Structural and maturity evolution of lignite using Raman spectroscopy and organic petrology of the Barmer basin, Rajasthan, India.

This study investigates the structural and thermal evolution of lignite from the Barmer Basin, Rajasthan, India, using Raman spectroscopy and organic petrology. Lignite samples were collected from active mines in Kapurdi, Giral, and Sonari and analyzed through proximate and ultimate analyses, petrography and Raman spectroscopic techniques to assess their rank and structural characteristics. Vitrinite reflectance measurements, ranging from 0.24% to 0.38%, to assess maturation of organic matter. The volatile matter, a conventional maturity parameter, ranges from 38.5% to 46.5%, indicating low rank and limited thermal evolution. Ash yield values vary from 2.32% to 28.24%. Petrographically, the lignite's are rich in huminite, ranging from 58.0% to 77.4%, with inertinite content between 3.6% and 13.6% and liptinite concentration from 5.4% to 13.0%. The maceral composition suggests carbonaceous materials in an early diagenetic phase. Raman spectroscopy reveal the presence of D and G bands, typical spectral signatures of carbon-rich natural materials. The Raman shift for the D band ranges from 1343 to 1391 cm^-1 and for the G band, from 1565 to 1588 cm^-1. The intensity ratio of the I_D/I_G band ranged from 0.71 to 0.97, while the (G-D1) parameter varied from 187 to 238 cm^-1, indicating sample heterogeneity. However, the AD₁/A_G ratio shows an irregular pattern, with values between 1.19 and 2.74. The study demonstrates that integrating Raman spectroscopy with Vitrinite reflectance (VRo) provides insights into the structural attributes of immature kerogen which denotes organic matter that has not yet undergone sufficient thermal evolution to generate oil or gas, which is typical of low-rank coals like lignite. The petrological, proximate and ultimate analyses collectively confirm the carbonaceous, coaly material in its early coalification stages. This study emphasizes the use of Raman spectroscopy and Organic petrology as a proxy to evaluate lignite thermal evolution, enhance the interpretation of carbon structural disorder (e.g., D/G band ratios) and offer deeper insights into the aromaticity, maturity and heterogeneity of lignite, comparing its results with traditional maturity indicators.