Clay minerals in sandstones: Controls on formation, distribution and evolution

This paper addresses the origin, distribution pattern and burial diagenetic evolution of clay minerals in sandstone: kaolin, smectite, illite, chlorite, berthierine, glauconite and mixed-layer illite-smectite and chlorite-smectite. Clay minerals may be co-deposited with sand grains as sand-sized argillaceous intra- and extra-clasts and as flocculated clays. These sand-sized argillaceous clasts are deformed by mechanical compaction into clay pseudomatrix. Detrital clay minerals may be incorporated into sandy deposits by bioturbation and infiltration of muddy waters. Diagenetic clay minerals form by alteration of unstable detrital silicates and by transformation of detrital and precursor diagenetic clay minerals. The most common eogenetic clay minerals are kaolinite, dioctahedral and trioctahedral smectite, berthierine, glauconite and, less commonly, Mg-rich clay minerals such as palygorskite. The distribution of eogenetic clay minerals is strongly related to depositional facies and sequence stratigraphic surfaces. Illite and chlorite dominate the mesogenetic clay minerals and usually grow at the expense of eogenetic clay minerals and detrital feldspars and lithic grains. Mesogenetic illite and chlorite can result from widely different reactants and processes. Clay minerals usually are assumed to be detrimental to sandstone reservoir quality because they can plug pore throats and some clay minerals promote chemical compaction. However, coats of chlorite on sand grains can preserve reservoir quality because they prevent quartz cementation. Adding oil to a sandstone stops clay diagenesis if the sandstone is oil-wet but probably only slows clay reactions if the sandstone is water-wet. Sandstones tend to be more oil-wet as the Fe-bearing clay content of the sand increases and as oil becomes more enriched in polar compounds.