INTRODUCTION Diagenetic modifications that occur after depo- sition at near-surface conditions (i.e. low T, P and surface-related fluids) and before signific- ant burial (e.g. < 2 km; Morad et al., 2000) are referred to here as eodiagenesis, whereas mod- ifications occurring after significant burial (e.g. > 2 km; Morad et al., 2000), under the influence of increasing P and T, and by modified dia- genetic fluids, are referred to as mesodiagenesis The role of the Cimmerian Unconformity (Early Cretaceous) in the kaolinitization and related reservoir-quality evolution in Triassic sandstones of the Snorre Field, North Sea J.M. KETZER 1 , S. MORAD 1 , J.P. NYSTUEN 2, * and L.F. DE ROS 3 1 Department of Earth Sciences, Uppsala, Villa vägen 16, S-752 36, Uppsala, Sweden, e-mail: marcelo.ketzer@geo.uu.se, sadoon.morad@geo.uu.se 2 Saga Petroleum ASA, 1301 Sandvika, Norway 3 Universidade Federal do Rio Grande do Sul, Instituto de Geociências, Av. Bento Gonçalves, 9500, CEP 91501-970, Porto Alegre, Brazil, e-mail: lfderos@pop.inf.ufrgs.br ABSTRACT Modal petrographic analysis, textural criteria and petrophysical data were used to unravel the origin and timing of kaolinite formation and its impact on reservoir properties of Triassic sandstones of the Lunde Formation, Snorre Field, northern North Sea. Kaolinite formation occurred by the replacement of detrital feldspar, mica, rock fragments, mud intraclasts and pseudomatrix. Some of the kaolinitization occurred shortly after deposition, but most occurred during the Early Cretaceous regional uplift and forma- tion of the late Cimmerian Unconformity. Kaolinite stable isotopic data (δ 18 O SMOW =+13.9‰ to +18.5‰, and δD SMOW =- 83‰ to - 69‰) support kaolinite formation from Early Cretaceous meteoric waters. Kaolinite con- tent in the sandstones increases from less than 5 to up to 20 vol.% within the first 200 m below the unconformity and average total porosity was enhanced by about 5%. Where mudstones were present between the uncon- formity and the Triassic sandstones, meteoric water flushing and, hence, kaolinite formation, were limited. However, in some cases, kaolinitization in sandstones buried under thick mudstones was aided by major normal faults that were hydraulically connected to the unconformity surface. The lack of a negative correlation between kaolinite and feldspar content is attributed to: (i) the presence of kaolinite sources other than feldspar (pseu- domatrix, mica, mud intraclast and rock fragments), (ii) the strong variations in the initial detrital mineralogical composition of the sandstones, and (iii) to mass transfer of Si and Al ions on scales greater than that of the thin-section. Int. Assoc. Sedimentol. Spec. Publ. (2003) 34, 361–382 * Present address: Department of Geology, Oslo Univer- sity, N-0316 Oslo, Norway. Clay Mineral Cements in Sandstones. Edited by Richard H. Worden and Morad © 2003 International Association of Sedimentologists. ISBN: 978-1-405-10587-3