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

¹ ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801-0998; clifford.c.walters@exxonmobil.com
² ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, Texas, 77252-2189
³ ExxonMobil Exploration Company, 233 Benmar, Houston, Texas, 77210-4778

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
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.

	INTERPRETATION OF END-MEMBER COMPOSITIONS

 
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 the sixth results from biodegradation, . . . [Full Text of this Article]

This article has been cited by other articles:

				J. Collister, R. Ehrlich, F. Mango, and G. Johnson Modifications of the petroleum system concept: Origins of alkanes and isoprenoids in crude oils: Reply AAPG Bulletin, September 1, 2005; 89(9): 1245 - 1250. [Full Text] [PDF]