A sequence of Tertiary shales from the Vienna basin, Austria, contains a series of illite-smectite (I/S) clays that are the source of catalytic activity as measured by the method developed by Goldstein in 1983. The source of this activity is Bronsted acidity resulting from the dissociation of water molecules bonded to exchangeable cations in the interlayers of the smectite and on the external basal surfaces of illite platelets. This specific catalytic activity (SCA) is exchangeable cation dependent and correlates directly with increasing interlayer charge and tetrahedral substitution accompanying the diagenetic formation of thin interlayer illite particles with high-charge external surfaces. As a result, diagenetic transformation of smectite to illite in this series results in increased catalytic activity. From consideration of cation exchange capacity and interlayer charge of end-member components of this I/S series, we show that a cation site resulting from tetrahedral substitution of Al3+ for Si4+ has an activity about 40 times greater than a site of octahedral origin. The endmember illite particles of this I/S series have calculated mean thicknesses of about 80Å (eight 2:1 mica layers). The linear relationship between SCA values and estimated platelet thickness for three different micas and illites confirms the role of cation sites on external basal surfaces as sources of catalytic activity.
The possibility of direct involvement of catalytically active diagenetic illite in natural kerogen pyrolysis, acting simultaneously with a fluid release mechanism and its geopressure development, may provide optimum conditions for hydrocarbon generation and migration during I/S diagenesis. The presence of high-surface-area illite in diagenetically altered reservoir sandstones suggests the possibility for modifying the hydrocarbon constitution of crude oils within buried reservoirs. The effects of exchange cation composition on catalytic properties of I/S clays show that formation water chemistry must be considered in evaluating the effectiveness of clay catalysts in hydrocarbon-forming reactions.