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Abstract

Petroleum hydrocarbons in reservoirs are subjected to different degrees of abiotic and biotic alteration processes, which often have strong economic consequences since they lead to a decrease in oil quality and reduce the total volume of petroleum in place. Moreover, producibility of the respective fluids strongly depends on physical properties and hence composition. To evaluate these microbial processes in more detail, different approaches can be followed. Here, the changes in stable carbon isotopic signatures of petroleum hydrocarbons are evaluated in detail as biodegradation processes are known to result in an enrichment of the heavier isotopes in the residual hydrocarbons. The change in the isotopic composition can be used to quantify biodegradation by applying the Rayleigh-equation. The Rayleigh-equation correlates the decrease in concentration and the enrichment in the isotopic composition by a fractionation factor (a). This factor depends on the degradation mechanism and the substrate molecule and can be derived from degradation experiments. Thus, the use of carbon isotope ratios has become a sensitive tool in identification and quantification of biodegradation processes in petroleum reservoirs. This quantification is demonstrated using a set of slightly to moderately biodegraded crude oils from the Gullfaks oil field which was analysed for δ13C of the light hydrocarbons.