Understanding of the hydrocarbon filling of the reservior is very important for hydrocarbon exploration. K-Ar dating of authigenic illite in reservior can provide very useful data about the hydrocarbon filling history. The separation of authigenic illite in hydrocarbon reservoirs and its K-Ar dating techniques are described in detail in this paper. Detrital K-bearing minerals such as potassium feldspar and illite in authigenic illite separate may lead to an erroneous age. For samples with detrital potassium feldspar and/or detrital illite, it is necessary to find a way to remove the contribution created by those impurities, even at low levels, to the authigenic illite K-Ar age. This paper concluded a detailed study on the impact of contaminant phases on K-Ar age, and as a result, a data processing technique has been developed and a new concept named “Corrected Age has been presented.