HYDROCARBON FINGERPRINTING FOR APPLICATION IN FORENSIC GEOLOGY - REVIEW WITH CASE-STUDIES

Forensic geology, the application of the science to the law has requir ed detailed classification, identification, and fingerprinting of hydr ocarbons. One does not need to be a chemist to understand practical co ncepts or to use the basic techniques of hydrocarbon fingerprinting. C urrently, the best overall tool for this is a chromatogram derived fro m capillary column gas chromatography (GC). just as hardness and cleav age identify rock minerals and x-ray angles help identify clay mineral s, 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 hydrocarb on mixtures such as gasoline, diesel fuel, or crude oil. Refined and c rude petroleum may be distinguished on a chromatogram by the range of n-paraffins in a mixture, the shape of the n-paraffin envelope, the pr esence 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 he avier hydrocarbons, Refined products have a more limited n-paraffin ra nge. With some notable exceptions, such as gasoline, the envelope of m ost refined products is bell shaped. Olefins are an artifact of the re fining process and are not present in crudes. Methylcyclohexane is rel atively abundant in crudes and condensates, and less abundant in gasol ines. Isooctane and aromatics are more abundant in premium gasolines t han in condensates and crudes. Fuel additives such as tetraethyl lead, methyl tertiary butyl ether, ethyl tertiary butyl ether, tertiary but yl alcohol, and ethanol do not exist in crudes. This paper uses case h istories to illustrate fingerprinting techniques. Case one matches the fingerprint of a plume to a specific source. Case two eliminates casi ng-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 contamin ation, and case four differentiates refined products.