Geochemical characterization and source investigation of oils discovered in Khoraghat–Nambar structures of the Assam-Arakan Basin, India

Abstract

We examined the geochemical compositions of the newly discovered Bokabil and Sylhet oils and of the extracts of potential source rock samples from the Khoraghat–Nambar area of the Assam-Arakan Basin, India. Stable carbon isotopic compositions and a number of biomarkers indicate that oils in the Bokabil Formation (Early to Middle Miocene) can be correlated across the Khoraghat and Nambar fields. However, oil from the Sylhet Formation (Middle to Late Eocene) is distinct from those in the Bokabil Formation. Late Eocene to Early Oligocene Barail coal-shale and Middle to Late Eocene Kopili shale sequences are identified as potential source rocks. Both strata contain organic matter predominantly derived from higher land plants; however, different organofacies exist for Barail coal-shale and Kopili shale sequences that can be distinguished by the stable carbon isotopic compositions and biomarker distributions. An oil-source correlation study shows that Barail rock extracts are genetically correlatable with the Bokabil oils, whereas Kopili rock extracts have similar source-specific parameters to those of the Sylhet oil. Thus, the study suggests the presence of two petroleum systems in the Khoraghat–Nambar area: the Barail-Bokabil and the Kopili-Sylhet petroleum systems. Barail and Kopili sediments from the study area are of lower maturity than the oils, suggesting that the actual source strata are more mature equivalents likely to be present in the Naga-Schuppen zone. Therefore, areas in and around the Naga-Schuppen zone, where better reservoir facies are expected, are favoured targets for further exploration.