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Petrological Investigations of Oligocene Coals from Foreland Basin of Northeast India

The results of the petrological investigations carried out on coal samples from the Oligocene deposits of Northeastern India (Assam, Arunachal Pradesh and Nagaland) have been discussed. These deposits occur in the Tikak Parbat Formation (Barail Group) and were probably deposited in the foreland basin. Petrographically, these coals are enriched in vitrinite macerals (65.8–96.6 vol %, mean 87.2 vol %) with variable concentrations of liptinite macerals (2.7–35.4 vol %, mean 11.9 vol %). They contain low concentration of inertinite (nil-3.3 vol %, mean 0.9 vol %) macerals. The microlithotype is dominated by vitrite (61.5–100 vol %, mean 90.8 vol %). An elevated volatile matter (44.15 – 60.20 wt %, mean 52.90 wt %, d.a.f. basis) and reflectance values (VRr 0.39–0.61%) put their rank as sub-bituminous type ‘C’ to high volatile bituminous type ‘C’. The GI and TPI values favour telmatic conditions of origin with high tree density and prolonged wet conditions. This is in agreement with the paleoenvironment interpreted from the vitrinite and liptinite-rich microlithotypes, which suggested a forest and reed facies. However, the presence of inertinite in some coal seams indicates tectonic upheavals / regression of the sea resulting to periodic and short span dryness of the basin.