Abstract. Carbon consists of two stable isotopes, carbon 12 and carbon 13. Carbon isotope ratios are determined by the CO₂ sample using the gas mass spectrometer, so the carbon of petroleum is transformed to the carbon dioxide. The carbon isotope ratios are reported as δ-values, where the δ-value is the difference between the isotope ratios of a sample and a standard, related to the isotope ratio of the standard used. To resolve the confusion due to expression of the isotopic data, we have expressed the data versus international PDB-standard, so we have normalized these data. Normalization is the process of stretching or shrinking an isotope ratio scale, so, that a second reference material is set to a defined δ-value relative to the first reference material.

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