Geology, petrology and utilisation potential of some Tertiary coals of the northeastern region of India

Abstract

The coal-bearing Tertiary sediments of the northeastern region of India range in age from Palaeocene to Oligocene. The coal deposits of Meghalaya and the Mikir and North Cachar hills of Assam contain generally thin seams of Eocene age and were formed under stable shelf condition in peripheral platform areas. The coal deposits of Oligocene age occur in a narrow, linear belt of overthrusts referred to as the ‘belt of Schuppen’, that extends from Nagaland through Assam to Arunachal Pradesh; they were deposited in near-shore, deltaic, wet forest swamps to marshy environments, close to a geosynclinal trough. The coal seams attain considerable thickness in the Makum and Namchik-Namphuk coalfields. Comparisons of coal type and rank variations in the Eocene and Oligocene coals indicate that the coals are broadly similar. The coals are vitrinite-rich (> 70 vol%, mineral-matter-free hereafter referred as mmf), with moderate amounts of liptinite (> 8 vol%, mmf) and inertinite (> 5 vol%, mmf). The coals are high in volatile matter (38–57 %, dry, mineral-matter-free basis, hereafter referred as dmf), sulphur (1–10 %) and hydrogen contents (4–9% dmf). The carbon content of the coals ranges from 68% to 85% (dmf). The coals have caking properties in restricted zones. The vitrinite reflectance of the Oligocene coals (% R r = 0.53–0.74) is slightly higher than that of the Eocene coals (% R r = 0.37-0.67). As per ASTM Standard, the Eocene coals are classified as sub-bituminous C to high volatile bituminous C, and the Oligocene coals as sub-bituminous A to high volatile bituminous B. Microscopic examination of the coals shows a number of features which indicate that the northeastern coals may have generated hydrocarbons. Most of the Eocene and Oligocene coals are suitable for combustion and conversion (e.g., liquefaction) processes. The coals with caking properties are being used at rates up to 5–10% in blends for metallurgical coke provided their sulphur content is below 3% and their ash content below 10%.