Paleodepositional environment and hydrocarbon generation potential of the Paleogene lignite in the Barmer Basin, Rajasthan, India

Abstract

The Barmer Basin in Rajasthan is considered one of the most important petroliferous sedimentary basins of India. Apart from a hydrocarbon-producing source rock, the basin also hosts some significant lignite deposits of Giral fen and Kapurdi bog developed during the late Paleocene to early Eocene, being currently under exploitation. Applying organic petrology, geochemical, mineralogical techniques, and Rock-Eval pyrolysis on samples obtained from seams in both mines, the paleodepositional settings in both areas were reconstructed, and the hydrocarbon generation potential was assessed. The Tmax (377–427 °C) and PI (0.01–0.09) values in the studied samples from the Barmer Basin indicate an immature nature of the organic matter. HI values range from 66 to 476 mg HC/g TOC, revealing that the Giral and Kapurdi lignite contains mostly Type III and mixed Type II-III kerogen and can generate mainly gaseous hydrocarbons, although few samples have liquid hydrocarbon generation potential. Significant values of TOC (20–60 wt%) reveal that they may act as a good source of hydrocarbons and make them favourable for the generation of hydrocarbons. The concentrations of major oxides and selected trace elements indicate that the Giral and Kapurdi lignite was developed in warm humid to some extent arid warm climate and oxic conditions prevailed in the basin. Petrographic indices suggest the peat accumulation occurred in a dry forest mire under limnic to limnotelmatic conditions. The indicators further suggest the lignite originated from the bog and fen under mesotrophic to ombrotrophic hydrological condition. The Giral and Kapurdi lignite were determined to be ortho-lignite or low-rank lignite C rank, immature for hydrocarbon generation. The lignite derived mainly from herbaceous vegetation with a higher contribution of woods in the Giral fen compared to the Kapurdi bog. Geochemical indicators suggest the presence of freshwater conditions and a shift from freshwater to brackish water conditions within the basin.