Clay Minerals (1995) 30, 27-38

T H E R M O G R A V I M E T R Y / E V O L V E D W A T E R A N A L Y S I S ( T G / E W A ) C O M B I N E D WITH XRD FOR I M P R O V E D

Q U A N T I T A T I V E W H O L E - R O C K A N A L Y S I S OF C L A Y M I N E R A L S IN S A N D S T O N E S

D. M . THORNLEY* AND T. J . PRIMMER

BP Exploration, Research and Engineering Centre, Chertsey Rd, Sunbury-on-Thames, Middx, TW16 7LN, UK

(Received 26 January 1994; revised 8 September 1994)

A B S T R A C T : Current methods of quantitative whole-rock clay mineral analysis of sandstones often provide little more than an estimate of clay mineral abundances, especially where the total clay mineral content is <10 wt% of the sandstone. More accurate determinations of clay mineral abundance in the whole rock can be made by combining thermogravimetry/evolved water analysis (TG/EWA) and X-ray diffraction (XRD) data. The TGA/EWA system incorporates a purpose built thermobalance linked to a water specific infrared detector which is used to measure quantitatively the clay mineral dehydroxylation water evolved from the whole rock when heated from 250~ to 900~ This gives a measure of the total hydroxyl content of the clay minerals in the whole rock which, when combined with XRD analysis of a separated clay size-fraction, enables individual clay mineral abundances in the whole-rock sample to be determined. Results on artificial sand/clay mineral mixtures prepared with known amounts of different clay minerals (chlorite, illite and kaolinite) show that the accuracy of the combined method is most influenced by the accuracy of the XRD data. Errors associated with TG/EWA were found to be negligible by comparison. A case study is included in which the technique has been used to determine accurately the illite abundance in the Magnus Sandstone Reservoir, Northern North Sea.

Clay minerals are common constituents of many hydrocarbon bearing reservoir sandstones and can, where such sandstones are of economic importance, play an important part in determining their productivity. The type, abundance and morphology of individual clay mineral particles and aggregates can all impose a profound effect on the quality of the reservoir and the means by which the volume of petroleum in place is evaluated (Waxman & Smits, 1968; Wilson & Pittman, 1977; Pallatt et al., 1984; W o r t h i n g t o n , 1985). I d e n t i f i c a t i o n o f the morphology and type of clay mineral present in sandstones is a relatively straightforward procedure using conventional scanning electron microscopy (Welton, 1984) and qualitative XRD techniques (Wilson, 1987), respectively. However, it is imperat ive that the pattern of clay mineral

* Present address: Postgraduate Research Institute of Sedimentology, University of Reading, PO Box 227, Whiteknights, Reading, Berks, RG6 2AB, UK

abundance throughout the unit of interest is also understood if reservoir performance is to be appraised in accurate and precise terms.

Unfortunately, no robust and reliable methods of clay mineral quantification in sandstones have ever been established. To date, whole-rock XRD has been the most widely used laboratory technique. However, the shortcomings in this technique were highlighted long ago. Pierce & Siegel (1969) concluded that until unified methodologies for sample preparation and treatment, peak selection and weighting factors were established, accurate quantification would be unattainable even though precision in any one combination of methods may be good. Despite this warning, little has changed over the last 20 years and whole-rock XRD methods still give at best only semi-quantitative estimates of absolute abundance, especially for sandstones in which clay mineral contents are often fairly low (<10-15 wt%). In order to circumvent the problem of low abundance, XRD

�9 1995 The Mineralogical Society

28 D. M. Thornley and T. J. Primmer

TABLE 1. Compositions of synthetic sandstone mixtures containing different amounts (in wt%) of clay minerals.

