Geochemical characterisation of Middle Devonian oils in NW Alberta, Canada: possible source and maturity effect on pyrrolic nitrogen compounds

Molecular geochemical compositions of a suite of Middle Devonian sourced and reservoired oils from the Rainbow–Zama–Shekilie sub-basins in N.W. Alberta, Canada, reveal the presence of at least two oil families in the study area. The distribution of each oil family is geographically restricted to a single sub-basin, consistent with the oils being sourced locally within a series of closed generation/migration/trapping systems. The diversity in the biomarker distributions of the oils indicates the wide range of depositional environments and source materials existing in each sub-basin, rather than mixing of end member oils across different sub-basins. Clear maturity differences are observed between the oils from the Rainbow and Zama subbasins. Pre-Cretaceous thermal anomalies along the reactivated regional Precambrian basement faults are proposed as one of the major causes for the relatively high maturity levels for the Middle Devonian source rocks in the study area. We infer that depositional environment and thermal maturity have had a strong impact on the geochemical characteristics of the saturated and aromatic hydrocarbons in the Rainbow–Shekilie–Zama oils, but may not influence the pyrrolic nitrogen compounds to a significant extent. However, recognition of possible source and maturity effects on pyrrolic nitrogen compounds in other studies suggests that these factors should be considered before the pyrrolic nitrogen compounds are used to characterize petroleum migration.