Exploring the role of organofacies in governing kerogen kinetics: A case study of the Permian Gondwana coals from India

This study investigates four Late Permian Gondwana coals from the Raniganj sub-basin, Damodar Valley and three Early Permian Gondwana coals from the Talcher sub-basin, Mahanadi Valley. The study aims to characterize the kerogen type, hydrocarbon generation potential, thermal maturity, and organic matter composition, as well as to explore the impact of organic matter characteristics on kerogen kinetic parameters, utilizing multi proxy approach. The study further investigates the influence of the mineral matrix on kerogen decomposition kinetics. Based on the Rock-Eval parameters, Fourier transform infrared spectroscopy (FTIR) parameters, and vitrinite reflectance ( R o ), the kerogen is primarily classified as immature/early mature Type Ⅲ kerogen derived from terrestrial land plants, with minor contribution from Type Ⅱ kerogen having mainly gas generating potential. The source of the organic matter, as determined by stable carbon isotopic composition (δ 13 C org ) values and C/N ratios indicates a primary input from C 3 terrestrial plants. The presence of enriched collotelinites and liptinites, such as sporinite and resinite, provides evidence for the input of terrestrial higher plants, including both herbaceous and arborescent species. Various indices derived from maceral abundances, including the gelification index ( GI ), tissue preservation index ( TPI ), and vegetation index ( VI ), collectively indicate diverse depositional conditions which range from wet forest swamps to shallow water-covered wet forest swamps and even dry forest swamps. The variation in vitrinite macerals influenced the kerogen type and consequently impacted the kinetic parameters, viz. the distribution of activation energies ( E a ), kerogen transformation ratio ( KTR ), and hydrocarbon generation rate ( HGR ). Among the Talcher coals, shaly coals exhibit different kerogen type and kerogen transformation. This dissimilarity can be attributed to variable maceral compositions. Unlike the Talcher samples, all Raniganj samples show consistent kinetic behaviour despite their sample type (coal/shaly coal) or kerogen type (Type Ⅲ/mixed Type Ⅱ/Ⅲ) owing to comparable liptinite/vitrinite maceral composition. This study reveals that whilst the presence of mineral matrix shifts the apparent E a of kerogen through interactions like adsorption and catalysis, it does not significantly affect the kerogen type, HGR , or KTR of the studied coals. • The Permian coals/shaly coals from Raniganj and Talcher sub-basins contain terrestrial Type Ⅲ/Ⅱ kerogen. • E a distribution/FTIR denote heterogeneity of OM. • Organofacies instead of sample nature influences the kerogen type/kinetic parameters. • Different sample nature but similar macerals may exhibit similar kerogen kinetics. • Similar FTIR spectra but different macerals may produce different kinetic outcomes.