Organic Geochemical Assessment and Characterisation of Extra-Peninsular Gondwana Coal From Kalijhora Basin, Darjeeling District, West Bengal, India

Abstract

Kalijhora in Darjeeling district of West Bengal is one of the few places in India where extra-peninsular Gondwana rocks are exposed. The Kalijhora lithostratigraphic column which is correlatable with part of Damuda Formation, Bhutan and with fining upward Upper Barakar Formation of peninsular Lower Gondwanas, exudes imprints of Himalayan geodynamism and is largely arenaceous with minor carbonaceous shale, sandy shale and coal. The present study primarily entails the organic geochemical assessment of the coal therein to ascertain their hydrocarbon generating potential through physico-chemical methods like proximate analysis, utimate/elemental analysis and rock eval pyrolysis along with an appraisal of significant organic petrographic attributes. The study reveals that Kalijhora coals are vitrinite rich with higher vitrinite reflectance values and depict the dominance of vitrite microlithotype indicating coal formation under fluvial control. They are bituminous in rank having low moisture and relatively high ash content along with moderately high sulphur, depicting marine influence in a near shore depositional environment. The organic matter in sediments is primarily subjugated by type-III kerogen matter which has the potential to expel gaseous hydrocarbon on liquefaction. Although the TOC and S₂ values are high, lower values of S₁ indicate poor source rocks, fact also supported by genetic potential findings. HI and atomic H/C analyses indicate the oil generation chances to be low. Source rocks are found to be over mature with presence of inert material. A higher level conversion or expulsion is also reflected from the findings. Kalijhora coal can be categorised as dominantly bituminous medium volatile ones and, para-bituminous as per Seyler’s classification. Overall analyses indicate poor preservation of the Kalijhora coal which was also impacted by increase in terrigenous input due to near shore deposition of Kalijhora sediments under fluctuating sea levels resulting in a lower level of hydrocarbon generating potential.