Mixture Illite Kaolinite Chlorite 'A' Chlorite 'B' Quartz

A 3.5 5.4 1.3 - 89.8 B 5.4 4.3 - - 90.3 C 1,4 8.3 - - 90.3 D 6.0 2.3 - 1.8 89.9 E 2.3 4.7 - 1.8 91.2

Chlorite 'A': CMS Special Clay CCa-1 Chlorite 'B': separated from Tuscatoosa Sandstone, Louisiana, USA

techniques have also been used to determine the relative proportions of clay mineral species contained within a concentrated clay size-fraction (e.g. <5 gm) extracted from the bulk rock. By concentrating the clay minerals thus and preparing an orientated smear mount, quantification of the relative proportions of different clay minerals can be made easier by exploiting the dominant 001 reflections and ignoring reflections due to non-clay minerals (Wilson, 1987). However, this form of XRD analysis will only give an indication of which clay minerals are more common than others in the bulk rock and it is important to remember that the actual abundance of each clay mineral in the original bulk rock sample is not measured or determined. It is not the purpose of this paper to discuss in any depth the merits of different whole- rock or clay fraction XRD techniques.

Thermal techniques have also been used in the quantitative analysis of clay minerals. Johnson et al. (1985) used weight loss and gain from thermo- gravimetry as a part of a complex multicomponent/ matrix inversion technique involving many analy- tical techniques for the quantitative mineralogical analysis of soils. Bloodworth et al. (1990) showed how evolved water analysis could be used to determine the kaolini te content of artificial mixtures of quartz and kaolinite and some sandstones. Reproducible analyses of kaolinite to levels as low as 0.25 wt% could be made using this technique. Unfortunately, mixtures of different clay minerals in sandstones cannot be resolved by thermal methods alone (Paterson & Swaffield, 1987).

This study describes how data obtained from standard XRD analysis of a separated clay size- fraction, along with data from TG/EWA of the corresponding whole rock, may be combined to

make quantitative determination of individual clay minerals in the whole-rock sample possible. To demonstrate the accuracy of the combined technique, a short programme of analyses of five different artificial clay mineral/sand mixtures was made. The mixtures were prepared from kaolinite, illite and chlorite reference clay minerals, mixed with known proportions of quartz sand, to give clay mineral concentrations typical of those found in reservoir sandstones. A case study conducted on sandstone core samples from the Magnus Reservoir, Northern North Sea is included to demonstrate the application of quantitative clay mineral analysis.

M A T E R I A L S A N D M E T H O D S

Sample p r e p a r a t i o n

Clay minerals for the artificial sand/clay mixtures were~-prepared from reference clay samples KGa-2 (kaolinite), IMt-1 (illite) and CCa-1 (chlorite) obtained from the Clay Minerals Society Source Repository (University of Missouri, USA). An additional chlorite sample (Tu-31) was extracted from a sample of Cretaceous Tuscaloosa Sandstone from the Louisiana Gulf Coast, USA. Samples of IMt-1, CCa-1 and Tu-31 were crushed, ultrasoni- cally disaggregated and dispersed in deionized water whereas KGa-2 was dispersed directly in deionized water. A <5 Ixm equivalent spherical diameter size-fraction was then separated by repeated centrifugation to render a pure clay mineral fraction. This was then concentrated by filtering through a 0.45 ~m membrane filter and dried at 105~ for 16 h. A small portion of each clay mineral separated was saved for reference and detailed characterization and purity check by XRD and TG/EWA. The pure clay minerals were then

Clay mineral quantification 29

mixed in various proportions and diluted by quartz sand (Fisons acid-washed fine sand, fired at 600~ for 48 h) to a ratio of approximately nine parts sand to one part clay mixture (Table 1) to produce five different artificial sandstone mixtures. Samples were thoroughly homogenized by gentle hand grinding in an agate mortar and pestle and by subsequent end-over-end shaking in glass bottles.

XRD analysis

Chung's (1974) adiabatic method, which incor- porates boehmite as an internal reference intensity standard was used for all XRD analysis (Wilson, I987). Prior to analysis of the sand/clay mixtures, reference intensity ratios for each of the pure clay minerals with respect to boehmite were made on orientated air-dried and glycolated specimens of a 50/50 mix of boehmite/clay mineral.

