Nature of Clay Minerals and Shale
Shale is a mixture of clay minerals and silt laid down in very low energy environments. The siit fraction* of shale consists of fine particles (<0.0625 mm), mostly quartz. On the other hand, the clay fraction of shale is made up of minerals which are hydrous aluminum sllicates with small amounts of magnesium, iron, potassium, and other elements. Olay particles have a layered platelet structure 5-10 Â(Â = 10−8 cm) thick and to 10,000 Â in length and width. Clay platelets are stacked one above the other with spacings of 20-100 Â. Clay minerals are very small particles (maximum size 0.0002 mm), about 10-100 times smaller than the average sand grain (Dewan, 1983).
Clay minerals can be classified into specific groups according to their crystal structure. Table 3is a list of significant clay minerals and illustrates clay properties that affect log readings. The first column in Table 3 gives the cation exchange capacity (CEC) of each clay mineral.
The cation exchange capacity (CEC) is the capacity of tire clay to exchange cations from exchangeable positions in the adsorbed water around the clay with cations in the free water in the pores, and with cations in the Interlayer of the clay minerals. Because of the higher cation exchange capacity (CEC) of montmorillonite and illite compared with chlorite and kaolinite (Table 3), shaly sands with large amounts of montmorillonite and illite have higher conductivity (i.e. lower resistivity) than shaly sands with kaolinite