EVOLUTION OF CLAY-MINERALS CRYSTALLOGRAPH IC STRUCTURE IN THE DUTCH ROTLIEGENDE SANDSTONE RESERVOIR

This paper gives a thorough characterization of the structural evoluti on of clay minerals that are present in the Rotliegende sandstone rese rvoir, lower Slochteren, of the Broad Fourteens Basin, the Netherlands (Southern North Sea), resulting from contrasting burial histories whi ch have affected several blocks.The early kaolinite crystallization is followed by a steady kaolinite-dickite diagenetic transformation affe cting both the structure and the morphology of kaolin-group minerals. Kaolinite ''books'' are first replaced by dickite pseudomorphs which a re subsequently replaced by dickite blocky crystals with increasing bu rial depth. Intermediate structures between both end-members are diffi cult to characterize with X-ray diffraction. However, both differentia l thermal analysis (DTA) and infrared spec spectroscopy allow an accur ate and fast characterization of this kaolinite-dickite evolution. The structural characteristics of kaolin-group minerals are related to th e burial history experienced by the sediments prior to the Cimmerian o rogeny. During the Cimmerian orogeny, kaolin-group minerals are sudden ly illitized. With the increase of temperature conditions prevailing d uring this hydrothermal-like event, I the morphology of illitic minera ls steadily evolves from hairy illite to lath-shaped particles and ult imately to isometric plates. The size of illitic crystals also increas es with increasing temperature conditions. At the same time, structura l characteristics of illitic minerals also evolve; this evolution is f ollowed by the decomposition oi experimental X-ray diffraction profile s. The decomposition method allows us to determine simultaneously the illite over I-S ratio, the illite content in I-S, and illite crystalli nity. The evolution of the three-dimensional structure oi illitic mine rals, from 1M with trans vacant sites towards 1M with cia vacant sites with increasing temperature, is determined by X-ray diffraction on ra ndomly oriented preparations. The morphological and structural charact eristics of illitic minerals do not represent the progress of a smecti te-illite transformation but these characteristics clearly reflect the temperature conditions prevailing during the illitization of kaolin ( similar to 155 My). Morphological and structural characteristics of bo th kaolin-group and illitic minerals may be characterized efficiently with X-ray diffraction, DTA, and infrared spectroscopy. Once the diage netic sequence oi clay mineral neoformation has been determined, these characteristics can be used to constrain estimations oi palaeoburial depths.