To simulate the analysis of artificial sand/clay mixtures as unknowns, a representative size-fraction

Cahn Balance..~

Dry Nitrogen ~

....--- Fu rnace

amp le

Infra-red Water L [ ~ I YapourAnalyser

Dry Nitrogen .~- . V Evolved Gas

Fro. 1. Schematic illustration of thermogravimetric/ evolved water analysis apparatus.

(<5 ~tm equivalent spherical diameter) of a 5g portion of each mixture was separated by standard dispersion and centrifugation techniques (Hardy & Tucker, 1988). This was then concentrated into a paste by filtering through a Gelman <0.45 gm diameter membrane filter. The paste was then mixed with c. 5 - 1 0 mg of boehmite and smeared onto glass slides (Gibbs, 1965).

Samples were analysed with a Siemens D500 powder diffractometer using Ni-filtered Cu-Ket radiation. Siemens/Socabim DIFFRAC-AT soft- ware was used to integrate and measure peak areas of interest on air-dried and glycolated specimens using the 001 reflection of iIlite, the 002 reflection of kaolinite and the 004 reflection of chlorite with the boehmite as an internal reference intensity standard. Where significant overlap of chlorite 004 and kaolinite 002 reflections occurred, the Siemens/Socabim DIFFRAC-AT software was used to resolve each reflection. Chung's (1974) adiabatic method was then used to quantify the relative proportions of each clay mineral contained in the <5 gm size-fraction extracted from each sand/clay mineral mixture.

Thermogravimetry/evolved water analysis ( TG/EWA )

The technique of TG/EWA involves accurately measuring the amount of water evolved from a sample whilst heating over a given temperature interval. A schematic of the instrument used is shown in Fig. 1. It comprises a purpose-built t he rmoba lance inco rpora t ing a Cahn 1000 Microbalance which records continuously the sample weight loss. Samples were heated using a Stanton Redcroft temperature programmer and ;'80 series water-cooled furnace. An ADC-225-Mk3 specific infrared water vapour analyser similar to that used by Morgan (1977) is connected to the base of the furnace to monitor continuously the water vapour concentration in the nitrogen canier gas. Calibration of the infrared detector is made prior to each run using an ADC water vapour generator which supplies a known concentration of water vapour in nitrogen to the detector.

Prior to structural water analysis, the removal of all surface adsorbed water was accomplished by heating a sample at 10~ to 250~ in the TG/ EWA apparatus, Samples were held isothermally until no further weight loss was recorded. At this temperature, no structural water was released from

30 D. M. Thornley and T. J. Primmer

Kaol in i te (KGa-2) m Chlor i te (CCa-1) Illite ( IMt-1)

/ / I /!i

/ l/i / / / \ , ,

/ :=/." . . . . . . . _ . . , , . ~ . - . z : . " . . . -~ ._ . _ ~ _ _ . : _ _ _ _ . . . . . . .

i i 1 i i i i 300 400 500 600 700 800 900

Temperature ( ~ )

T g t.~

"6 (D "O

0 ~-~ (0 >

FIG. 2. Evolved water profiles from reference portions of three of the clay minerals used in the artificial sand/clay mixtures prepared for this study.

the clay minerals. Samples were then heated at 10~ from 250~ to 900~ and both weight loss and evolved water were monitored continu- ously. The TG/EWA profiles for three of the clay minerals used are shown in Fig. 2. Most reservoir rocks also contain carbonate minerals in varying amounts, which decompose on heating between 250~ and 900~ with the evolution of carbon dioxide. Therefore, when applying the technique to reservoir rocks, the evolved water has to be measured using the infrared water vapour analyser. Use of just weight loss data alone would invalidate the results.

