Depositional environment and factors controlling β-carotane accumulation: A case study from the Jimsar Sag, Junggar Basin, northwestern China

A combined investigation of bulk geochemistry and molecular geochemistry was carried out on 31 source rock samples from the Lower Permian Lucaogou Formation, in the Jimsar Sag of the Junggar Basin, northwestern China. Geochemical characterization reveals the depositional environments and other factors favorable for the generation and accumulation of β-carotane, an important hydrocarbon precursor. Many core samples have a total organic carbon content > 1%, and Rock-Eval S2 values > 2 mg HC/g rock, suggesting relatively high hydrocarbon generation potential. Biomarkers in the Lucaogou source rock extracts are characterized by short- to long-chain n-alkanes, a wide range of Pr/Ph ratios (0.74-1.42) and β-carotane index values (β-carotane/Cmax, 0.16-1.92), high concentrations of C29 sterane, relatively low tricyclic terpane content, and moderate gammacerane index values (0.14-0.29). This is consistent with a weakly oxidizing to weakly reducing depositional environment, with fresh to brackish water conditions, rapid input of terrigenous organic matter, and a relatively small contribution of aquatic algal-bacterial organic matter. Cross-plots of biomarker parameters and the β-carotane index show that β-carotane in the Lucaogou source rocks is mainly derived from terrigenous organic matter, and is found predominantly in sediments deposited in weakly reducing, brackish environments. It does not appear to have been affected by thermal maturation.