Petroleum Geochemistry of Upper Assam Shelf- a Case Study

Pahari, S.¹, A.K.S. Madhavan², I.V.S.V. Prasad¹, M. Varshney³, A. K. Sinha¹, B.L. Sharma³, A. Mishra⁴, A. Banerjee⁵ (1) KDM Institute of Petroleum Exploration, Oil & Natural Gas Corporation Ltd, Dehradun, India (2) ONGC Ltd, India (3) KDM Institute of Petroleum Exploration, Oil & Natural Gas Corporation ltd, Dehradun, India (4) IDT, India (5) ONGC Ltd, Rajamundry, India

The present study aims to resolve the ambiguities associated with oil generation, migration and entrapment theories from the Tertiary sediments of Upper Assam Shelf; a southeast dipping shelf over thrust by the Naga Hills is a foreland part of Assam-Arakan Basin. The kinetic parameters were derived from Optkin software and applied on Genex software for modeling besides source rock identification, maturity assessment, oil to oil and oil to source correlations.

Source rocks mainly in BCS, Kopili and Sylhet formations of the Schuppen belt charged reservoirs in Nazira area and Dhanasiri valley since Late Miocene. Reactivation of structures and developing folds in the foreland possibly created impedance to lateral migration from the Schuppen Belt since Pleistocene. Migration is primarily updip to the northwest along the northeast- trending slope and occurs through reactivated basement-rooted faults. In Nazira area, hydrocarbon charge from local Barail, Kopili and Tura SRs of the foreland has caused diversity in oil characteristics. In Dhanasiri valley, uniformity in oil characteristics is due to lack of local generation.

Geochemical analyses on BCS identified a mixed assemblage comprising lignin-based vitrinite-rich facies, polymethylene-rich algal facies and polycadinene-based resinite-rich facies. These mixed kerogen assemblages exhibit kinetic diversity providing evidences for the early oil expulsion. Optimised kinetic parameters of Kopili Shale and BCS also confirm early break-down nature of respective source OM.

Three oil groups are evident; predominantly matching to source rocks of Early Eocene Sylhet and Oligocene Barail – dominated by terrestrial OM, and Late to Middle Eocene Kopili Formation – dominated by marine OM.