Abstract

In Permian Rotliegend sandstones in the Dutch part of the Southern Permian Basin illite can be a practical annoyance because it strongly influences reservoir properties. Despite its practical importance, the origin and in particular the prediction of the distribution of authigenic illite in siliciclastic sandstone is still under discussion.

Three main types of authigenic illite exist, each of which result from different diagenetic processes at different stages during burial.

The earliest illite is tangential illite, parallel to the grain surfaces. It has formed by infiltration of suspended clay-size particles into loose surficial sands and can be interpreted as cutanic clay. The clay cutans were occasionally formed in situ but are mostly inherited with grains having been reworked from interdune and sabkha areas by fluvial and aeolian action.

The second illite type is authigenic and occurs on top of the clay cutans. Commonly two or three illite generations with different crystal habit are present, ranging from platy to hairy. Clay cutans were nucleation sites and generated the mass for the illite rim cement. Different amounts and habits of illite rim cement are linked to differences in the thickness and completeness of the clay cutans. The more iron oxide in the cutans, the less authigenic illite formed. The illite rim cement precipitated after mechanical compaction, and is probably related to illitization of smectitic clay in the cutans. Illite rim cement is thus related to the local paleogeography of the basin, including aridisol development and erosion of such soils.

The third type of authigenic illite occurs in secondary pores after dissolution of detrital K-feldspar or rock fragments containing K-feldspar. The latter illite is not the product of kaolinite illitization but it precipitated as a primary mineral without a kaolinite precursor. The illite in secondary pores after feldspar dissolution thus depends on the detrital composition.

Only the illite rim cement reduces intergranular porosity to microporosity, thereby increasing the specific surface area. This results in a high irreducible water content. In consequence the permeability for a given porosity can be reduced by up to two orders of magnitude. The other two types of illite have no significant influence on reservoir properties.