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Refining compositional affinity of Williston Basin family C oilsusing multivariate statistical analysis of saturate biomarkers

Pasadakis Nikos, K.G. Osadetz, M. Obermajer

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Summary

This study presents preliminary results of a Principal Component Analysis (PCA) performed on mass spectrometry data obtained from analyses of family C oils produced in the Canadian sector of the Williston Basin. Molecular methods based on saturated hydrocarbon biomarkers are commonly considered the most powerful oil-oil correlation tool, but these techniques often rely on a limited number of molecular characteristics that may obscure the significance of a biomarker signature. Here, PCA is used to maximize diagnostic information carried in a broader terpane and sterane spectrum. The family C oils are distinguished from other oil families occurring in this basin primarily based on differences in the compositions of their medium and higher molecular weight (C12-C40) saturate fractions. Although a number of commonly used biomarker characteristics indicate a high compositional affinity within the family, there is a minor but persistent tendency for the oils to separate into two groups corresponding to the two main geographical areas of their occurrence. This separation pattern is more evident when PCA is applied to biomarker data. Both biomarker compositional and ratio models show a more significant division between oils from the southeast (Cse) and the southwest (Csw) compared with the standard biomarker cross-plots. Factor loadings indicate that the variance in terpane compositional model PC1 is controlled by the relative abundance of tricyclic and pentacyclic terpanes. There is a greater internal variability of the PC1 within the Cse oils, suggesting a greater compositional diversity of their source kerogen and a mixed contribution from algal and microbial precursors. In contrast, the Csw oils are probably derived from a more homogeneous source. Variations in PC2 are controlled by the relative amounts of trisnorhopanes, C29 and C30 hopanes, and C24 tetracyclic terpanes. Such compositional differences are commonly attributed to unique source rock paleodepositional environments. The subdivision of the C oils into two groups determined from a terpane compositional model is substantiated further by PCA models using sterane compositional data and a combination of biomarker ratios. While this subgrouping is delineated mainly by the sterane ratios, variability in terpane ratios defines internal diversity within the Cse oils. There is a general tendency for the Cse oils to show a greater internal variation compared with a relatively smaller variation within the Csw oils. This suggests much greater source heterogeneity for the Cse oils implying mixing of multiple-sourced oils or a progressive change in organic facies. The biomarker contrast between the two groups of oils likely reflects differences in the primary composition of the source organic matter, slightly amplified by the regional differences in oil thermal maturity. Overall, PCA of biomarker compositional signatures, interpreted separately or in combination, enhances the oil-oil correlation and interpretation of petroleum systems, especially when combined with geochemical information from the other petroleum fractions and local geology.

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