C O M B I N E D A N A L Y S I S

Calculat ion

From TG/EWA of the whole-rock sample and XRD analysis of the clay fraction, the amount of structural water that may be attributed to each specific clay mineral is given by

Wei . Xel E w e i = E n = l ( W c i . X e i ) E w t (1 )

where Ewc i is the amount of structural water from a given (ith) clay mineral (mg), Wci is the weight fraction of structural water in a given (ith) clay

m i n e r a l , X c i is the relative proportion of given (ith) clay mineral in total clay fraction, and Ewt is the total amount of structural water evolved from sample (mg).

Then using measured values of Wci (Table 3), the absolute weight of any clay mineral in the whole rock can be determined. Also, as the initial sample weight is also recorded, the % by weight of each clay mineral in the whole rock can be calculated using,

100. Ewci wt% specific clay mineral - - T w ' W e i ( 2 )

where Tw is the amount of sample used (mg).

Substituting in eqn. (1) for Ewcl yields a working equation to calculate the wt% of a specific clay mineral in the bulk sample,

wt% specific clay mineral in bulk sample

Xci wt% structural water (3)

~-~.i~l(Wel. Xci) lost from sample

where:

wt% structural water 100 �9 Ewt lost from sample Tw

Clay mineral quantification

TABLE 2. Results of XRD analysis of representative size-fractions (<5 Ixm) separated from each synthetic sandstone mixture.

31

Mixture Relative proportions Relative proportions weighed in determined by XRD

A chlorite ' A' 0.13 0.05 illite 0.34 0.39 kaolinite 0.53 0.56

B illite 0.56 0.62 kaolinite 0.44 0.38

C illite 0.14 0.14 kaolinite 0.86 0.86

D chlorite 'B' 0.18 0.15 illite 0.59 0.62 kaolinite 0.23 0.23

E chlorite 'B' 0.20 0.24 illite 0.26 0.28 kaolinite 0.54 0.48

The total (wt%) clay mineral content in the whole rock can be calculated by summing the values determined for each clay mineral present.

R E S U L T S A N D D I S C U S S I O N

Results of the XRD analysis of the separated <5 gm size-fractions are shown in Table 2 and compared with the relative proportions weighed into each synthetic clay mineral/sandstone mixture. The evolved structural water for individual pure clay minerals used to make up the mixtures are given in Table 3, along with theoretical structural water calculated from structural formulae for comparison.

Structural water analyses of the mixtures are shown in Table 4, with the wt% evolved water by infrared analysis (EWA) tabulated in column 1. An infrared evolved water analysis profile for sand/clay mixture A is given in Fig. 3. It is clear from this profile that it is not possible to resolve the contributions from individual clay minerals, and that clay mineral analysis cannot be made using TG/EWA alone.

As this study was carried out on samples of known composit ion without the presence of carbonate minerals, weight loss in the mixtures may be attributed solely to structural water. Therefore structural water calculated from weight loss (TG) is tabulated in column 2 for comparison.

TABLE 3. Comparison of theoretical water contents derived from structural formulae and measured water contents determined by TG/EWA on reference portion of pure clays.

Clay mineral Structural formula Theoretical water Measured water content (wt fraction) content (wt fraction)

chlorite 'A' (Mg4.sFesA12.5)(Sis.sAlzs)O20(OH) 16 0. I 134 0.1029

chlorite 'B' (Mgz.TFes.3A13.z)(Si6.zAll.s)O20(OH) 16 0.1105 0.1026

illite K 1.4(A12.SMg0.6Fe0.6)(Siy.3A10.7)O20(OH)4 0.0455 0.0500

kaolinite AI4Si4Olo(OH)8 0.1395 0.1318

32 D. M. Thornley and T. J. Primmer

TABLE 4. Results of TG/EWA on each synthetic sandstone mixture.

