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

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 formation of the late Cimmerian Unconformity. Kaolinite stable isotopic data (δ18OSMOW=+13.9‰ to +18.5‰, and δDSMOW=−83‰ to −69‰) support kaolinite formation from Early Cretaceous meteoric waters. Kaolinite content in the sandstones increases from less than 5 to up to 20vol.% within the first 200m below the unconformity and average total porosity was enhanced by about 5%. Where mudstones were present between the unconformity 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 (pseudomatrix, 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.