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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.
