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The chemical composition of authigenic illite within two sandstone reservoirs as analysed by ATEM

Published online by Cambridge University Press:  09 July 2018

E. A. Warren
Affiliation:
Department of Geology, University of Sheffield, Mappin Street, Sheffield S3 7HF
C. D. Curtis
Affiliation:
Department of Geology, University of Sheffield, Mappin Street, Sheffield S3 7HF

Abstract

Analytical transmission electron microscopy (ATEM) was used to obtain chemical analyses of single illite crystals from Upper Carboniferous reservoir sandstones of the Bothamsall Oilfield, East Midlands, UK, and from Rotliegendes (Permian) sandstones of the North Sea. All samples were found to be highly aluminous, with only minor Mg and Fe, and to have near-ideal dioctahedral sheet totals. No significant variation in chemical composition of illite was found within the Bothamsall reservoir rocks, irrespective of paragenesis or stratigraphic horizon. Similar results were obtained from the Rotliegendes samples. Variation was found, however, between Bothamsall and Rotliegendes analyses populations. The Rotliegendes illites were distinctly more K-rich, which is the result of greater charge deficiency in the octahedral sheet. These results indicate that all the illite in both reservoirs precipitated in equilibrium with the reservoir pore-fluid. Furthermore, they imply that the physico-chemical composition of the pore-fluid did not evolve significantly between the different illite generations in the Bothamsall samples. These data, when compared with published analyses of authigenic illite, indicate that the compositional field for illite in sandstones is restricted in comparison with that of illite in mudrocks.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1989

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