Abstract

Catagenesis of the <0.2 μm and 2.0-0.2 μm clay fractions has been investigated in samples from six wells and one formation in the Sverdrup and Beaufort-Mackenzie Basins of the Canadian Northwest Territories.

Upon burial of the sediments, the concentration of expandable 2:1 layer silicates, kaolinite and amorphous inorganic material decreases while illite increases in concentration in the 2.0 μm fraction. The same trend is observed in the < 0.2 μm fraction in which the interstratified layer silicates are concentrated. The first dehydration of the interstratified clays is made permanent by isomorphic substitution for Si⁴⁺ by Al³⁺ and the ensuing absorption of K⁺ and occurs prior to the onset of hydrocarbon generation from the sedimentary organic matter. Water is not only released to the pore system by clay dehydration but also by dissolution of amorphous inorganic matter and destruction of layer silicate minerals. Only the last of these mechanisms is operative within the zone of oil generation.