Petroleum geochemistry of a part of Upper Assam Basin, India: a brief overview

The Upper Assam Tertiary sedimentary basin is an important petroleum province with reservoir rocks ranging in age from Upper Paleocene to Miocene. The bulk of the oil production is from the Barail (Oligocene) and Tipam (Miocene) formations. Promising hydrocarbon pools have been discovered recently in Upper Paleocene-Lower Eocene formations. Prolific source rocks occur within the Barail (Oligocene) Formation, Langpar Formation (Upper Paleocene) and Lakadong-Theria Member of Sylhet Formation of Upper Paleocene-Lower Eocene age. The source rocks contain predominantly Type II organic matter and are thermally immature to early mature. Biomarker ratios indicate the dominance of land plant derived kerogen. The Barail and Tipam oils have moderate gravity and significant wax contents. The physical and chemical characteristics of the Paleocene-Eocene crude oil is similar to that produced from younger horizons within the basin with the exception of minor quantities of heavy viscous crude oils observed in a few wells. Multivariate analysis, including principal components analysis and factor analysis, of fluorescence data and principal components analysis of biomarker data indicate that the Barail (Oligocene) and Tipam (Miocene) oil have been derived from Barail source rocks with a possible contribution from Eocene source rocks. These preliminary studies have not provided conclusive evidence for the source of the Eocene oils although geological factors indicate that the source rocks are probably the interbedded shales/coals occurring within the Upper Paleocene-Lower Eocene sediments.