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Compositional variations in diagenetic chlorites and illites, and relationships with formation-water chemistry

Published online by Cambridge University Press:  09 July 2018

J. S. Jahren  and
P. Aagaard
J. S. Jahren
Affiliation:
Department of Geology, University of Oslo, PO Box 1047, Blindern, N-0316 Oslo 3, Norway
P. Aagaard
Affiliation:
Department of Geology, University of Oslo, PO Box 1047, Blindern, N-0316 Oslo 3, Norway
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Abstract

Authigenic illites and chlorites from clastic reservoirs, offshore Norway, have been studied by analytical transmission electron microscopy (ATEM). Present-day reservoir conditions range from 1600 m burial depth and 70°C to 4400 m and 160°C. The reservoir units have experienced continuous subsidence since early Cretaceous, and are presently at maximum burial. Textural and morphological evidence indicates that the authigenic illites and chlorites investigated are late diagenetic products, except for an early-formed berthierine in the shallowest reservoir. While the illites show very limited chemical variability at temperatures between 140° and 160°C the chlorites exhibit definite compositional trends with increasing temperatures. Tetrahedral Al increases substantially from 100°–160°C, and the octahedral vacancy decreases correspondingly. These observations indicate that continuous recrystallization of authigenic clays has taken place. This is also supported by the chemical composition of present-day porewaters, which are close to equilibrium with the clay mineral assemblage.

Type
Research Article
Information
Clay Minerals , Volume 24 , Issue 2 , June 1989 , pp. 157 - 170
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1989

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