Evaluating the hydrocarbon generation potential of the Paleocene–Eocene carbonaceous rocks in the Barmer and Bikaner-Nagaur Basins, western Rajasthan, India

Abstract

The Bikaner-Nagaur and Barmer Basins (Rajasthan) are the most important petroliferous sedimentary basins in India. For over a decade, the exploration and extraction of hydrocarbons in these basins. Paleocene-Eocene age rocks bear organic-rich sediments in these basins, including lignite and carbonaceous shale deposits. The present research investigates the source rock properties, petroleum potential and thermal maturity of the carbonaceous shale partings from the lignite mines of Gurha (Bikaner-Nagaur Basin) and Kapurdi (Barmer Basin) using petrographical and geochemical tools. The carbonaceous shales have high organic matter (OM), with considerable total organic carbon (TOC) contents ranging from 13% to 39%. Furthermore, they contain hydrogen-rich kerogen, including types II and II/III, as evidenced by the Rock-Eval and elemental analysis results. The existence of these kerogen types indicates the abundance of reactive (vitrinite and liptinite) macerals. However, the carbonaceous shales from the Bikaner–Nagaur Basin have oil generation potentials, with a high hydrogen index (up to 516 mg HC/g TOC) and a H/C ratio (up to 1.5) along with a significant presence of oil-prone liptinitic macerals. Apart from the geochemical and petrological results, the studied shales have low huminite reflectance (0.31%–0.48%), maximum temperature (S₂ peak; T_max) between 419°C and 429°C, and low production index values (PI: 0.01–0.03). These results indicate that these carbonaceous shales contain immature OM, and thereby, they cannot yet release commercial amount of oil. This immaturity level in the studied outcrop section is due to the shallow burial depth. Geochemical proxies further indicate the presence of both oil and gas-prone source rocks.