Tracing the Late Permian Gondwana surface environments using sedimentary biomarkers from the Raniganj Formation, Damodar Valley, Eastern India

Sedimentary biomarkers are useful proxies for determining the source rock properties and its paleo-environmental conditions of deposition. The Upper Permian shales of the Raniganj Formation in the Jharia sub-basin of the Damodar Valley were analyzed for their biomarker distribution patterns using gas chromatography–mass spectrometry, with the aim to decipher the Late Permian Gondwana surface environment, organic provenance and source rock properties, including the thermal maturity. The shales are characterized by acyclic isoprenoids, n-alkane distributions ranging between nC₁₂ to nC₃₅ with no odd–even predominance, hopanes, steranes and polyaromatic hydrocarbons. The organic-matter input, primarily from terrestrial Type-III kerogen, got deposited in open water or swampy environments. Stratigraphically, a shift in the depositional environment from marine/aquatic to terrestrial was marked from the depth to the top of the Raniganj Formation. This leads to the inference of a geographic location close to or along the shoreline, potentially within a peri-tidal environment, where sedimentation was influenced significantly by waves. The organic-rich shales are thermally mature in the oil-generation stage and exhibit increasing maturity with depth but have not undergone sufficient burial for the gas generation. Overall, there is a combination of organic-matter inputs, from both terrestrial and aquatic sources, along with a discernible transition in the depositional environment from marine/aquatic to terrestrial in the area, leading to deposition of organically rich and thermally mature Raniganj shales.