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Abstract

Information gained from standard X-ray powder diffraction (XRD) analyses of Gulf Coast Tertiary and North Sea Mesozoic shales are compared with backscattered electron microscopy and electron microprobe data to illustrate the potential of backscattered electron imagery (BEI) for diagenetic studies of shales. The XRD analyses of Gulf Coast shales showed that one of the principal changes is the increased illitization of mixed-layer clays with depth. The electron microprobe data confirmed this change, but the BEI data revealed that closed microenvironments also exist within the shales, in which other diagenetic processes have taken place, e.g., the formation of authigenic kaolinite and chlorite. The XRD analyses of Mesozoic North Sea shales showed abrupt changes in the nature of illite/smectite compositions with stratigraphie position, suggesting that the compositional changes could be due to differences in detrital source materials. The BEI and probe analyses of these North Sea samples indicate a variety of diagenetic reactions in different microenvironments including dissolution, precipitation, and alteration involving phyllosilicate, feldspar, carbonate, and sulfide phases. The diagenetic reactions were found to vary significantly depending on the nature of the original sediments.

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