Lacustrine Shales and Oil Shales from Stellarton Basin, Nova Scotia, Canada: Organofacies Variations and Use of Polyaromatic Hydrocarbons as Maturity Indicators

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The Post-Acadian Stellarton Basin is a small pull-apart basin in northern Nova Scotia. The Coal Brook Member, which is the main focus of this study, contains the thickest and most extensive development of oil shales within the Late Carboniferous Stellarton Formation. Bulk and molecular geochemistry indicate that the organic-rich oil shales and black shales contain Type I/II kerogen with a mixed assemblage of aquatic and terrestrial organic matter that is dominated by the former. Biomarker evidence indicates that these sediments were deposited under deep, anoxic and fresh-to-brackish water lake conditions. Organofacies distinctions were made on the basis of several parameters that are independent of maturity and therefore suggest organic matter changes. The organofacies identified are the result of water-level fluctuations that are climate controlled. All of the aromatic compounds evaluated as maturity parameters were seen to experience alterations in proportion to temperature in the form of either depth or geologic age, that can be correlated with maturity. These alterations are believed to result from changes in molecular composition. Maturity parameters based on the alkylbenzothiophenic and alkylnaphthalenic compounds are shown to be effective maturity parameters in lacustrine sequences, particularly over the higher maturity ranges where standard maturity parameters based on steranes and hopanes are not effective.



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