Abstract: The timing of hydrocarbon generation is of particular concern in resource exploration. When were the solid organic matters heated to form oil and gas? Because illite forms in rocks in response to heating in the same temperature range as oi1 formation, its K-Ar age is all important parameter. Illite in reservoir sedimentary rocks is a mixture of detrital and diagenetic components. Defining reliable diagenetic ages of illitization in sedimentary rocks still remains a formidable challenge because separation of the diagenetic component is impossible, even if it is carried out down to the grain size as small as