This work tried to determine the nature of source rocks' depositional environments in the North Sea and the Kimmeridge using the Pristane/Phytane ratio (Pr/Ph), and at the same time define and explain the Carbon Preference Index (CPI) used as a maturity parameter. Geochemical analysis of a rock sample from the Kimmeridge in Dorset North England, and Oil sample from the North Sea were carried out. The extractable hydrocarbon (Bitumen) from the Kimmeridge rock was extracted by the Soxhlet extraction method and the extract was later separated using Thin Layer Chromatography (TLC) into fractions (Aliphatic, Aromatic and Polar). The Oil sample was also separated into same fractions using open column liquid chromatography (column packed with slurry of Silica and Alumina). The Aliphatic fractions of the oil and the rock extract were analyzed by Temperature Programmed Gas-Chromatography. Study of the n-alkanes distribution in the two GC chromatograms and also the Pr/Ph ratio and CPI suggest that the North Sea oil is from a matured source rock that might be deposited in an oxic environment with some evidences of terrestrial contributions, whereas the Kimmeridge source rock was immature marine origin organic matter, which might be deposited under anoxic conditions. The limitations of the applications of these parameters were also highlighted, since there are some influences that are proven to impact on these parameters, and therefore redox conditions might not be the reason for these differences but rather the effect of difference in maturity or inputs from other sources.