Mixture Wt% structural Wt% structural Wt% structural Wt% structural water calculated water calculated water calculated water calculated

from evolved from weight loss from measured from theoretical water (EWA) (TG) Wc and known Wc and known

amounts of each amounts of each clay mineral clay mineral

A 0.99 1.06 1.03 1.07 B 0.81 0.86 0.83 0.84 C 1.15 1.20 1.17 1.22 D 0.73 0.76 0.76 0.79 E 0.91 0.92 0.91 0.96

Theoretical Wc: derived from structural formulae (see Table 3). Measured Wc: obtained by TG/EWA on reference portion of pure clay (see Table 3).

Also shown in column 3 are results calculated using values of measured water content of each clay mineral (Wc) and the known amounts of clay minerals weighed into the mixtures. However, in the analysis of reservoir rock samples it is not possible to perform TG/EWA analysis on pure clay extracts. The structural water content for each clay mineral is an unknown and would have to be calculated or estimated from appropriate structural formulae. Column 4 shows the results calculated using theoretical Wc from structural formulae and the known amounts of clay mineral weighed into the mixtures.

The results of combining the XRD analysis (Table 2, column 2) and the TG/EWA (Table 4, column 1) for the c lay/sands tone mixtures according to eqn. (3) are shown in Table 5. These values are compared with the known composition of the mixtures from Table 1.

Accuracy, precis ion and sources o f error

Differences between wt% clay mineral analysed by the combined method and wt% clay mineral weighed into the original sand/clay mixture range

100

% %

99.5 - - ~ ~ Weight loss

Z _", Evo,v . . . . . . . . . . . . . .

98.5 -

I I I I I I I

300 400 500 600 700 800 900

T e m p e r a t u r e ( oC )

FIG. 3. An example of evolved water and weight loss profiles of sand/clay mixture A.

t - O

Clay mineral quantification

TABLE 5. The results of combined XRD/TG/EWA on each synthetic sandstone mixture compared with the actual amounts of each clay mineral weighed into each mixture.

33

Mixture Wt% clay mineral Wt% clay mineral weighed in analysed

A chlorite 'A' 1.3 0.5 illite 3.5 3.9 kaolinite 5.4 5.6

B illite 5.4 6.2 kaolinite 4.3 3.8

C illite 1.4 1.3 kaolinite 8.3 8.2

D chlorite 'B' 1.8 1.4 illite 6.0 5.9 kaolinite 2.3 2.2

chlorite 'B' 1.8 2.2 illite 2.3 2.5 kaolinite 4.7 4.3

between 0.1 and 0.8 wt%. The precision of repeat determinations of wt% structural water by TG/EWA in mixtures A, B & C for 20 analyses over a 12 month period was <5%. Measured values of wt% structural water shown in Table 4, column 1 are all within 4% of the values calculated from measured Wc and known amounts of clay minerals weighed into the mixtures (Table 4, column 3), indicating that a reasonable level of accuracy in TG/EWA can be achieved. This suggests that the greatest source of error must occur in the XRD analysis of the clay fractions. Figure 4 shows that the largest errors in the amount of each clay mineral analysed compared to the amount weighed in, correspond to mixtures where the largest errors in the relative proportion of each clay mineral determined by XRD occur. Similarly, where errors in XRD analysis of the clay fraction are small, errors in the wt% clay mineral analysed in the sand/clay mixtures by the combined method are small. Errors associated with the analysis of separated clay fractions by XRD have been discussed elsewhere (e.g. Reynolds, 1989) and it is not the purpose of this paper to examine them further. However, as quantitative XRD analysis of a clay fraction separated from a sandstone is frequently attempted, use can be made of the XRD data in the context of the whole rock by combining TG/EWA data as described above,

without incurring significant additional error in the final result.

