Clay minerals of the Permian Rotliegend Group in the North Sea and adjacent areas

The nature, distribution and origin of clay minerals in the hydrocarbon-bearing Permian Rotliegend sandstones of the North Sea and the adjacent areas of the Netherlands and Germany are reviewed. The clay minerals occur as detrital coatings of smectite and smectite-illite on the surfaces of sandgrains, and as later diagenetic cements of kaolinite, chlorite (two varieties), and illite in the pore spaces of those sandstones. Two diagenetic clay mineral assemblages are predominant in the Rotliegend of the North Sea. The kaolinite-illite assemblage is restricted to the Rotliegend of shelf areas which underwent shallow burial followed by strong Jurassic/Cretaceous (Late Cimmerian) structural inversions, whereas the illite-chlorite assemblage is associated with basinal areas that underwent deep and rapid burial throughout the Mesozoic. The factors controlling mineralogy, crystal chemistry and morphology of those diagenetic clay minerals, as well as their regional distribution and origin, are numerous, complicated, and interrelated. Evidence suggests that the following aspects were important parameters: (1) variations in the original depositional and desert environment; (2) the chemistry and flow patterns of the porewaters; (3) temperature and timing of clay mineral formation; (4) local burial history; (5) the presence or absence of meteoric water; and (6) the structural setting of the Rotliegend sandstones. Oxygen isotope data indicate that the illite cements formed over a wide range of temperatures (24-140 degrees C) that is consistent with the deep burial conditions prevailing in the palaeo-basins. In contrast, oxygen isotopes indicate that kaolinite cements formed over a more restricted temperature range (40-80 degrees C) and under the influence of meteoric water penetrating the sandstones of the shelf areas as a result of their Late Cimmerian uplift and associated erosion. Hypotheses suggesting that the absence of kaolinite cement from the deeply buried Rotliegend sandstones is caused by its illitization during burial, and that the chlorite cements have formed by the alteration of earlier smectite, smectite-chlorite and corrensite cements, are not supported by evidence.