Modification of the petroleum system concept: Origins of alkanes and isoprenoids in crude oils: Discussion

Abstract

In their recent paper, Collister et al. (2004) presented a statistical analysis of C10–C28 n-alkane and C16–C21 regular isoprenoid distributions of oils and rock extracts from the Timan-Pechora basin. They found that all unaltered to moderately biodegraded samples could be described as mixtures of six end-member compositions that were assigned to five unique biological sources and to one reservoir process (biodegradation). Because the end members appeared to yield no indications of thermal maturation, Collister et al. surmised that these hydrocarbons were generated not from the thermal cracking of kerogen, but from the liquefaction of biopolymers. If this theory is true, our current paradigms of oil generation and source rock potential need to be changed drastically because it suggests that a significant quantity of liquid hydrocarbons can be generated directly from unaltered biological precursors instead of the thermal cracking of kerogen. We respectfully disagree with the conclusions and hypotheses reached by Collister et al. We regard their interpretation of the statistical analysis of the hydrocarbon distribution data to be flawed and their hypotheses concerning the generation of C10+ n-alkanes and isoprenoids during diagenesis from biopolymers to be incorrect. The mathematical procedure (polytopic vector analysis) used by Collister et al. has the ability to extract end-member spectra from mixed systems. They found that six end members were needed to explain most of the variance in the data. This is not a unique solution, however, because the inclusion of additional end members will always improve the overall goodness of fit. Whether these six end-member fluids truly are representative of specific biological precursors, geologic processes, or are merely mathematical constructs cannot be resolved from the statistical analysis alone. Although Collister et al. claim that five of the end members bear the signature of specific biological sources and …