A P P L I C A T I O N O F C O M B I N E D X R D / T G / E W A

The influence o f illite abundance on the permeabili ty o f the Magnus Sandstone Reservoir

During appraisal of the Magnus oil field in the UK sector of the Northern North Sea, permeabilities calculated from well test data (Heaviside et al., 1983) were found to vary by over two orders of magnitude both vertically from the oil to water leg and laterally from crest to flank (Table 6). The low permeabi l i ty of the aquifer rock had major implications on development plans as future water injection into the water zone of the reservoir would not be viable. A further problem arose in that a comparison of well test permeability values with those der ived from rout ine gas permeabi l i ty measurements on dried core plugs showed unusual discrepancies. This was most noticeable in the water zone where measured core permeabilities were 20 times higher than permeabilities derived from well tests (Table 6). A detailed laboratory core

34

-0.1

Chlorite

Illite

Kaolinite

D. M. Thornley and T. J. Primmer

Difference in Relative Proportions of Clay Minerals Determined by XRD and Known Proportions Weighed into Each Mixture

+0.1 -0.1 +0.1 -0.1 +0.1 -0.1 +0.1

[ZI IN

i!I! ............... :%;::::::::::;:

i i i i i i i i i i i i i i

iiiiii N~ iN N |

I []

-0.1 +0.1

I i/| ~ . "

Nt

N

Difference in Weight % of Clay Minerals Determined by Combined XRD/TG/EWA and Known Amounts Weighed into Each Mixture

-1

Chlorite

IIlite

1 +1 -1 +1 -1 +1 -t +1 -1 +1

Kaolinite

.....................

iiiiiiiiiiiiiiiiilii

iI IN IN

1 F

D IN D Fro. 4. A comparison of errors in XRD analysis of separated clay fractions and errors in combined XRD/TG/

EWA analysis of associated sand/clay mixtures, A-E .

analysis study made on preserved core showed very similar trends to the well test and routine gas permeability results (Table 7). It is important to note here that laboratory-derived brine permeability values (column 1, Table 7) were obtained from preserved core that had undergone negligible structural damage prior to measurement. On this basis, brine permeability measurements are consid- ered to be the c loses t to in si tu reservoi r permeability.

At this stage it was considered that the presence of sensitive clay minerals could be responsible for

the unusual effects observed. Standard XRD analysis was performed to determine any differ- ences in clay mineralogy and/or abundance, but the results were inconclusive as the amount of illite present was so low. Analysis by SEM was also undertaken on samples that were prepared using a critical point drying technique that would retain the original clay mineral morphology as found in the reservoir (Pallatt et al., 1984). This revealed an extensive network of wispy clay mineral structures br idg ing across pore- throats . Compar i son of samples from the different zones indicated that

Clay mineral quantification 35

TABLE 6. Comparison of permeability estimates made from well tests with routine core analysis measurements.

Well Zone Permeability Gas permeability Core/well test from well test from routine core permeability

data (mD) analysis (mD) ratio

211/12-1 oil 500 450 0.9

211/12-5 residual oil 100 200 2

211/12a-9 water 3.5 70 20

the extent of the wispy illite appeared much greater in the water zone core, and was limited to the smaller pore-spaces in core from the oil zone. It was concluded that rock, especially from the water zone, contained pore-bridging filamentous illite (Fig. 5), and that the collapse of illite onto the framework grains during cleaning and drying was responsible for the large increase in gas perme- ability values obtained in the water zone samples.

X R D / T G / E W A resu l t s

Combined XRD/TG/EWA was employed to see if the variation in reservoir permeability could be attributed to variations in illite abundance. Thirty one core samples from five wells were analysed and are shown as a composite depth vs. illite and kaolinite abundance plot (Fig. 6). Samples were selected to represent all three zones. There is no sys t ema t i c va r i a t ion of e i the r clay mine ra l abundance with depth or from crest to flank location. More importantly, there is no increase in illite abundance in the water zone.

FIG. 5. SEM photomicrograph of a critical-point dried sandstone containing delicate fibres of pore-bridging

illite.

TABLE 7. Comparison of brine and gas permeability measurements made on preserved core.

