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Forensic geology, the application of the science to the law, has required detailed classification, identification, and fingerprinting of hydrocarbons. One does not need to be a chemist to understand practical concepts or to use the basic techniques of hydrocarbon fingerprinting. Currently, the best overall tool for this is a chromatogram derived from capillary column gas chromatography (GC). Just as hardness and cleavage identify rock minerals and x-ray angles help identify clay minerals, retention time on a chromatogram can help identify key hydrocarbons, such as normal paraffins. Similarly, as suites of minerals are used to classify rocks, n-paraffin ranges can be used to classify hydrocarbon mixtures such as gasoline, diesel fuel, or crude oil.
Refined and crude petroleum may be distinguished on a chromatogram by the range of n-paraffins in a mixture, the shape of the n-paraffin envelope, the presence or absence of olefins, and the presence and relative abundance of certain hydrocarbon additives. Crude oils tend to have a wide range of n-paraffins whose envelope is asymmetric and includes a tail of heavier hydrocarbons. Refined products have a more limited n-paraffin range. With some notable exceptions, such as gasoline, the envelope of most refined products is bell shaped. Olefins are an artifact of the refining process and are not present in crudes. Methylcyclohexane is relatively abundant in crudes and condensates, and less abundant in gasolines. Isooctane and aromatics are more abundant in premium gasolines than in conde sates and crudes. Fuel additives such as tetraethyl lead, methyl tertiary butyl ether, ethyl tertiary butyl ether, tertiary butyl alcohol, and ethanol do not exist in crudes.
This paper uses case histories to illustrate fingerprinting techniques. Case one matches the fingerprint of a plume to a specific source. Case two eliminates casing-head condensate as the source of a plume and tags processed natural-gas liquids as the probable source. Case three illustrates how other organic compounds may be mistakenly identified as hydrocarbon contamination, and case four differentiates refined products.
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