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Sedimentation, Maturity and Petroleum Potential of the Oligocene Oil Shale Deposits of the Assam Basin, North-East India: A Review

Jha, Prakash; Chaubey, Ravi S.
Centre for Petroleum & Applied Geology, Panjab University, Chandigarh, India.

This review article presents some of the characteristics of the oil shale deposit in the Assam Basin of Northeast India. Presently, India is not producing oil shale, but in the preliminary stage of prospecting of the oil shale. The Assam Basin is located in the north-east region of the India and consisting of Precambrian metamorphic and igneous basement and filled mainly with more than 4 km terrigenous sediments of the Tertiary age. The carbonaceous shale of Oligocene age occurs in association with coal in Assam and neighboring area of the Arunachal Pradesh. The coal shale unit extends laterally towards south of the oil fields in a region called "Belt of Schuppen". The potential locations for finding oil shale in Northeast India are Assam shelf deposit; Naga Schuppen Belt and Assam-Arakan fold Belt. The coal-shale units were probably deposited in a regressive phase in brackish water lagoons or swamps on a prograding delta complex. These coal and carbonaceous shale constitutes the principal source rock in Assam Basin. The well known Digboi oil field is located in the vicinity. Average vitrinite reflectance (VRO) measurements on the oil shale range from 0.5 to 0.7%. The studied oil shale are characterized by high total organic carbon (TOC) value upto 77.02% and (SI) value indicates upto 8.76 mg/g and (S2) value ranges from 288.38 mg/g indicating excellent source rocks. Rock Eval Pyrolysis value (Tmax.Oc ) are less than 435°C which indicate low thermal maturity i.e. oil shale are immature to early mature stage for hydrocarbon generation. The organic matter (Kerogen) is predominately Type- II & III . Biomarker ratio indicates a dominance of Terrestrial derived kerogen. The sulphur content is high upto 1.5 to5%. The oil shale reserves in India is estimated upto more than 15 billion tones, especially those in Assam and Arunachal Pradesh, which are likely to produce 140 million tones per year of the next 100 years.


AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012