Compositional Variations in Diagenetic Chlorites and Illites, and Relationships with Formation-Water Chemistry

Abstract

Authigenic illites and chlorites from elastic reservoirs, offshore Norway, have been studied by analytical transmission electron microscopy (ATEM). Present-day reservoir conditions range from 1600 m burial depth and 70~ to 4400 m and 160~ 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~ the chlorites exhibit definite compositional trends with increasing temperatures. Tetrahedral Ai increases substantially from 100~176 and the octahedral vacancy decreases correspondingly. These observations indicate that continuous recrystalliza- tion 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. Illites and chlorites are diagenetic minerals of considerable importance both because of their general geochemical reorganization during diagenesis and also their influence on the production of hydrocarbons from elastic reservoirs. A better understanding of the control and mechanisms of their formation is needed in order to model their distribution in sedimentary basins. This paper focuses on the formation of iUites and chlorites from kaolinites in sandstones. Illites forming from kaolinite precursors are normally characterized by low octahedral Mg and Fe (Srodon & Eberl, 1984), and the same is true for hydrothermal illites. No compositional trends have been demonstrated for these illites in the diagenetic regime, although Cathelineau (1987) has used the compositions of hydrothermal illites coexisting with chlorites as geothermometers. Velde & Medhioub (1988) have recently discussed the compositional range of diagenetic chlorites within both sandstones and mudstones from the equilibrium standpoint. This study was based on microprobe analyses of chlorites from a series of samples taken from two wells covering temperature ranges of respectively 30~ ~ and 80~176 They concluded that these diagenetic chlorites appear to form under equilibrium conditions, and that they repeatedly recrystallized during increasing diagenetic grade. We have adopted a similar approach in this study. Samples from Jurassic clastic reservoirs offshore Norway, representing a wide range of burial depths and temperatures, have been examined to establish both the occurrence of diagenetic chlorites and illites, and their compositional variation. Any thermodynamic discussion of diagenetic clay minerals must refer to the activity- composition relations of these minerals (Aagaard & Helgeson, 1983; Tardy & Fritz, 1981;