Carbon isotopic ratios of volatile hydrocarbon fractions of marine oils are diagnostic of organic facies and depositional environments of source rocks. For carbonate oils, low-molecular-weight volatile hydrocarbons (< C₉) are isotopically lighter than high-molecular-weight volatile hydrocarbons (C₉-C₁₇). In contrast, for deltaic oils, low-molecular-weight volatile hydrocarbons are isotopically heavier than high-molecular-weight volatile hydrocarbons. Marine shale oils show patterns intermediate between carbonate and deltaic oils. This relative variation of carbon isotopic ratios among volatile hydrocarbons of oils is explained by earlier expulsion of marine oils derived from isotopically homogeneous (algal-bacterial) kerogens in rich so rce rocks, and secondary cracking of petroleum prior to expulsion for marine oils derived from isotopically heterogeneous (terrestrial) kerogens in lean source rocks. In basins with multiple source rocks, carbon isotopic ratios of volatile hydrocarbons are useful for determining oil-oil correlation and for inferring oil-source rock relationship.