Forensic Geochemistry Methods to Trace Sources of Oil and Gasoline Pollution

Ian R. Kaplan

When crude or refined oil products enter soil, ground water, or an aqueous environment (river, lakes or ocean), they begin to be degraded by numerous microbiological or physical processes. The result of such changes is to alter the molecular composition of the product so that its source is unrecognizable. Numerous methods have been devised in the petroleum exploration industry to characterize source rock bitumens and reservoired hydrocarbons. A modification of these methods has been successfully applied to identify the source of the fugitive hydrocarbons.

The methods use four different chemical classes for identification.

(1) For mildly altered products, a detailed statistical comparison is made of the paraffinic or n-alkane distribution between C₁₂-C₃₅ for heavy products, C₄-C₇ for the gasoline range, and C₁-C₅ for the gas range products.

(2) For highly altered products, a search is made for complex organic molecules that have undergone the least alteration. Among such groups are the long-chain polycyclic aromatic hydrocarbons and the polycyclic paraffinic hydrocarbons, frequently referred to as biomarks. The aromatic products often contain nitrogen or sulfur. The nonaromatic compounds have many isomeric forms characteristic of their sources.

(3) Element composition, especially sulfur, vanadium, and nickel, for crude oil and lead or manganese for refined gasoline products, can help identify sources.

(4) The isotopic ratio of ¹³C/¹²C and D/H have been shown to be very useful natural tracers for gasoline and heavy oil. The ³⁴S/³²S ratio may also be useful for heavy oils.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.