Organo-petrographic and geochemical characteristics of Gurha lignite deposits, Rajasthan, India: Insights into the palaeovegetation, palaeoenvironment and hydrocarbon source rock potential

The sedimentary sequence containing lignite deposits in Gurha quarry of the Bikaner-Nagaur Basin (Rajasthan) has been investigated. The samples from lignite and allied shale horizons were evaluated for petrographical, palynological, palynofacies and organic geochemical inferences, to depict the source flora and to reconstruct the palaeodepositional conditions prevailed during the sedimentation. An assessment for the hydrocarbon generation potential of these deposits has also been made. The results revealed the dominance of huminite macerals and phytoclasts organic matter (OM) indicating the existence of forested vegetation in the vicinity of the depositional site. A relatively high terrigenous/aquatic ratio (TAR) and the carbon preference index (CPI) are also suggesting the contribution of higher plants in the peat formation. However, the n-alkane distributions, maximizing at n-C17 and n-C29, showed inputs from the algal communities along with the higher plant derived organic matters. Recovered palynomorphs of the families Onagraceae, Meliaceae, Arecaceae, Rhizophoraceae, Rubiaceae, Ctenolophonaceae, etc. together with oleanene and ursane types of triterpenoids suggest the contribution from angiosperms source vegetation. Interestingly, the presence of Araucareaceae and Podocarpaceae pollen grains shows the existence of gymnosperms vegetation. Further, the presence of tetracyclic diterpanes; demethylated entbeyerane, sandaracopimarane, pimarane, and Kaurane type of compounds confirms the contribution of conifers. The variation in the values of the coefficient of non-equality (H: 0.68%–7.56%), the standard deviation (δ: 0.04%–0.16%) and the coefficient of variability (V: 16.10%–46.47%), also shows the heterogeneity in the source organic matter. The various petrographical indices, palynological entities, and geochemical parameters indicate that the peatforming vegetation was accumulated under a mixed environment and fluctuating hydrological settings. The interpretation of palynofacies data on APP (Amorphous organic matter-Phytoclast-Palynomorphs) diagram suggests that the accumulation of organic matter occurred in a dysoxic-suboxic condition in a proximal (to land) setting with the shift to an anoxic condition in distal setting towards the termination of sedimentation. The huminite (ulminite) reflectance (Rr) values (av. 0.28%) showed a good relationship with average Tmax value (414
C), suggesting the immaturity. The TOC content ranges of 13–59 wt.%, and HI values vary between 101 and 546 mg HC/g TOC in the studied samples. Collectively, the studied lignite and shale samples have the admixed kerogens (Type III–II) and exhibit the ability to generate the gaseous to oil hydrocarbons upon maturation.