Optical properties of some Tertiary coals from northeastern India: their depositional environment and hydrocarbon potential

Abstract

In northeastern India, the coalfields of Arunachal Pradesh, Assam and Nagaland are disposed along a linear belt of overthrusts, known as the “belt of Schuppen”, where Tertiary strata have been folded and dispersed into a number of thrust slices. The coal-bearing sediments in this belt are associated with the Baragolai and Tikak Parbat formations of the Barail Group (Oligocene). In Meghalaya, the Tertiary sediments were deposited on a stable shelf along the periphery of the Shillong Plateau. They are almost undisturbed and sub-horizontal in attitude. The coal seams are associated with the Sylhet/Tura formations (Palaeocene) of the Jaintia Group. The chemical properties of these coals show characteristics of both low (on volatile matter content) and high (on moisture and calorific value) ranks. The coals have a low softening temperature and a high swelling index, volatile matter, hydrogen and sulphur contents besides having the highest tar yield among Indian coals. The syncrude produced from these coals shows similarities with the identical fraction of crude oil from Nahorkatiya Oilfield, Assam. The coals from the “belt of Schuppen” and Meghalaya are non banded and bright in nature. Microscopically, their humic components are strongly decomposed with a common to frequent association of framboidal pyrite and calcite concretions. The coals are vitrinite-rich (52.0–80.0%) with moderate amounts of liptinite macerals (5.8–16.2%). The inertinite content is low (4.6–11.0%) to occasionally high (30.0%). Fluorescing vitrinite and liptinite macerals constitute 46.5 to 83.5% by volume (m.m.f.). Perhydrous vitrinite, liptodetrinite and resinite are the most common fluorescing macerals. The rank of the coal samples ( R 0 max: 0.57–0.74%) ranges between High Volatile Bituminous C and B stages. The Assam coals are higher in rank than those of Nagaland and Meghalaya. In northeastern India during the Palaeocene and Oligocene Epochs, peat-swamps formed in situ from tropical mangrove-rich angiospermous forest vegetation. The Palaeocene coals in Meghalaya were formed in estuaries or lagoons. However, the coal forming phase ceased shortly due to prolonged marine transgression. The Oligocene coals in the “belt of Schuppen” were formed during a regressive phase in back-water lagoons or brackish water swamps on a prograding delta complex. The hydrocarbon potential (mg CH/g C org.) of the coal samples from northeastern India has been theoretically deduced on the basis of their kerogen types following Mukhopadhyay et al. (1985). The presumed hydrogen index varies from 250 to 430 and 300 to 470 mg CH/g C org. respectively for coals in the “belt of Schuppen” and Meghalaya. Evidence indicates that in Arunachal Pradesh, Assam and Nagaland oil has been generated from kerogen types II B and II A in the Oligocene mudstone-siltstone-shale association of the Coal-Measures of Barail Group. In Meghalaya, no oil is known so far. Nevertheless, the predominance of kerogen types II A and II B in coals indicates a good oil potential in the Palaeocene sediments of the Jaintia Group.