Palaeotemperature - Palaeodepth Study from Selected Coal Basins of Damodar Valley Coalfields, India

Selected coal basins from Raniganj to South Karanpura (four adjoining basins of the Darnodar Valley coalfields) of Gondwana age have been investigated from the study of coal samples collected from the deepest boreholes of the region to reveal the tectono - sedimentary and geothermal history of the Peninsular Gondwana. Quantitative estimation and modelling of accumulated sediments and their subsequent partial removal, in relation to sinking and upheaval of basin floor as well as validation study of development of the present day slope have been attempted.

Maturation study of organic matter has revealed that maximum paleodepth/subsidence of the basin floor was reached in Jharia (3920 m) and minimum in South Kmpura (3040 m) during a span of 55 million years. Assuming 135 million years as soaking time/stabilisation period for the organic matter of Barakar Formation, the palaeotemperature attained is maximum in Jharia (155°C) and minimum in South Karanpura (110°C).

The geothermal gradient is maximum in Jharia (5.7°C/100 m) and minimum in South Karanpura (3.1°C 1100 m), but the average geothermal gradient is around 4°C/100 m. The higher geothermal province (Jharia, East Bokaro, Raniganj) of which Jharia is the locus for the entire belt, is different from the lower geothermal region occurring further west (South Karanpura). Calculated rate of accumulation of sediment and subsidence of the basin floor of four basins are in agreement with the palaeodepth, pdaeotemperature and maturation condition of the organics.

Sinking, stabilisation and later uplift of the sedimentary pile were followed by fierce erosional activity which was maximum in Raniganj and minimum in South Karanpura. The degree of erosion is in accordance with the present day slope of the drainage system i.e., towards the Darnodar river and its tributaries. The error percentage between the calculated and accepted pre-erosional stratigraphic thickness of the four basins is found to lie between 0.56 (South Karanpura) and 4.74 (Jharia), which is remarkably low and indicates the strength of the analyses.