Clay Minerals (1991) 26, 141-145 NOTE FIBROUS CLAY MINERAL COLLAPSE PRODUCED BY BEAM DAMAGE OF CARBON-COATED SAMPLES DURING SCANNING ELECTRON MICROSCOPY Authigenic fibrous clays often occur in the pore systems of sandstones in hydrocarbon reservoirs. Owing to their morphology and high surface-area to volume ratio, these clays significantly reduce sandstone permeability (Stadler, 1973; Seeman, 1979; Kantorowicz, 1990) and cause major problems during enhanced recovery (Kantorowicz et al., 1986). Consequently, there is an increasing interest in predicting the occurrence of these clays prior to exploration, and understanding how they react during production. The need for this information, combined with recent advances in analytical techniques, has resulted in a move towards more studies related to the clay mineral chemistry, and away from purely textural observations. This paper reports the results of a scanning electron microscope (SEM) study of fibrous illite in a reservoir sandstone. It highlights a potential problem of incorrect textural interpretation of fibrous clay minerals caused by beam damage of carbon-coated samples. Samples and methodology The materials used in this study were samples of aeolian sandstone from the Lower Permian Rotliegend of the southern North Sea taken from a depth of ~3 km. Each sample was split in two and the fresh fracture surfaces mounted facing upwards on SEM stubs. One stub was sputter-coated with gold using an Edwards E5000C (PS3) sputter coater, whilst the other sample was coated with carbon using an Edwards E6100 vacuum coater. The thicknesses of the gold and carbon coatings were -370 fit and 200 fit, respectively. It should be noted that the samples were chips off the side of unpreserved core and received no additional treatment, having been air-dried naturally. Samples were examined by X-ray diffraction (XRD) to confirm clay mineral type. XRD analysis of the <2 /~m fraction was carried out using a Philips PWll30 generator and PW1380 goniometer under the following conditions: radiation Cu-Kcr, divergence slit 1 ~ receiving slit 0.25 ~ scan speed l~ voltage 35 kv, current 55 ma. Samples were run airdried, after glycolation, and after heating to 375~ for 30 min. The relative abundances of the clay minerals were calculated using the method of Weir et al. (1975). The gold- and carbon-coated fracture surfaces were examined using a JEOL JXA-840 SEM using an accelerating voltage of 20 kv. Resul~ XRD of orientated samples of the <2/~m fraction in an air-dried state show an illite peak in the 9-9-10.04 fit range which is reduced in intensity on glycolation and increases on heating (Fig. 1). Peaks at 7 and 14 fit remain constant for all the treatments and reflect the presence of chlorite. Quantification using the Weir et al. (1975) method shows illite to be the 9 1991 The Mineralogical Society
142 Note T ~-.'..." J..... Degrees~ FIG. 1.XRDpatterns ofthe<2 #mclayfraction after air-drying (solid line), glycolation (dashed line) and heating 375~ line). Cu-Kolradiation. dominant clay mineral (62%), with subordinate expandable clay (33%) and minor chlorite (5%). The gold-coated sample analysed in secondary electron (SE) mode shows an extensive network of fine fibres and ribbons (Fig. 2a). The fibres extend into open pore-space and are often < 1 ffm in width and 20-30/tm in length (Fig. 2b). Occasionally thickers "plates" of illite extend from detrital grain surfaces and are up to 20 #m in length (Fig. 2c). In contrast, micrographs of the carbon-coated sample show poor resolution with local areas of charging. Similar features to the gold-coated sample are present but are F1G. 2. Scanning electron micrographs of fracture surfaces of Rotliegend (a-f) and Skagerrak (g-h) core. (a) (b) (c): gold-coated samples showing an extensive network of fibres and "plates" of iuite; (d) (e): carbon-coated samples showing illite fibre collapse and local areas of charging, resulting in poor quality micrographs; (f): sample coated with carbon prior to gold-coating, showing similar features to samples that have been gold-coated only; (g): carbon-coated sample of sandstone from the Skagerrak formation showing poorly resolved grain-coating chlorite-smectite; (h): gold-coated sample showing better definition in morphology of the grain-rimming chlorite-smectite. Scale bar 10,um in all micrographs.
