Evaluation of hydrocarbon potential with insight into climate and environment present during deposition of the Sonari lignite, Barmer Basin Rajasthan

Abstract

Lignite samples from the Sonari mines of Barmer Basin, Rajasthan, India have been studied for the first time by applying advanced petrochemical techniques. The objective of the study was to reconstruct Paleomire conditions and to examine the source rock potential of the lignite-bearing sequence. Based on the mean huminite/vitrinite reflectance values (0.24–0.31%Ro), all of the samples studied are lignite B in rank. Geochemical analysis results show, the atomic ration H/C and O/C ranges from 0.03–0.15 and 0.27–0.61 respectively, hydrogen index (HI) and oxygen index (OI) ranges from 173 to 256 mg HC/g TOC and 29–55 mg CO2/g TOC respectively, with Tmax value (415–416 °C), entails these lignites are thermally immature and contain type III kerogen. The studied lignites have total organic carbon (TOC), and S2 values vary from 41.24–49.19wt% and 78.53–119.81 mg HC/g rock, respectively, which indicate that these lignites may be served as good hydrocarbon source rock. The lignites are principally composed of huminite (mean 47.5 vol%) and inertinite (mean 31.4 vol%) group of macerals with a low concentration (mean 8.5 vol%) of liptinite macerals. The predominance of attrinite in the Sonari lignite samples suggests that woody vegetation has undergone significant degradation. Petrographic indices such as TPI (Tissue preservation index; 0.92–2.13), GI (Gelification index; 0.97–8.25), GWI (Groundwater influence index; 0.34–1.50), and VI (Vegetation index; 0.16–1.84) indicate deposition in a wet forest paleomire and peat formation under ombrotrophic to mesotrophic hydrogeological conditions. Ternary plots based on maceral and microlithotype entails fluctuating water table condition during peat deposition. The GC-MS (Gas chromatography–mass spectrometry) data shows that the ratios, Pr/Ph pristine/phytane (˂1), Pr/C17 (0.57), Ph/C18 (0.25) and carbon preference index (CPI) value 3.73 along with δ13C (−25.04%) values indicate that the Sonari lignites are composed primarily of gymnosperms taxa which formed under relatively dry and cold paleoclimatic conditions. Besides, FT-IR (Fourier-transform infrared spectroscopy) spectra give the signature of a C–H aliphatic stretching region and predominate methylene group in the aliphatic carbon region, which is also observed in the NMR (Nuclear magnetic resonance).

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