Sedimentary and organic geochemical investigation of tertiary lacustrine oil shale in the central Tibetan plateau: Palaeolimnological and palaeoclimatic significances

The Zhuonai Lake and Lunpori oil shale, located in the central Tibetan Plateau, were deposited in a Tertiary lacustrine environment in the Hoh Xil and Lunpola Basins, respectively. A combined investigation of sedimentary, organic geochemistry, and organic carbon isotope was performed to define source rock potential, the palaeoenvironment, and source of the organic matter. Three types of facies sequences are developed in the Zhuonai Lake section and oil shale mainly occurs in the oil shale-marl sequence. The oil shale-silty claystone and oil shale-marl facies sequences are developed in the Lunpori section. Both oil shales are finely stratified especially for the Lunpori oil shale with thickness of the laminae about 10 μm which contains abundant pyrite crystals and fossilized fish bones.Both the Zhuonai Lake and Lunpori oil shale contain high values of total organic matter (TOC), S2, and hydrogen index (HI), indicating the oil shales have good oil source rock potential. Tmax values show an immature stage of organic matter for the Zhuonai Lake oil shale and thermally marginally mature for the Lunpori oil shale because of difference in their burial history. Lunpori oil shale shows the characteristics of amorphous organic matter (AOM) dominance, odd-over-even predominance, maximum n-alkanes peak at nC25 or nC23, higher proportion of C29 sterane, light δ13Corg values (− 30.0‰ to − 27.4‰), high concentrations of phytane (Pr/Ph ratio ranging from 0.02 to 0.18), high values of gammacerane index (up to 25.26), and presence of β-carotane, which are consistent with a reducing, stratified, and hypersaline palaeo-lake with main contribution of algae and bacteria to the organic matter (OM). Oil shale samples from the lower and upper part of the Zhuonai Lake section are characterized by low H/C ratios and high O/C ratios, amorphous humic OM and vitrinite dominance, odd-over-even predominance with long-chain n-alkanes dominance, heavy organic carbon isotope, and C29 > C27 sterane distribution, in agreement with a main contribution of land plant. The heavy organic carbon isotope suggests the expanding of C4 land plant during the early Miocene. While the most samples show characteristics of AOM dominance, maximum peak at nC29 or nC31, and high C27 sterane concentration, consistent with a main contribution from algae to the OM. The Pr/Ph ratio cannot be applied as a measure for redox conditions in the section because of maturity effects. Biomarker analysis indicates that the salinity during the deposition of the Zhuonai Lake oil shale was not enhanced.The Oligocene Lunpori oil shale and the Early Miocene Zhuonai Lake oil shale recorded the variation of palaeolimnological water chemistry. The development history of the palaeo-lakes from the Oligocene saline to the Early Miocene fresh water reflects the variation of palaeoclimate from dry to humid in the central Tibetan Plateau.

Research highlights
► Sedimentary and organic geochemistry of Zhuonai Lake and Lunpori oil shale are included.
► Organic geochemical data indicated algae and bacteria are the main contribution to the organic matter of the Lunpola oil shale; while land plants are the dominant organic matter in the lower and upper Zhuonai Lake oil shale.
► The palaeo-lakes varied from the Oligocene saline water to the Early Miocene fresh-water reflects the variation of palaeoclimate from dry to humid in the central Tibetan Plateau.