Note 143
144 Note "collapsed". Fibrous and ribbon-like illite is present on the detrital grains as a tangled mat that does not extend into the pore space (Fig. 2d). At higher magnification (Fig. 2c) it can be seen that the individual illite fibres have "crumpled" and collapsed on to the pore walls. Discussion The major advantage of coating samples with gold is that it is more effective in eliminating charge effects and thereby is used primarily for obtaining good resolution photographs (Trewin, 1988). Gold-coating, however, has the disadvantage that if energydispersive analysis (EDA) is undertaken, the large gold peaks may obscure those produced by elements within the sample. In order to carry out useful chemical analyses by EDA, samples are usually carbon-coated, as carbon is not detected using standard beryllium window detectors. One possible explanation for the clay textures observed in the SEM is related to the thicker coating of gold on those samples compared to the carbon-coated samples. As well as eliminating charging effects, gold is also more efficient at producing secondary electrons (McHardy, personal communication), and thus gold-coated fibres will be more visible in the SEM than carbon-coated ones. In addition, the thicker gold coating would artificially increase the thickness of the illite fibres (McHardy, personal communication). Although this explanation may account for the greater visibility of the gold-coated fibres, it does not account for the observed collapse of the carbon-coated samples. The texture of the gold- and carbon-coated samples is similar to that reported by McHardy et al. (1982) for gold-coated samples of fibrous illite that had been critical-point dried and air-dried. Whereas critical-point dried samples showed extensive ribbons and fibres of illite, the air-dried samples showed extensive evidence of having collapsed, resulting in a film of clay adhering to the detrital grain surfaces. McHardy et al. (1982) concluded that the morphology of fibrous illite observed by SEM was dependent upon the manner of drying of the specimen. It can be seen, however, that the drying technique is not the cause of the collapsed illite structure observed in the carbon-coated illite in this study. The fracture surfaces analysed all originated from the same sample which had been airdried, the only difference in sample preparation being the coating prior to SEM examination. In order to see if the illite collapse takes place in the carbon-coater, a sample was carboncoated and then gold-coated prior to examination by SEM. The carbon and gold-coated sample shows similar features to the gold-coated sample (Fig. 2f) and thus suggests that clay mineral collapse does not take place within the carbon-coater. Consequently, the collapse of the fibrous iuite in the carbon-coated sample is believed to have been caused by beam damage and charging effects within the SEM. The illite fibres prior to their collapse can be seen to charge up, with adjacent fibres repelling and attracting each other. Owing to their fragile morphology, the ribbons and fibres break and form a matted tangle of crumpled illite on the detrital grain surface. The gold coat, as previously mentioned, eliminates the charge effects more efficiently, and consequently very delicate illite structures can be observed. To reduce the charging effect, some samples were treated with several coats of carbon, but this appeared to have little effect. Although the relatively high accelerating voltage used (20 kv) allows analysis by EDA, it has resulted in charging and beam damage of finegrained particles. This can be decreased by reducing the accelerating voltage to 10-15 kv. In order to understand the effect of clay morphology on permeability it is important to know if the clay samples viewed in the SEM represent the original morphology when the
Note 145 rock was in the reservoir. Core recovered from -3 km under the sea will suffer a substantial drop in pressure on the way to the surface as well as a major change in pore-fluid due to drilling mud invasion of the core. Sample preparation, such as drying or cleaning samples for permeability analysis may also damage the clay morphology (Kantorowicz, personal communication). Consequently, beam damage produced by analysing carbon-coated samples in the SEM adds another potential problem to morphology-based interpretations. However, this problem can be reduced by examining both gold- and carbon-coated samples prior to EDA analysis, or by reducing the accelerating voltage. The effect of carbon-coating described above has also been observed when analysing other authigenic clays. Chlorite-smectite clays observed after carbon-coating (Fig. 2g) are more poorly defined than those that are gold-coated (Fig. 2h), and consequently give a false impression of the morphology of these clays. Conclusion When analysing clay minerals by SEM using carbon-coated samples, the possible effect of sample damage should be taken into account in order to avoid the problem of incorrectly assessing the extent and morphology of the clays. ACKNOWLEDGMENTS The author would like to thank Shell/Esso for providing funds for this research, and staff at PRIS for helpful comments. J.D. Kantorowicz and W.J. McHardy are also thanked for their helpful reviews. University of Reading PRIS contribution No. 074. Postgraduate Research Institute for Sedimentology, Reading University, Whiteknights, Reading RG6 2AB, UK. Received 24 May 1990; revised 29 June 1990 K. PURVIS REFERENCES KANTOROWlCZ J.D. (1990) The influence of variations in illite morphology on the permeability of Middle Jurassic Brent Group sandstones, Cormorant Field, UK North Sea. Mar. Petrol. Geol. 7, 66--74. KANTOROWICZ J.D., LIEVAART L., EYLANDER J.G.R. & EIGNER M.R.P. (1986) The role of diagenetic studies in production operations. Clay Miner. 21, 769-780. MCHARDY W.J., WILSON M.J. & TAIT J.M. (1982) Electron microscope and X-ray diffraction studies of filamentous illitic clay from sandstones of the Magnus Field. Clay Miner. 17, 23-39. SEEMAN U. (1979) Diagenetically formed interstitial clay minerals as a factor in Rotliegend sandstone reservoir quality in the North Sea. J. Petrol. Geol. 1, 55--62. STADLER P.J. (1973) Influence of crystallographic habit and aggregate structure of authigenic clay minerals on sandstone permeability. Geol. Mijnbouw 53, 217-220. TREWlN N. (1988) Use of the scanning electron microscope in sedimentology. Pp. 229-273 in: Techniques in Sedimentology (M. Tucker, editor). Blackwell Scientific Publications, Oxford. WEIR A.H., ORMEROD E.C. & EL MANSEY I.M.I. (1975) Clay mineralogy of sediments of the western Nile Delta. Clay Miner. 10, 369-386.