The biomarker characterisation of Paleocene-Eocene, Oligocene and Miocene reservoir oils of Assam Basin, eastern India has been studied. The oils are waxy and terrestrially sourced. All the oils have triterpenoids biomarkers such as bicadinanes and oleanoids derived from angiosperm precursors. Paleocene-Eocene reservoir oils contain diterpenoids such as ent-beyerane, rosane and pimarane, derived from conifer sources which are absent from Oligocene-Miocene reservoir oils. These oils have higher triterpenoids indices signifying difference in organofacies and change in source vegetation from Eocene to Oligocene. They also contain unsaturated triterpenes or olefins, which suggests that Paleocene-Eocene reservoir oils show evidences of primary migration or migration contamination, therefore different diagenetic, migration, maturation history from Oligocene-Miocene reservoir oils. Comprehensive GCxGC TOFMS is a powerful tool for studying petroleum biomarkers. It successfully separates rosane and ent-beyerane, a tricyclic and tetracyclic diterpane present in Paleocene-Eocene oils, which co-elutes in conventional GC-MS. It also groups compound classes based on the retention time and polarity. Oligocene-Miocene reservoir oils have higher maturities evident from the higher degree of aromatisation of aromatic triterpenoids. This could be a result of thrusting of Oligocene source rocks or the presence of two different source kitchens.


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