**3.2 Classification of clay minerals**

Hughes [34] was one of the earliest researchers to attempt classification of clay minerals relative in the petroleum industry. This classification was done a few years after commercialization of X-Ray Diffraction (XRD) technology, which was hitherto used by petroleum companies as a method of studying clay minerals. Hughes [34] classified clay minerals into: Kaolinite, Smectite, Illite and Chlorite groups as shown on **Table 1**. He also indicated other classes which are mainly mixed layers of the four groups of clays. Descriptions by Hughes [34] are captured below:

