Organic geochemical and petrographical characteristics of the Nagaur lignites, Western Rajasthan, India and their relevance to liquid hydrocarbon generation

Abstract

The article presents the geochemical and petrographical characteristics of Paleocene lignite deposits of the Nagaur Basin to appraise the regional rank variation, organic matter maturity, and liquid hydrocarbon generative potential. The petrographical investigation indicated that the analyzed lignites contain abundant huminite group macerals with a significant amount of the liptinite group macerals while the concentration of inertinite group is less in comparison to huminite and liptinite group. The huminite reflectance (%R0) indicates that lignite samples of Nagaur Basin are thermally immature in nature and lignite to sub-bituminous B in rank. Immaturity of samples has a significant influence on the proximate results of the samples, especially on moisture and volatile matter concentration and low fixed carbon percentage. The analyzed lignites have TOC and S2 values that range from 30.4 to 43.0 and 58.0 to 130.7 mg HC/g rock, respectively, which shows that these lignites have excellent hydrocarbon generation potential and make them industrially important, considering the huge lignite reserve in the study area region. The Rock-Eval pyrolysis and ultimate analysis also reveal that these lignite samples mainly contained mixed type II–III kerogens with significant potential for both liquid hydrocarbon and gas generation. These kerogens are dependable on the maceral composition that was derived from the microscopic study. The presence of the high huminite with significant amount of liptinite (more than 15%) macerals also suggests that these lignites can act as a good source rock for hydrocarbon generation.