Molecular structure and thermal decomposition kinetics of kerogen from the Paleocene oil-shale facies in the Bikaner–Nagaur Basin, western India

The bitumen shale facies in the Paleocene Palana Formation serves as a key oil-shale resource in the Bikaner-Nagaur Basin, Western India. It has a high organic matter concentration, which provides the necessary foundation for significant oil accumulation. Therefore, the purpose of this work is to characterize the molecular structure of kerogen within the Palana oil shale facies in order to assess the potential of shale oil resources and comprehend the mechanisms of hydrocarbon generation. In this study, comprehensive chemical analyses, including elemental analysis (CHNS), FTIR, TG/DTA, Py-GC, and kinetics of the kerogen decomposition, incorporated with microscopic investigation, were employed to decipher the elemental composition and molecular structure of Palana's kerogen. The microscopic analysis of kerogen indicates that the Palana oil shale sediments are characterized by high abundance marine organic matter assemblages, including bituminite, fluorescence AOM, and algae, consistent with the presence of hydrogen-rich kerogen. The predominance of hydrogen-enriched kerogen, primarily classified as Type II, with moderate-to-low sulfur content is substantiated by the kerogen's elemental profile and molecular structure. This includes a high hydrogen-to-carbon atomic ratio (H/C ˃1.40), low sulfur-to-carbon atomic ratio (S/C < 0.04), and predominance of aliphatic compounds with relatively low concentrations of aromatic compounds. The petroleum type of the Palana's kerogen is corresponding to P-N-A oils with high wax content, and it is released at a subsurface temperature regime of 107 to 153 °C, which is consistent with the computed vitrinite reflectance values of 0.62-1.07%VR_o, as demonstrated by the bulk and compositional kinetic results. The highlight results of the elemental composition and molecular structure provide a strong basis for further in-situ conversion processes and development of the shale oil system, as well as the mechanism of petroleum generation in the oil shale of the Palana Formation.