Abstract

This study examines and assesses a number of commonly used biomarker ratios that are routinely applied as proxies for thermal maturity. Using the organic-rich shale of the Bakken Formation and uncontaminated core cut from a single borehole within southern Saskatchewan, Canada, a series of detailed biomarker depth profiles were created based upon 18 samples sampled over an interval of 3.98 m (13 ft). Detailed analysis shows a high degree of variability in T_s/(T_s + T_m), the C₂₉ 20S/(20S + 20R) sterane, C₂₉αββ/(ααα + αββ) sterane, and the C₃₂ 22S/(22S + 22R) homohopane biomarker thermal maturity ratios, although T_s/(T_s + T_m) and the C₂₉ 20S/(20S + 20R) sterane ratio show the greatest degree of variation. There is no agreement as to the precise level of thermal maturity over the sampled interval, with expressions of thermal maturity that range from very immature, through early mature, to peak maturity depending upon which biomarker index is used. Furthermore, when compared to the absolute abundance for each biomarker isomer, all biomarker ratios have the highest expressions of thermal maturity over intervals of depth in which absolute abundance is very low. Generally, observed variations in biomarker ratio cannot be ascribed to the effects of thermal maturation. A comparative examination of absolute abundance against gammacerane and other paleodepositional proxies indicates that the paleodepositional environment and variations in source input have a very strong influence upon these biomarker parameters.

A lack of consistency in most biomarker ratios highlights the potential inaccuracies that could arise when biomarker-based assessments are derived from a single sample, or very limited number of samples per formational unit. Results from this study call into question the universal assumption that the transformation of biological isomer, or configuration, to that of a geological isomer or configuration is solely governed by the thermal conversion of a reactant and a product. This study clearly supports a growing body of evidence that many biomarker thermal maturation parameters are subject to diagenetic effects (i.e., paleodepositional) or a paleoecological overprint, and that depth-wise variations in a given biomarker ratio may not be due to catagenesis resulting from burial depth and increasing thermal maturity.