LATE-STAGE DIAGENESIS OF ILLITIC CLAY-MINERALS AS SEEN BY DECOMPOSITION OF X-RAY-DIFFRACTION PATTERNS - CONTRASTED BEHAVIORS OF SEDIMENTARYBASINS WITH DIFFERENT BURIAL HISTORIES

The smectite-to-illite diagenetic transformation has been documented i n 5 different sedimentary basins using X-ray diffraction (XRD). Interm ediate reaction products coexisting because of the effect of kinetics on this reaction have been characterized using decomposition of XRD pa tterns and comparison with calculated patterns. The nature and relativ e abundances of the various subpopulations of particles are shown to v ary as a function of the geothermal gradient and of the age of the sed iment. In all sedimentary basins that experienced a low steady geother mal gradient the physico-chemical characteristics (coherent scattering domain size [CSDS], junction probabilities) of intermediate mixed-lay ered illite-smectites (I-S) are similar. However, both the relative ab undance and the crystallinity of the end-member illite increase as a f unction of the age of the sediment. In basins that have experienced a higher geothermal gradient, the CSDS of the I-S subpopulation is highe r for a given illite content, indicating a slightly different reaction pathway. This difference in the characteristics (peak position and wi dth) of elementary peaks may be used to infer the presence of such a h igh geothermal gradient when no other data are conclusive. In this cas e the growth of the illite end-member is favored over the growth of in termediate I-S phases even in young basins. Illitic phases formed from the alteration of kaolin minerals exhibit characteristics similar to the reaction products of the smectite-to-illite diagenetic transformat ion in the case of a high geothermal gradient. In contrast with what i s observed in shale diagenesis, the characteristics of the illitic sub populations describe a continuum with absolutely no gap in between sub populations. In sandstone reservoirs, the various subpopulations cryst allize simultaneously from a kaolin precursor. As a consequence, no ki nship is expected between the various subpopulations.