Clay argillaceous rock composed of fine grained (finer than 3 µm) clastic sediments; consists of minute, flaky crystals of minerals produced by chemical weathering of crystalline rocks. Its cohesive and plastic properties are derived from thin films of water enveloping flaky particles. When heated and water is driven off, it becomes hard and stonelike. It has adsorption and ion exchange properties and comprises clay minerals, with varying amounts of quartz, feldspars, other silicates, carbonates and ferruginous or organic matter.
In Bangladesh, Permian and Tertiary clays include quartz, illite, kaolinite, chlorite and illite/smectite mixed layer clay as the major component, feldspar as the minor component and calcite, dolomite and siderite in trace amounts. However, in Permian clays illite-smectite mixed layer clay mineral does not occur. Randomly interstratified illite/smectite mixed layer clay mineral shows progressive diagenetic illitization with decreasing expandability of illite/smectite clays estimated by quantitative method and also by the decreasing 17Å glycolated peak intensity with increasing stratigraphical and burial depths. Significant volumes of smectite diagenesis derived water should have been available at this level to migrate from smectite interlayers to the pore water system. It is suggested that this water has contributed to the generation of overpressures, as well as facilitated the primary migration of hydrocarbons from mudstones encountered in the subsurface of Bengal Foredeep and Fold Belt areas. [Sifatul Quader Chowdhury]