Well Zone Brine Gas permeability permeability after soxhlet

(mD) extraction and drying (mD)

Gas/brine permeability

ratio

211/12-5 & 211/12a-9 oil 139

211/12a-9 residual oil 40

211/112a-9 water 3.1

251

74

31

1.8

1.8

10

36 D. M. Thornley and T. J. Primmer

2900

3000

%-.

3100 E c-

o 3200 1=i >

3300

3400

0

I

131 I I

wt % clay mineral

I I I I

It�9 [] I

I

4 6 i I m

ml i I

ml

[] I I

m I I I

,~

I

I I o �9 ,e.

[]

I ! I I

I I I I i

OWC

�9 �9

illite [] I

FIG. 6. Composite plot of illite and kaolinite abundance in the oil and water zones of the reservoir against true

vertical depth.

D i s c u s s i o n

Recent work by Emery et al. (1993), has shown that quartz cement abundance varies from <2% by volume in the oil leg at the crest of the field to >8% downflank in the water leg. Given the invariance in clay mineral abundance, the range in quartz cement must be the primary diagenetic inf luence on absolute permeability variation within the reservoir sandstone rather than illite. However, variation in quartz cement abundance alone cannot account for the differences in the ratio of laboratory-derived gas

Fit. 7. Cartoon to illustrate the combined effect of small amounts of illite and variable quantities of quartz on the nature of the pore system and its permeability.

to brine permeability measurements between oil and water zones.

Clearly it is the combined effect of an increase in quartz cement with a relatively constant illite abundance that is controlling the permeabili ty character of these sandstones. This is illustrated schematically in Fig. 7. On the crest of the reservoir, high in the oil leg, there is very little quartz cement (<2%). The pore throats are large and permeabili ty from well test data is high (500 mD, see Table 6). Any damage to the illite structure on drying of the preserved core is unlikely to have much affect on the routine gas permeability, which was very similar at 450 mD.

Downflank in the water zone, however, the proportion of quartz cement increases to 8% with a corresponding loss in porosity. The same amount of authigenic illite (c. 0.75%) now occupies a smaller available pore volume and bridges smaller pore throats that have been significantly narrowed by the presence of quartz cement. Well test and brine permeability data on preserved core from the quartz cemented water zone are very similar at 3.5 mD and 3.1 mD, respectively. However, the collapse of illite during the oven drying process, prior to routine gas permeability measurements, creates less restriction through the pore throats. This gives rise to a l0 to 20 fold increase in permeability.

In this case study, very small levels of illite abundance have been analysed quantitatively by XRD/TG/EWA. This work, along with recent

Clay mineral quantification 37

evidence for variations in the pattern of quartz cementation (Emery et al., 1993), allows a new model to be put forward to explain the permeability characteristics of the Magnus Field reservoir sandstone.

C O N C L U S I O N S

The use of a whole rock TG/EWA technique combined with routine XRD analysis of a clay size-fraction separated from the whole rock has shown' that it is possible to quantify mixtures of small quantities of clay minerals in laboratory- prepared synthetic sandstone mixtures, The degree of accuracy reported is greater than that which might be routinely achieved by many traditional whole-rock XRD techniques. Using the combined methodology on unknown sandstones, the main source of error in the calculated clay mineral content of the whole rock would be that associated with XRD analysis of a clay size-fraction extracted from the sandstone. However, if errors associated with the XRD analysis of the clay size-fraction can be minimized, additional TG/EWA data can be combined, without incurring significant additional error, to obtain quantitative analysis of clay mineral abundance in whole-rock samples of sandstones.

This makes the XRD/TG/EWA method ideal for detailed studies on the distribution of clay minerals in petroleum reservoir sandstones. This has been demonstrated in a study to determine quantitatively the illite and kaolinite abundance of a suite of samples from reservoir sandstones in the Magnus oilfield, from the UK sector of the Northern North Sea. The results showed that a revised petrological interpretation of the sandstone was required to explain better the permeability characteristics of the reservoir.

