Geochemical and organic petrographic characteristics of low-rank coals from Thar coalfield in the Sindh Province, Pakistan

Abstract

Pakistan's largest coal reserves are in the Sindh Province. The organic geochemical characteristics and petro- graphic characteristics of the Tertiary coals in the Thar coal- field, south-eastern Sindh Province, were investigated to their hydrocarbon generative potential and regional rank and to reconstruct the palaeoenvironment conditions during peat ac- cumulation. The Thar coals are lignite to subbituminous C rank, possessing huminite reflectance in the range of 0.26- 0.39 % and high moisture and volatile matter contents. The coals have total organic carbon (TOC) content in the range of 47-67 wt%, and hydrogen index values between 105 and 437 mg HC per gram TOC indicate that their organic matter type is dominated by type III and mixed types II-III kerogens, whereby the coals were derived from plant materials of terrig- enous origin. If subjected to appropriate burial and heating, the hydrocarbon potential of these coals would be considered to be mainly gas-prone with limited oil-generative capacity. Diagnostic macerals and petrographic facies show that the Thar coals are humic and characterised by predominant huminite with significant amounts of liptinite and low amounts of inertinite macerals, representing predominantly topogenous mires deposited under anaerobic conditions, with limited thermal and oxidative tissue destruction. The palaeoenvironment conditions of the coals are generally interpreted as a lower deltaic plain wet peat-swamp deposi- tional setting, which are generally characterised by low Tissue Preservation Index (TPI) and high Gelification Index (GI) values. These plotted on the marsh field of the Diessel's diagram, consistent with generate relatively high ash yield. The organic facies study also shows that the main coal seams of the Thar coalfields were deposited in limnic freshwater environment, generally wet limno-telmatic zone as supported by relatively low sulphur contents.