MECHANISM AND KINETIC CONSTRAINTS FOR ILL ITIZATION REACTIONS OF SEDIMENTARY CLAYS, INFERRED FROM WATER-ROCK MODELING

The nature and timing of diagenetic reactions in marine sandstone/shal e formations were modelled for a 50-120 degrees C temperature range, a ssuming at first chemical reactions in closed systems, in order to hav e a better understanding of the factors that control the illitization reaction when K-feldspar coexists with aluminous clay. The original se diment is constituted of quartz, muscovite, K-feldspar, Al-clays (kaol inite, beidellite, montmorillonite), with and without organic matter m aturation. We compared the stable mineral parageneses predicted with t hose observed in natural sandstones and shales. We also tested the eff ects of an energy barrier on illite growth by allowing or not allowing the muscovite/illite precipitation reaction to occur and by using sev eral illitization reaction rates. When compared with data for natural mineral assemblages, the results suggested that the illitization react ion depends on the nature of the reacting clays. Kaolinite conversion to end member illite involves high energy conditions which are not met when the pore water equilibrates with the mineral matrix from undersa turated conditions. To overcome this barrier, the fluid should be over saturated with respect to the K-feldspar. An external source of potass ium or a pH increase appears to be the most possible driving force in such reactions. Kaolinite conversion to end member illite appears then as a marker of fluid circulations and does not affect the K-feldspars when present. We also discuss the composition of the end member illit e as an alternative explanation for the metastability oi the quartz-po tassic feldspar-kaolinite assemblage. On the other hand, a smectite-il lite conversion involves more limited energy requirements and the reac tion progresses spontaneously in closed systems (as observed in most s hales) by dissolving K-feldspars (the source oi potassium) and produci ng quartz overgrowth. The aqueous species of organic origin do not aff ect these reactions significantly.