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Authigenic Illite Morphology: Appearances Can Be Deceiving

Joshua D. Cocker

Authigenic filamentous illite can severely reduce permeability despite relatively high porosities in sandstones. The effect of illite on rock properties is dependent on the illite's morphology. By combining careful core preservation, critical-point drying, and scanning electron microscopy examination, it can be shown that illite has various morphologies, both natural and artifactual. The native-state growth form of illite is filamentous into open, brine-filled pore space. If the illite grows as relatively thick filaments, its morphology is stable, unaltered by changes in liquid saturations or core drying. However, thin illite is delicate, and its filamentous form can be modified naturally and artificially, by the passage of interfaces between fluids. Hydrocarbons entering a reservoir modify the fiber distribution, giving rise to various illite textures. In a North Sea Brent reservoir, filaments are preserved in the water zone and in isolated water-saturated sandstones high in the oil zone, but in oil-saturated samples the illite is matted into flake and honeycomb forms. Many textures in the oil zone are similar to those of conventionally air-dried samples, which have artifactual illite morphologies. Without appropriate core preservation and critical-point drying, interpreting the textures as natural or artifactual is difficult--for example, some air-dried illite forms resemble those of smectite or detrital clays. Understanding the various illite morphologies is important for interpreting clay origins, core analysis, and potential productivity or injectivi y of a formation.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.