ACKNOWLEDGMENTS

We wish to thank Mark Hopkins, Geoff Cade and Alison Fry for their help with sample preparation and the XRD analyses. Nadia Pallatt for supplying the sample of Tuscaloosa Sandstone and BP Exploration for permission to publish this paper.

REFERENCES

BLOODWORTH A.J., HURST A. & MORGAN D.J. (1990) Detection and estimation of low levels of kaolinite

by evolved water vapour analysis. Mem. Sci, Geol. Strasbourg 89, 143-148.

CHUNG F.H. (1974) Quantitative interpretation of X-ray diffraction patterns of mixtures. II. Adiabatic principle of X-ray diffraction analysis of mixtures. J. Appl. Crystallogr. 7, 526-531.

EMERY D., SMALLEY P.C. & OxToav N.H. (1993) Synchronous oil migration and cementation in sandstone reservoirs demonstrated by quantitative description of diagenesis. Phil. Trans. R. Soc, Lond. A, 334, 115-125.

GraBS R.J. (1965) Error due to segregation in quantitative clay mineral X-ray diffraction mount- ing techniques. Am. Miner. 50, 741-751,

HARDY R.G. & TUC~:ER M.E. (1988) X-ray powder diffraction of sediments. Pp. 191-228 in: Techni- ques in Sedimentology (M.E. Tucker, editor). Blackwell, London.

HEAVISIDE J., LANGLEY G.O. & PALLATT N. (1983) Permeability characteristics of Magnus reservoir rock. 8th European Formation Evaluation Sym. Paper A, 29 pp.

JOHNSON L.J., CHU C.H. & HUSSEY G.A. (1985) Quantitative clay mineral analysis using simulta- neous linear equations. Clays Clay Miner. 33, 107-117.

MORGAN D.J. (1977) Simultaneous DTA-EGA of minerals and natural mineral mixtures. J. Thermal Anal. 12, 245-263.

PALLATr N., WILSON M.J. & MCHARDY W.J. (1984) The relationship between permeability and the morphol- ogy of diagenetic illite in reservoir rocks. J. Petrol. Tech. 36, 2225-2227.

PATERSON E. & SWAFFIELO R. (1987) Thermal analysis. Pp. 99-132 in: A Handbook of Determinative Methods in Clay Mineralogy (M.J. Wilson, editor). Blackie, London.

PIERCE J.W. & SIE6EL F.R. (1969) Quantification in clay mineral studies of sediments and sedimentary rocks. J. Sed. Pet. 39, 187-193.

REYNOLDS R.C. (1989) Principles and techniques of quantitative analysis of clay minerals by X-ray powder diffraction. Pp. 4 -37 in: CMS Workshop Lectures, Vol. 1, Quantitative Mineral Analysis of Clays (D.R. Pevear & F.A. Mumpton, editors). The Clay Minerals Society, Evergreen, Colorado.

WAXMAN M.H. & SMITS L.J.M. (1968) Electrical conductivities in oil-bearing shaly sands, Soc. Pet. Eng. J. 8, 107-122.

WELTON J.E. (1984) SEM Petrology Atlas, pp. 38-100. American Association of Petroleum Geology, Tulsa, Oklahoma.

WILSON M.D. & PITTMAN E.D. (1977) Authigenic clays in sandstones: recognition and influence on reservoir properties and paleoenvironmental analysis. J. Sed. Pet. 47, 3-31.

WILSON M.J. (1987) X-ray powder diffraction methods.

38 D. M. Thornley and T. J. Primmer

Pp. 26-98 in: A Handbook of Determinative Methods in Clay Mineralogy (M.J. Wilson, editor). Blackie, London.

WORTHINGTON P.F. (1985) The evolution of shaly-sand concepts in reservoir evaluation. Log Analyst, 26, 23 -40.