ILLITIZATION OF DIAGENETIC KAOLINITE-TO-DICKITE CONVERSION SERIES - LATE-STAGE DIAGENESIS OF THE LOWER PERMIAN ROTLIEGEND SANDSTONE RESERVOIR, OFFSHORE OF THE NETHERLANDS

This paper describes the diagenetic evolution of clay minerals in the Rotliegend sandstone reservoir under contrasting burial histories in t he Broad Fourteens basin (Dutch sector, Southern North Sea), The diage netic modifications affecting the crystal structure of clay minerals ( both kaolin-group and illitic minerals) were studied using X-ray diffr action (XRD), The XRD study includes oriented and random mounts of var ious size fractions, numerical processing (decomposition) of XRD profi les, and simulation of one-dimensional and three-dimensional XRD patte rns, Petrographic observations, differential thermal analysis, K/Ar ge ochronology, and geochemical considerations complement the XRD study a nd allow determination of the sequence of mineral crystallization and the morphological evolution of clay minerals and place further constra ints on the absolute timing of diagenetic events and on the nature of the fluids responsible for clay-mineral crystallization. From depositi on time (similar to 275 Ma) to the Kimmerian orogeny (similar to 155 M a), crystallization of kaolinite at the expense of K-feldspars was fav ored by acid fluids from the underlying Carboniferous Coal Measures so urce rocks; kaolinite crystallization is followed by a steady kaolinit e-to-dickite transformation affecting both the structure and the morph ology of kaolin group minerals, The structural characteristics of kaol in-group minerals are related to the burial history of the sediments p rior to the Kimmerian orogeny, During the Kimmerian orogeny, rapid ill itization of kaolin-group minerals was favored both by increased heat flow in the sedimentary pile and by widespread presence of faults, whi ch permitted significant fluid how probably from the Zechstein Formati on, The morphological and structural characteristics of illitic minera ls, i.e., illite content of illite/smectite mixed layer (I/S), ratio o f illite to I/S, and three-dimensional structure of illitic minerals, do not represent the progress of a smectite-to-illite transformation, but these characteristics clearly reflect the temperature during illit ization of kaolin.