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200 J. M. Huggett and H. F. Shaw ~l~tm Fro. 3. Micrographs of a Palaeocene mudrock succession, central North Sea. (A) BSEM of a mudrock buried to 2 kin. Organic matter (black) and mica (e.g. arrow) define the laminated fabric. The diffuse matrix is predominantly clay, but individual particles are not resolved; (B) CSEM of same mudrock sample shown in (A); (C) FESEM image of the same sample as (B); (D) CSEM of mudrock buried to approximately 3 km; (E) FESEM image of the same sample as (D), showing 'illitic' overgrowths not visible at the lower magnification; (F) HRTEM lattice fringe image of illite plate with overgrowth shown in (E) nucleated at a dislocation site in the plate.
esistance at 20 mA, was used to provide the gold- palladium and platinum coatings. The electron microscopes used in our studies were a JEOL 6400F with an Oxford Instruments high purity germanium energy dispersive X-ray detector at the Centre for Microscopy and Microanalysis, University of Queensland, and a Hitachi $800 (without a microanalyser) at the Natural History Museum, London. EXAMPLES Authigenic illite formation in mudrocks Until recently, X-ray diffraction (XRD) has been the principal investigative tool in mudrock diagen- esis, with the result that identification of authigenic minerals has often been by inference rather than observation. Now, FESEM, BSEM, CSEM and high-resolution transmission electron microscopy (HRTEM) have been used to examine Palaeocene mudrocks from a North Sea well, which have been buried to between 2 and 3 km (see Huggett, 1995). The XRD analysis of the clay fractions of these mudrocks indicates that they are composed predominantly of illite and illite-smectite, with minor kaolinite and chlorite. The composition of the illite-smectite is typically 40-50% smectite and is regularly interstratified. The texture and relation- ships between the matrix, silt-size grains and organic matter are best observed in BSEM images (Fig. 3a). However, from Fig. 3a it is clear that BSEM is inadequate to observe individual clay particles. At 2 km depth the clay comprises flakes with the ragged appearance typical of detrital clay in both CSEM (Fig. 3b) and FESEM images (Fig. 3c). At ~ 3 km depth the clay fabric appears indistinguishable in CSEM images from that seen at 2 km (Fig. 3d). However, in FESEM images of samples at 3 km depth, wispy overgrowths on illitic clay particles are a ubiquitous, if minor, component of the mudrocks (Fig. 3e). The precise chemical composition of these particles is uncertain as the particles were too small for collection of X-ray spectra, though HRTEM lattice fringe images confirm that they are indeed illite overgrowths on platy particles (Fig. 3f). The FESEM examination of samples from intermediate depths indicates that there is an overall increase in the proportion of wispy clay overgrowths with depth. The FESEM analyses have thus revealed the formation of diagenetic illites in these mudrocks that increases Field emission electron microscopy 201 with depth, which conventional XRD and SEM analysis had previously failed to show. Multiple generations (?) of authigenic fibrous illite in sandstones The FESEM examination of authigenic fibrous illite in Westphalian sandstones from wells in the southern North Sea has revealed the existence of at least three morphological types (Figs. 4a & b). Previous conventional SEM analysis of these samples had not revealed these variations in illite morphology. It is possible that these morphological variations represent different generations of illite formation. If this interpretation is correct, it could have important petrographic significance regarding authigenic illite formation. It could also affect the interpretation of K/Ar dating and stable isotope analyses of authigenic illite separates. Such analyses are made on the basis that the separated illites represent a single illite formation event in any one sample. The FESEM observations of the illites in the Westphalian sandstones show that there could be several illite formation events. Effects of KC1 drilling muds on shale fabrics The effects of KC1 drilling muds on a range of shales were examined during work carried out jointly with Eniricerche in Milan, Italy. Figures 5a and b show samples of a shale composed predominantly of detrital illite-smectite, examined before and after being reacted with KC1 drilling mud at 60~ for two weeks. One effect of the treatment is shown in Fig. 5b with small fibrous overgrowths on the platy illite-smectite particles. The composition of the fibrous overgrowths is uncertain, as they are too small to be analysed in situ or to be separated for XRD analysis. Small and co-workers have demonstrated how fibrous and platy illite can be grown experimentally in sandstones in the presence of K-beating solutions (Small et al., 1992; Small & Manning, 1993). Using these experimental data as a precedent, it seems possible that the KC1 muds might have interacted with the detrital illite-smectite to produce the fibrous (illitic?) clay overgrowths observed in the FESEM photomicrographs. However, all that can be reported here is the observation of these fibrous overgrowths and any interpretation of the processes leading to their formation can only be conjectural at this stage.
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