c). The proportion of K decreases slightly with depth, reflecting the leaching of micas and their replacement by kaolinite. The sharp decrease in Fe reflects the replacement of biotite and chlorite by kaolinite. In the Abernant borehole there is generally higher Fe and less K than in Wells 1 to 3 (Fig. 12d), reflecting the greater proportion of chlorite and scarcity of muscovite in the Abernant samples.

As in Fig. 11, the sandstones show a greater compositional spread than the mudstones because the grains are larger and analyses were obtained more from a single phase than fine-scale polyphase intergrowths in the mudstones.

C O N C L U S I O N S

This brief study shows that it is possible to make direct petrographic observations of mudstones and thus directly compare mudstone and sandstone diagenesis.

Diagenesis has further diversified initially similar ranges of phyllosilicate types in interbedded sandstones and mudstones. In the sandstones, chemical diagenesis has proceeded more rapidly than in the mudstones due to higher porosities and permeabilities and consequent greater pore-fluid flow.

In the East Midlands wells, greater flow of freshwater through the sandstones than the mudstones had produced a higher degree of alteration of muscovite and chlorite to kaolinite in the sandstones. The two kaolinite morphologies may indicate two mechanisms of formation: the pore-tilling books having precipitated from solution, and the intergrowths with other phyllosilicates being an in situ replacement. The degree of kaolinization increases with well depth, though whether the increased burial is a genuine cause of the reaction is uncertain. The kaolinization may be very early, have occurred at the Permian unconformity, or may be recent. The latter two are unlikely to be the only causes because this would have resulted in a reduced degree of leaching with depth.

The fact that chlorite occurs in the two shallower wells but is absent from the deepest one further suggests that unconformity leaching is not the prime cause of kaolinization. Hence early kaofinite formation seems the most probable mechanism.

SEM phyllosilicate diagenesis in sandstones and mudstones 615

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In the Abernant samples chlorite is present in both the mudstones and sandstones. In the latter it appears to have replaced biotite; in the former it is too fine-grained to discern whether it is detrital or authigenic. It would not be unreasonable to expect chlorite formation during the Hercynian orogeny, when a high diagenetic grade was reached in the Abernant area. However, the possibility of a detrital source cannot be discounted. The illitization of much of the mica and chlorite probably occurred during the post-orogenic phase of uplift and erosion. The scarcity of kaolinite reflects an absence of significant freshwater leaching.

A C K N O W L E D G M E N T S

l am grateful to BP Petroleum Development Limited and the NCB (South Wales) for permission to publish this paper. I particularly wish to thank Mr Steve Dumpleton (NCB) for supplying core material.

R E F E R E N C E S

DEER W.R., HOWIE R.A. & ZtrSSMAN J. (1966) An Introduction to the Rock-Forming Minerals. Longmans, 528 pp.

GILL W.D., KnALAF F.I. & MASSOUD M.S. (1979) Organic matter as indicator of the degree of metamorphism of the Carboniferous rocks in the South Wales Coalfields. J. Pet. GeoL L 39-62.

HUGGETT J.M. (1982) The growth and origin of authigenic clay minerals in sandstones. PhD Thesis, University of London.

1-IUGGETT J.M. (1984) An SEM study of phyllosilicates in a Westphalian Coal Measures sandstone using back-scattered electron imaging and wavelength dispersive spectral analysis. Sediment. Geol. 40, 233-247.

VELDE B. (1977) Clays and Clay Minerals in Natural and Synthetic Systems. Elsevier, Amsterdam, 218 pp. WHITE S.H., HUGGETr J.M. & SHAW H.F. (1985) Electron optical studies of phyllosilicate intergrowths in

sedimentary and metamorphic rocks. Mineral. Mag. 49, 413-423.